Move ERC test utilities to common file

* lisp/erc/erc-common.el (erc--define-catalog): Update name of
reference to convenience command now located in `erc-tests-common'.
* test/lisp/erc/erc-button-tests.el: Require common test-util library
`erc-tests-common', located under test/lisp/erc/resources.
; (erc-button-alist--url,
; erc-button-tests--erc-button-alist--function-as-form,
; erc-button-tests--erc-button-alist--nil-form,
; erc-button--display-error-notice-with-keys): Use common helper
; `erc-tests-common-init-server-proc' from test-utils library.
* test/lisp/erc/erc-fill-tests.el: Require `erc-tests-common'.
(erc-fill-tests--wrap-populate): Use helper
`erc-tests-common-init-server-proc'.
(erc-fill-tests--save-p): Remove.  See replacement
`erc-tests-common-snapshot-save-p' in erc-tests-common.
(erc-fill-tests--graphic-dir): Add trailing slash.
(erc-fill-tests--compare): Move body to generalized utility
`erc-tests-common-snapshot-compare' in erc-tests-common.
* test/lisp/erc/erc-goodies-tests.el: Require `erc-tests-common'.
(erc--get-inserted-msg-beg/readonly,
erc--get-inserted-msg-end/readonly,
erc--get-inserted-msg-bounds/readonly): Move here from erc-tests.el.
* test/lisp/erc/erc-networks-tests.el: Load `erc-tests-common'.
(erc-networks-tests--create-live-proc): Defer to
`erc-tests-common-init-server-proc' and drop optional buffer param.
(erc-networks-tests--clean-bufs): Defer to
`erc-tests-common-kill-buffers'.
(erc-networks--rename-server-buffer--existing--live): Call
`erc-networks-tests--create-live-proc' in server buffer.
* test/lisp/erc/erc-scenarios-internal.el: Load `erc-tests-common'.
(erc-scenarios-internal--run-graphical-all): Use
`erc-tests-common-create-subprocess' to create process.
* test/lisp/erc/erc-scenarios-sasl.el
(erc-scenarios-sasl--plain-fail): Silence error message.
* test/lisp/erc/erc-stamp-tests.el: Require `erc-tests-common'.
(erc-stamp-tests--insert-right, erc-timestamp-intangible--left): Use
`erc-tests-common-init-server-proc'.
(erc-tests--assert-get-inserted-msg/stamp,
erc-stamp-tests--assert-get-inserted-msg/stamp): Move from
erc-tests.el, renaming to latter.
(erc--get-inserted-msg-beg/stamp,
erc--get-inserted-msg-beg/readonly/stamp,
erc--get-inserted-msg-end/stamp,
erc--get-inserted-msg-end/readonly/stamp,
erc--get-inserted-msg-bounds/stamp,
erc--get-inserted-msg-bounds/readonly/stamp): Move here from
erc-tests.el.
* test/lisp/erc/erc-tests.el: Require `erc-tests-common'.
(erc-with-server-buffer): Use renamed test-helper utility
`erc-tests-common-init-server-proc'.
(erc-tests--send-prep, erc-tests--set-fake-server-process): Move to
`erc-tests-common' library and rename to
`erc-tests-common-prep-for-insertion' and
`erc-tests-common-init-server-proc', respectively.
; (erc-hide-prompt, erc--refresh-prompt,
; erc-setup-buffer--custom-action, erc--parsed-prefix,
; erc--update-channel-modes, erc--channel-modes,
; erc--channel-modes/graphic-p, erc-ring-previous-command): Use
; `erc-tests-common-prep-for-insertion' instead of
; `erc-tests--send-prep', and use `erc-tests-common-init-server-proc'
; instead of `erc-tests--set-fake-server-process'.
(erc-tests--with-process-input-spy): Move to `erc-tests-common' and
rename `erc-tests-common-with-process-input-spy'.
; (erc--check-prompt-input-functions, erc-send-current-line,
; erc--check-prompt-input-for-multiline-blanks,
; erc-send-whitespace-lines): Use renamed
; `erc-tests-common-with-process-input-spy' and
; `erc-tests-common-init-server-proc'.
; (erc-process-input-line): Use renamed
; `erc-tests-common-init-server-proc'.
(erc-tests--get-inserted-msg-setup,
erc-tests--assert-get-inserted-msg,
erc-tests--assert-get-inserted-msg/basic,
erc-tests--assert-get-inserted-msg-readonly-with): Move to
`erc-tests-common' and rename with "common" prefix, using single
instead of double hyphen.
(erc-tests--assert-get-inserted-msg/stamp): Move to `erc-stamp-tests'
and rename with "stamp" prefix.
(erc--get-inserted-msg-beg/stamp,
erc--get-inserted-msg-beg/readonly/stamp,
erc--get-inserted-msg-end/stamp,
erc--get-inserted-msg-end/readonly/stamp,
erc--get-inserted-msg-bounds/stamp,
erc--get-inserted-msg-bounds/readonly/stamp): Move to
`erc-stamp-tests'.
(erc--get-inserted-msg-beg/readonly,
erc--get-inserted-msg-end/readonly,
erc--get-inserted-msg-bounds/readonly): Move to `erc-goodies-tests'.
; (erc--get-inserted-msg-beg/basic,
; erc--get-inserted-msg-end/basic,
; erc--get-inserted-msg-bounds/basic): Use common helpers.
; (erc--route-insertion): Use renamed helper functions
; `erc-tests-common-with-process-input-spy' and
; `erc-tests-common-init-server-proc'.
(erc-tests--make-server-buf): Move to `erc-common-tests' and rename
with "common" prefix.
(erc-tests--make-client-buf): Remove unused function without supplying
replacement.
; (erc-handle-irc-url): Use renamed `erc-tests-common-make-server-buf'
; utility function.
; (erc-tests--assert-printed-in-subprocess): Use helper from common lib
; `erc-tests-common-create-subprocess code' to do the heavy lifting.
(erc-tests--string-to-propertized-parts,
erc-tests-pp-propertized-parts): Move to `erc-tests-common' and rename
with "common" prefix.
* test/lisp/erc/resources/erc-tests-common.el: New file containing
helper utilities and fixtures used by multiple files in test/lisp/erc.

10 files changed, 449 insertions, 364 deletions

diff --git a/lisp/erc/erc-common.el b/lisp/erc/erc-common.el
index 64312e51f41..6c101dea4e3 100644
--- a/lisp/erc/erc-common.el
+++ b/lisp/erc/erc-common.el
@@ -551,10 +551,10 @@ Use the CASEMAPPING ISUPPORT parameter to determine the style."
   "Define `erc-display-message' formatting templates for NAME, a symbol.
 
 See `erc-define-message-format-catalog' for the meaning of
-ENTRIES, an alist.  Also see `erc-tests-pp-propertized-parts' in
+ENTRIES, an alist, and `erc-tests-common-pp-propertized-parts' in
 tests/lisp/erc/erc-tests.el for a convenience command to convert
-a literal string into a sequence of `propertize' forms, which
-are much easier to review and edit."
+a literal string into a sequence of `propertize' forms, which are
+much easier to review and edit."
   (declare (indent 1))
   (let (out)
     (dolist (e entries (cons 'progn (nreverse out)))
diff --git a/test/lisp/erc/erc-button-tests.el b/test/lisp/erc/erc-button-tests.el
index 34ad06b7eb8..be11b76bd2e 100644
--- a/test/lisp/erc/erc-button-tests.el
+++ b/test/lisp/erc/erc-button-tests.el
@@ -21,12 +21,15 @@
 
 ;;; Code:
 
+(require 'ert-x) ; cl-lib
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-tests-common)))
+
 (require 'erc-button)
 
 (ert-deftest erc-button-alist--url ()
-  (setq erc-server-process
-        (start-process "sleep" (current-buffer) "sleep" "1"))
-  (set-process-query-on-exit-flag erc-server-process nil)
+  (erc-tests-common-init-server-proc "sleep" "1")
   (with-current-buffer (erc--open-target "#chan")
     (let ((verify
            (lambda (p url)
@@ -65,9 +68,7 @@
   (apply #'erc-button-add-button rest))
 
 (defun erc-button-tests--erc-button-alist--function-as-form (func)
-  (setq erc-server-process
-        (start-process "sleep" (current-buffer) "sleep" "1"))
-  (set-process-query-on-exit-flag erc-server-process nil)
+  (erc-tests-common-init-server-proc "sleep" "1")
 
   (with-current-buffer (erc--open-target "#chan")
     (let* ((erc-button-tests--form nil)
@@ -102,9 +103,7 @@
      (apply #'erc-button-add-button r))))
 
 (defun erc-button-tests--erc-button-alist--nil-form (form)
-  (setq erc-server-process
-        (start-process "sleep" (current-buffer) "sleep" "1"))
-  (set-process-query-on-exit-flag erc-server-process nil)
+  (erc-tests-common-init-server-proc "sleep" "1")
 
   (with-current-buffer (erc--open-target "#chan")
     (let* ((erc-button-tests--form nil)
@@ -228,11 +227,9 @@
           (inhibit-message noninteractive)
           erc-modules
           erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
-      (erc-mode)
-      (setq erc-server-process
-            (start-process "sleep" (current-buffer) "sleep" "1"))
-      (set-process-query-on-exit-flag erc-server-process nil)
-      (erc--initialize-markers (point) nil)
+      (erc-tests-common-prep-for-insertion)
+      (erc-tests-common-init-server-proc "sleep" "1")
+
       (erc-button-mode +1)
       (should (equal (erc-button--display-error-notice-with-keys
                       "If \\[erc-bol] fails, "
diff --git a/test/lisp/erc/erc-fill-tests.el b/test/lisp/erc/erc-fill-tests.el
index 5e5b1d332ac..df83466cbc3 100644
--- a/test/lisp/erc/erc-fill-tests.el
+++ b/test/lisp/erc/erc-fill-tests.el
@@ -24,6 +24,10 @@
 
 ;;; Code:
 (require 'ert-x)
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-tests-common)))
+
 (require 'erc-fill)
 
 (defvar erc-fill-tests--buffers nil)
@@ -58,9 +62,7 @@
         erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
     (cl-letf (((symbol-function 'erc-server-connect)
                (lambda (&rest _)
-                 (setq erc-server-process
-                       (start-process "sleep" (current-buffer) "sleep" "1"))
-                 (set-process-query-on-exit-flag erc-server-process nil))))
+                 (erc-tests-common-init-server-proc "sleep" "1"))))
       (with-current-buffer
           (car (push (erc-open "localhost" 6667 "tester" "Tester" 'connect
                                nil nil nil nil nil "tester" 'foonet)
@@ -106,10 +108,9 @@
             (when set-transient-map-timer
               (timer-event-handler set-transient-map-timer))
             (set-window-buffer (selected-window) original-window-buffer)
-            (when noninteractive
-              (while-let ((buf (pop erc-fill-tests--buffers)))
-                (kill-buffer buf))
-              (kill-buffer))))))))
+            (when (or noninteractive (getenv "ERC_TESTS_GRAPHICAL"))
+              (erc-tests-common-kill-buffers erc-fill-tests--buffers)
+              (setq erc-fill-tests--buffers nil))))))))
 
 (defun erc-fill-tests--wrap-check-prefixes (&rest prefixes)
   ;; Check that prefix props are applied over correct intervals.
@@ -134,74 +135,21 @@
       (should (equal (get-text-property (1- (pos-eol)) 'wrap-prefix)
                      '(space :width erc-fill--wrap-value))))))
 
-;; Use this variable to generate new snapshots after carefully
-;; reviewing the output of *each* snapshot (not just first and last).
-;; Obviously, only run one test at a time.
-(defvar erc-fill-tests--save-p (getenv "ERC_TESTS_FILL_SAVE"))
-
 ;; On graphical displays, echo .graphic >> .git/info/exclude
-(defvar erc-fill-tests--graphic-dir "fill/snapshots/.graphic")
+(defvar erc-fill-tests--graphic-dir "fill/snapshots/.graphic/")
 
 (defun erc-fill-tests--compare (name)
-  (let* ((dir (expand-file-name (if (display-graphic-p)
-                                    erc-fill-tests--graphic-dir
-                                  "fill/snapshots/")
-                                (ert-resource-directory)))
-         (expect-file (file-name-with-extension (expand-file-name name dir)
-                                                "eld"))
-         (erc--own-property-names
-          (seq-difference `(font-lock-face ,@erc--own-property-names)
-                          `(field display wrap-prefix line-prefix
-                                  erc--msg erc--cmd erc--spkr erc--ts erc--ctcp
-                                  erc--ephemeral)
-                          #'eq))
-         (print-circle t)
-         (print-escape-newlines t)
-         (print-escape-nonascii t)
-         (got (erc--remove-text-properties
-               (buffer-substring (point-min) erc-insert-marker)))
-         (repr (string-replace "erc-fill--wrap-value"
-                               (number-to-string erc-fill--wrap-value)
-                               (prin1-to-string got))))
-    (with-current-buffer (generate-new-buffer name)
-      (push (current-buffer) erc-fill-tests--buffers)
-      (with-silent-modifications
-        (insert (setq got (read repr))))
-      (erc-mode))
-    ;; LHS is a string, RHS is a symbol.
-    (if (string= erc-fill-tests--save-p (ert-test-name (ert-running-test)))
-        (let (inhibit-message)
-          (with-temp-file expect-file
-            (insert repr))
-          ;; Limit writing snapshots to one test at a time.
-          (message "erc-fill-tests--compare: wrote %S" expect-file))
-      (if (file-exists-p expect-file)
-          ;; Ensure string-valued properties, like timestamps, aren't
-          ;; recursive (signals `max-lisp-eval-depth' exceeded).
-          (named-let assert-equal
-              ((latest (read repr))
-               (expect (read (with-temp-buffer
-                               (insert-file-contents-literally expect-file)
-                               (buffer-string)))))
-            (pcase latest
-              ((or "" 'nil) t)
-              ((pred stringp)
-               (should (equal-including-properties latest expect))
-               (let ((latest-intervals (object-intervals latest))
-                     (expect-intervals (object-intervals expect)))
-                 (while-let ((l-iv (pop latest-intervals))
-                             (x-iv (pop expect-intervals))
-                             (l-tab (map-into (nth 2 l-iv) 'hash-table))
-                             (x-tab (map-into (nth 2 x-iv) 'hash-table)))
-                   (pcase-dolist (`(,l-k . ,l-v) (map-pairs l-tab))
-                     (assert-equal l-v (gethash l-k x-tab))
-                     (remhash l-k x-tab))
-                   (should (zerop (hash-table-count x-tab))))))
-              ((pred sequencep)
-               (assert-equal (seq-first latest) (seq-first expect))
-               (assert-equal (seq-rest latest) (seq-rest expect)))
-              (_ (should (equal latest expect)))))
-        (message "Snapshot file missing: %S" expect-file)))))
+  (let ((dir (expand-file-name (if (display-graphic-p)
+                                   erc-fill-tests--graphic-dir
+                                 "fill/snapshots/" )
+                               (ert-resource-directory)))
+        (transform-fn (lambda (got)
+                        (string-replace "erc-fill--wrap-value"
+                                        (number-to-string erc-fill--wrap-value)
+                                        got)))
+        (buffer-setup-fn (lambda ()
+                           (push (current-buffer) erc-fill-tests--buffers))))
+    (erc-tests-common-snapshot-compare name dir transform-fn buffer-setup-fn)))
 
 ;; To inspect variable pitch, set `erc-mode-hook' to
 ;;
diff --git a/test/lisp/erc/erc-goodies-tests.el b/test/lisp/erc/erc-goodies-tests.el
index ca02089eb7c..bdd197fa5cb 100644
--- a/test/lisp/erc/erc-goodies-tests.el
+++ b/test/lisp/erc/erc-goodies-tests.el
@@ -20,6 +20,10 @@
 ;;; Commentary:
 ;;; Code:
 (require 'ert-x)
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-tests-common)))
+
 (require 'erc-goodies)
 
 (defun erc-goodies-tests--assert-face (beg end-str present &optional absent)
@@ -420,4 +424,21 @@
        (goto-char (overlay-start erc--keep-place-indicator-overlay))
        (should (looking-at (rx "*** This buffer is for text")))))))
 
+(ert-deftest erc--get-inserted-msg-beg/readonly ()
+  (erc-tests-common-assert-get-inserted-msg-readonly-with
+   #'erc-tests-common-assert-get-inserted-msg/basic
+   (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
+
+(ert-deftest erc--get-inserted-msg-end/readonly ()
+  (erc-tests-common-assert-get-inserted-msg-readonly-with
+   #'erc-tests-common-assert-get-inserted-msg/basic
+   (lambda (arg) (should (= 11 (erc--get-inserted-msg-end arg))))))
+
+(ert-deftest erc--get-inserted-msg-bounds/readonly ()
+  (erc-tests-common-assert-get-inserted-msg-readonly-with
+   #'erc-tests-common-assert-get-inserted-msg/basic
+   (lambda (arg)
+     (should (equal '(3 . 11) (erc--get-inserted-msg-bounds arg))))))
+
+
 ;;; erc-goodies-tests.el ends here
diff --git a/test/lisp/erc/erc-networks-tests.el b/test/lisp/erc/erc-networks-tests.el
index d0f1dddf6b3..7d9424d7430 100644
--- a/test/lisp/erc/erc-networks-tests.el
+++ b/test/lisp/erc/erc-networks-tests.el
@@ -20,25 +20,21 @@
 ;;; Code:
 
 (require 'ert-x) ; cl-lib
-(require 'erc)
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-tests-common)))
 
 (defun erc-networks-tests--create-dead-proc (&optional buf)
   (let ((p (start-process "true" (or buf (current-buffer)) "true")))
     (while (process-live-p p) (sit-for 0.1))
     p))
 
-(defun erc-networks-tests--create-live-proc (&optional buf)
-  (let ((proc (start-process "sleep" (or buf (current-buffer)) "sleep" "1")))
-    (set-process-query-on-exit-flag proc nil)
-    proc))
+(defun erc-networks-tests--create-live-proc ()
+  (erc-tests-common-init-server-proc "sleep" "1"))
 
 ;; When we drop 27, call `get-buffer-create with INHIBIT-BUFFER-HOOKS.
 (defun erc-networks-tests--clean-bufs ()
-  (let (erc-kill-channel-hook
-        erc-kill-server-hook
-        erc-kill-buffer-hook)
-    (dolist (buf (erc-buffer-list))
-      (kill-buffer buf))))
+  (erc-tests-common-kill-buffers))
 
 (defun erc-networks-tests--bufnames (prefix)
   (let* ((case-fold-search)
@@ -1442,10 +1438,12 @@
   (let* (erc-kill-server-hook
          erc-insert-modify-hook
          (old-buf (get-buffer-create "FooNet"))
-         (old-proc (erc-networks-tests--create-live-proc old-buf))) ; live
+         ;;
+         old-proc) ; live
 
     (with-current-buffer old-buf
       (erc-mode)
+      (setq old-proc (erc-networks-tests--create-live-proc))
       (erc--initialize-markers (point) nil)
       (insert "*** Old buf")
       (setq erc-network 'FooNet
diff --git a/test/lisp/erc/erc-scenarios-internal.el b/test/lisp/erc/erc-scenarios-internal.el
index 4ec94cedf0e..b6c4d1ba27f 100644
--- a/test/lisp/erc/erc-scenarios-internal.el
+++ b/test/lisp/erc/erc-scenarios-internal.el
@@ -24,9 +24,12 @@
   (when (and (getenv "EMACS_TEST_DIRECTORY")
              (getenv "EMACS_TEST_JUNIT_REPORT"))
     (setq ert-load-file-name (or (macroexp-file-name) buffer-file-name)))
-  (let ((load-path (cons (expand-file-name "erc-d" (ert-resource-directory))
-                         load-path)))
-    (load "erc-d-tests" nil 'silent)))
+  (let ((load-path `(,(expand-file-name "erc-d" (ert-resource-directory))
+                     ,(ert-resource-directory)
+                     ,@load-path)))
+    ;; Run all tests in ./resources/erc-d/erc-d-tests.el.
+    (load "erc-d-tests" nil 'silent)
+    (require 'erc-tests-common)))
 
 ;; Run all tests tagged `:erc--graphical' in an "interactive"
 ;; subprocess.  Time out after 90 seconds.
@@ -45,13 +48,9 @@
                           (with-current-buffer ert--output-buffer-name
                             (kill-emacs (ert--stats-failed-unexpected
                                          ert--results-stats)))))
-         (args `("erc-interactive-all" ,(current-buffer)
-                 ,(concat invocation-directory invocation-name)
-                 "-Q" "-L" "." "-l" "ert"
-                 ,@(let (o) (while libs (push (pop libs) o) (push "-l" o)) o)
-                 "-eval" ,(format "%S" program)))
-         (proc (apply #'start-process args)))
-    (set-process-query-on-exit-flag proc nil)
+         (proc (erc-tests-common-create-subprocess program
+                                                   '( "-L" "." "-l" "ert")
+                                                   libs)))
 
     (erc-d-t-wait-for 90 "interactive tests to complete"
       (not (process-live-p proc)))
diff --git a/test/lisp/erc/erc-scenarios-sasl.el b/test/lisp/erc/erc-scenarios-sasl.el
index 74075b1aaf3..ecabc365adb 100644
--- a/test/lisp/erc/erc-scenarios-sasl.el
+++ b/test/lisp/erc/erc-scenarios-sasl.el
@@ -151,6 +151,7 @@
        (erc-sasl-mechanism 'plain)
        (erc--warnings-buffer-name  "*ERC test warnings*")
        (warnings-buffer (get-buffer-create erc--warnings-buffer-name))
+       (inhibit-message noninteractive)
        (expect (erc-d-t-make-expecter)))
 
     (with-current-buffer (erc :server "127.0.0.1"
diff --git a/test/lisp/erc/erc-stamp-tests.el b/test/lisp/erc/erc-stamp-tests.el
index fd2e7000c0e..3f17e36e002 100644
--- a/test/lisp/erc/erc-stamp-tests.el
+++ b/test/lisp/erc/erc-stamp-tests.el
@@ -21,6 +21,10 @@
 
 ;;; Code:
 (require 'ert-x)
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-tests-common)))
+
 (require 'erc-stamp)
 (require 'erc-goodies) ; for `erc-make-read-only'
 
@@ -44,9 +48,7 @@
       (erc-mode)
       (erc-munge-invisibility-spec)
       (erc--initialize-markers (point) nil)
-      (setq erc-server-process (start-process "p" (current-buffer)
-                                              "sleep" "1"))
-      (set-process-query-on-exit-flag erc-server-process nil)
+      (erc-tests-common-init-server-proc "sleep" "1")
 
       (funcall test)
 
@@ -223,13 +225,13 @@
         (erc-timestamp-intangible t) ; default changed to nil in 2014
         (erc-hide-timestamps t)
         (erc-insert-timestamp-function 'erc-insert-timestamp-left)
-        (erc-server-process (start-process "true" (current-buffer) "true"))
         (erc-insert-modify-hook '(erc-make-read-only erc-add-timestamp))
         msg
         erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
     (should (not cursor-sensor-inhibit))
-    (set-process-query-on-exit-flag erc-server-process nil)
+
     (erc-mode)
+    (erc-tests-common-init-server-proc "true")
     (with-current-buffer (get-buffer-create "*erc-timestamp-intangible*")
       (erc-mode)
       (erc--initialize-markers (point) nil)
@@ -307,4 +309,44 @@
       (should (equal (call-interactively #'erc-echo-timestamp)
                      "1983-09-26 21:00:00 -07")))))
 
+(defun erc-stamp-tests--assert-get-inserted-msg/stamp (test-fn)
+  (let ((erc-insert-modify-hook erc-insert-modify-hook)
+        (erc-insert-timestamp-function 'erc-insert-timestamp-right)
+        (erc-timestamp-use-align-to 0)
+        (erc-timestamp-format "[00:00]"))
+    (cl-pushnew 'erc-add-timestamp erc-insert-modify-hook)
+    (erc-tests-common-get-inserted-msg-setup))
+  (goto-char 19)
+  (should (looking-back (rx "<bob> hi [00:00]")))
+  (erc-tests-common-assert-get-inserted-msg 3 19 test-fn))
+
+(ert-deftest erc--get-inserted-msg-beg/stamp ()
+  (erc-stamp-tests--assert-get-inserted-msg/stamp
+   (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
+
+(ert-deftest erc--get-inserted-msg-beg/readonly/stamp ()
+  (erc-tests-common-assert-get-inserted-msg-readonly-with
+   #'erc-stamp-tests--assert-get-inserted-msg/stamp
+   (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
+
+(ert-deftest erc--get-inserted-msg-end/stamp ()
+  (erc-stamp-tests--assert-get-inserted-msg/stamp
+   (lambda (arg) (should (= 19 (erc--get-inserted-msg-end arg))))))
+
+(ert-deftest erc--get-inserted-msg-end/readonly/stamp ()
+  (erc-tests-common-assert-get-inserted-msg-readonly-with
+   #'erc-stamp-tests--assert-get-inserted-msg/stamp
+   (lambda (arg) (should (= 19 (erc--get-inserted-msg-end arg))))))
+
+(ert-deftest erc--get-inserted-msg-bounds/stamp ()
+  (erc-stamp-tests--assert-get-inserted-msg/stamp
+   (lambda (arg)
+     (should (equal '(3 . 19) (erc--get-inserted-msg-bounds arg))))))
+
+(ert-deftest erc--get-inserted-msg-bounds/readonly/stamp ()
+  (erc-tests-common-assert-get-inserted-msg-readonly-with
+   #'erc-stamp-tests--assert-get-inserted-msg/stamp
+   (lambda (arg)
+     (should (equal '(3 . 19) (erc--get-inserted-msg-bounds arg))))))
+
 ;;; erc-stamp-tests.el ends here
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index ffc96eb4f1d..2d6eda6a24c 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -22,7 +22,10 @@
 ;;; Code:
 
 (require 'ert-x)
-(require 'erc)
+(eval-and-compile
+  (let ((load-path (cons (ert-resource-directory) load-path)))
+    (require 'erc-tests-common)))
+
 (require 'erc-ring)
 
 (ert-deftest erc--read-time-period ()
@@ -113,7 +116,7 @@
 
 (ert-deftest erc-with-server-buffer ()
   (setq erc-away 1)
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-init-server-proc "sleep" "1")
 
   (let (mockingp calls)
     (advice-add 'buffer-local-value :after
@@ -155,34 +158,22 @@
                     (when (cl-evenp c) (push c out)))))
     (should (equal out '(?f ?d ?b)))))
 
-(defun erc-tests--send-prep ()
-  ;; Caller should probably shadow `erc-insert-modify-hook' or
-  ;; populate user tables for erc-button.
-  (erc-mode)
-  (erc--initialize-markers (point) nil)
-  (should (= (point) erc-input-marker)))
-
-(defun erc-tests--set-fake-server-process (&rest args)
-  (setq erc-server-process
-        (apply #'start-process (car args) (current-buffer) args))
-  (set-process-query-on-exit-flag erc-server-process nil))
-
 (ert-deftest erc-hide-prompt ()
   (let ((erc-hide-prompt erc-hide-prompt)
         ;;
         erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
 
     (with-current-buffer (get-buffer-create "ServNet")
-      (erc-tests--send-prep)
+      (erc-tests-common-prep-for-insertion)
       (goto-char erc-insert-marker)
       (should (looking-at-p (regexp-quote erc-prompt)))
-      (erc-tests--set-fake-server-process "sleep" "1")
+      (erc-tests-common-init-server-proc "sleep" "1")
       (set-process-sentinel erc-server-process #'ignore)
       (setq erc-network 'ServNet)
       (set-process-query-on-exit-flag erc-server-process nil))
 
     (with-current-buffer (get-buffer-create "#chan")
-      (erc-tests--send-prep)
+      (erc-tests-common-prep-for-insertion)
       (goto-char erc-insert-marker)
       (should (looking-at-p (regexp-quote erc-prompt)))
       (setq erc-server-process (buffer-local-value 'erc-server-process
@@ -190,7 +181,7 @@
             erc--target (erc--target-from-string "#chan")))
 
     (with-current-buffer (get-buffer-create "bob")
-      (erc-tests--send-prep)
+      (erc-tests-common-prep-for-insertion)
       (goto-char erc-insert-marker)
       (should (looking-at-p (regexp-quote erc-prompt)))
       (setq erc-server-process (buffer-local-value 'erc-server-process
@@ -318,10 +309,10 @@
 
     (ert-info ("Server buffer")
       (with-current-buffer (get-buffer-create "ServNet")
-        (erc-tests--send-prep)
+        (erc-tests-common-prep-for-insertion)
         (goto-char erc-insert-marker)
         (should (looking-at-p "ServNet 3>"))
-        (erc-tests--set-fake-server-process "sleep" "1")
+        (erc-tests-common-init-server-proc "sleep" "1")
         (set-process-sentinel erc-server-process #'ignore)
         (setq erc-network 'ServNet
               erc-server-current-nick "tester"
@@ -353,7 +344,7 @@
 
     (ert-info ("Channel buffer")
       (with-current-buffer (get-buffer-create "#chan")
-        (erc-tests--send-prep)
+        (erc-tests-common-prep-for-insertion)
         (goto-char erc-insert-marker)
         (should (looking-at-p "#chan 9>"))
         (goto-char erc-input-marker)
@@ -546,7 +537,7 @@
 
 (ert-deftest erc-setup-buffer--custom-action ()
   (erc-mode)
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-init-server-proc "sleep" "1")
   (setq erc--server-last-reconnect-count 0)
   (let ((owin (selected-window))
         (obuf (window-buffer))
@@ -677,7 +668,7 @@
 
 (ert-deftest erc--parsed-prefix ()
   (erc-mode)
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-init-server-proc "sleep" "1")
   (setq erc--isupport-params (make-hash-table))
 
   ;; Uses fallback values when no PREFIX parameter yet received, thus
@@ -755,7 +746,7 @@
         erc-server-users (make-hash-table :test #'equal)
         erc--isupport-params (make-hash-table)
         erc--target (erc--target-from-string "#test"))
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-init-server-proc "sleep" "1")
 
   (let ((orig-handle-fn (symbol-function 'erc--handle-channel-mode))
         calls)
@@ -845,7 +836,7 @@
         erc-server-parameters
         '(("CHANMODES" . "eIbq,k,flj,CFLMPQRSTcgimnprstuz")))
 
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-init-server-proc "sleep" "1")
 
   (cl-letf (((symbol-function 'erc-update-mode-line) #'ignore))
     (erc--update-channel-modes "+bltk" "fool!*@*" "3" "h2"))
@@ -890,7 +881,7 @@
                            '(:erc--graphical)))
   (unless (display-graphic-p) (ert-skip "See non-/graphic-p variant"))
 
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-init-server-proc "sleep" "1")
   (setq erc--isupport-params (make-hash-table)
         erc--target (erc--target-from-string "#test")
         erc-server-parameters
@@ -1200,7 +1191,7 @@
 (ert-deftest erc-ring-previous-command ()
   (with-current-buffer (get-buffer-create "*#fake*")
     (erc-mode)
-    (erc-tests--send-prep)
+    (erc-tests-common-prep-for-insertion)
     (setq erc-server-current-nick "tester")
     (setq-local erc-last-input-time 0)
     (should-not (local-variable-if-set-p 'erc-send-completed-hook))
@@ -1381,29 +1372,8 @@
     (should (equal '("" "" "") (split-string "\n\n" p)))
     (should (equal '("" "" "") (split-string "\n\r" p)))))
 
-(defun erc-tests--with-process-input-spy (test)
-  (with-current-buffer (get-buffer-create "FakeNet")
-    (let* ((erc--input-review-functions
-            (remove #'erc-add-to-input-ring erc--input-review-functions))
-           (erc-pre-send-functions
-            (remove #'erc-add-to-input-ring erc-pre-send-functions)) ; for now
-           (inhibit-message noninteractive)
-           (erc-server-current-nick "tester")
-           (erc-last-input-time 0)
-           erc-accidental-paste-threshold-seconds
-           erc-send-modify-hook
-           ;;
-           calls)
-      (cl-letf (((symbol-function 'erc-process-input-line)
-                 (lambda (&rest r) (push r calls)))
-                ((symbol-function 'erc-server-buffer)
-                 (lambda () (current-buffer))))
-        (erc-tests--send-prep)
-        (funcall test (lambda () (pop calls)))))
-    (when noninteractive (kill-buffer))))
-
 (ert-deftest erc--check-prompt-input-functions ()
-  (erc-tests--with-process-input-spy
+  (erc-tests-common-with-process-input-spy
    (lambda (next)
 
      (ert-info ("Errors when point not in prompt area") ; actually just dings
@@ -1438,9 +1408,9 @@
 ;; These also indirectly tests `erc-send-input'
 
 (ert-deftest erc-send-current-line ()
-  (erc-tests--with-process-input-spy
+  (erc-tests-common-with-process-input-spy
    (lambda (next)
-     (erc-tests--set-fake-server-process "sleep" "1")
+     (erc-tests-common-init-server-proc "sleep" "1")
      (should (= 0 erc-last-input-time))
 
      (ert-info ("Simple command")
@@ -1519,9 +1489,9 @@
   '("Stripping" "Padding"))
 
 (ert-deftest erc--check-prompt-input-for-multiline-blanks ()
-  (erc-tests--with-process-input-spy
+  (erc-tests-common-with-process-input-spy
    (lambda (next)
-     (erc-tests--set-fake-server-process "sleep" "10")
+     (erc-tests-common-init-server-proc "sleep" "10")
      (should-not erc-send-whitespace-lines)
      (should erc-warn-about-blank-lines)
 
@@ -1600,9 +1570,9 @@
                        rv ))))))
 
 (ert-deftest erc-send-whitespace-lines ()
-  (erc-tests--with-process-input-spy
+  (erc-tests-common-with-process-input-spy
    (lambda (next)
-     (erc-tests--set-fake-server-process "sleep" "1")
+     (erc-tests-common-init-server-proc "sleep" "1")
      (setq-local erc-send-whitespace-lines t)
 
      (ert-info ("Multiline hunk with blank line correctly split")
@@ -1697,7 +1667,7 @@
         (erc-default-recipients '("#chan"))
         calls)
     (with-temp-buffer
-      (erc-tests--set-fake-server-process "sleep" "1")
+      (erc-tests-common-init-server-proc "sleep" "1")
       (cl-letf (((symbol-function 'erc-cmd-MSG)
                  (lambda (line)
                    (push line calls)
@@ -1755,120 +1725,19 @@
 
           (should-not calls))))))
 
-(defun erc-tests--get-inserted-msg-setup ()
-  (erc-mode)
-  (erc--initialize-markers (point) nil)
-  (let ((parsed (make-erc-response :unparsed ":bob PRIVMSG #chan :hi"
-                                   :sender "bob"
-                                   :command "PRIVMSG"
-                                   :command-args (list "#chan" "hi")
-                                   :contents "hi"))
-        (erc--msg-prop-overrides '((erc--ts . 0))))
-    (erc-display-message parsed nil (current-buffer)
-                         (erc-format-privmessage "bob" "hi" nil t)))
-  (goto-char 3)
-  (should (looking-at "<bob> hi")))
-
-;; All these bounds-finding functions take an optional POINT argument.
-;; So run each case with and without it at each pos in the message.
-(defun erc-tests--assert-get-inserted-msg (from to assert-fn)
-  (dolist (pt-arg '(nil t))
-    (dolist (i (number-sequence from to))
-      (goto-char i)
-      (ert-info ((format "At %d (%c) %s param" i (char-after i)
-                         (if pt-arg "with" "")))
-        (funcall assert-fn (and pt-arg i))))))
-
-(defun erc-tests--assert-get-inserted-msg/basic (test-fn)
-  (erc-tests--get-inserted-msg-setup)
-  (goto-char 11)
-  (should (looking-back "<bob> hi"))
-  (erc-tests--assert-get-inserted-msg 3 11 test-fn))
-
-(defun erc-tests--assert-get-inserted-msg/stamp (test-fn)
-  (require 'erc-stamp)
-  (defvar erc-insert-timestamp-function)
-  (defvar erc-timestamp-format)
-  (defvar erc-timestamp-use-align-to)
-  (let ((erc-insert-modify-hook erc-insert-modify-hook)
-        (erc-insert-timestamp-function 'erc-insert-timestamp-right)
-        (erc-timestamp-use-align-to 0)
-        (erc-timestamp-format "[00:00]"))
-    (cl-pushnew 'erc-add-timestamp erc-insert-modify-hook)
-    (erc-tests--get-inserted-msg-setup))
-  (goto-char 19)
-  (should (looking-back (rx "<bob> hi [00:00]")))
-  (erc-tests--assert-get-inserted-msg 3 19 test-fn))
-
-;; This is a "mixin" and requires a base assertion function to work.
-(defun erc-tests--assert-get-inserted-msg-readonly-with (assert-fn test-fn)
-  (defvar erc-readonly-mode)
-  (defvar erc-readonly-mode-hook)
-  (let ((erc-readonly-mode nil)
-        (erc-readonly-mode-hook nil)
-        (erc-send-post-hook erc-send-post-hook)
-        (erc-insert-post-hook erc-insert-post-hook))
-    (erc-readonly-mode +1)
-    (funcall assert-fn test-fn)))
-
 (ert-deftest erc--get-inserted-msg-beg/basic ()
-  (erc-tests--assert-get-inserted-msg/basic
-   (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
-
-(ert-deftest erc--get-inserted-msg-beg/readonly ()
-  (erc-tests--assert-get-inserted-msg-readonly-with
-   #'erc-tests--assert-get-inserted-msg/basic
-   (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
-
-(ert-deftest erc--get-inserted-msg-beg/stamp ()
-  (erc-tests--assert-get-inserted-msg/stamp
-   (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
-
-(ert-deftest erc--get-inserted-msg-beg/readonly/stamp ()
-  (erc-tests--assert-get-inserted-msg-readonly-with
-   #'erc-tests--assert-get-inserted-msg/stamp
+  (erc-tests-common-assert-get-inserted-msg/basic
    (lambda (arg) (should (= 3 (erc--get-inserted-msg-beg arg))))))
 
 (ert-deftest erc--get-inserted-msg-end/basic ()
-  (erc-tests--assert-get-inserted-msg/basic
+  (erc-tests-common-assert-get-inserted-msg/basic
    (lambda (arg) (should (= 11 (erc--get-inserted-msg-end arg))))))
 
-(ert-deftest erc--get-inserted-msg-end/readonly ()
-  (erc-tests--assert-get-inserted-msg-readonly-with
-   #'erc-tests--assert-get-inserted-msg/basic
-   (lambda (arg) (should (= 11 (erc--get-inserted-msg-end arg))))))
-
-(ert-deftest erc--get-inserted-msg-end/stamp ()
-  (erc-tests--assert-get-inserted-msg/stamp
-   (lambda (arg) (should (= 19 (erc--get-inserted-msg-end arg))))))
-
-(ert-deftest erc--get-inserted-msg-end/readonly/stamp ()
-  (erc-tests--assert-get-inserted-msg-readonly-with
-   #'erc-tests--assert-get-inserted-msg/stamp
-   (lambda (arg) (should (= 19 (erc--get-inserted-msg-end arg))))))
-
 (ert-deftest erc--get-inserted-msg-bounds/basic ()
-  (erc-tests--assert-get-inserted-msg/basic
+  (erc-tests-common-assert-get-inserted-msg/basic
    (lambda (arg)
      (should (equal '(3 . 11) (erc--get-inserted-msg-bounds arg))))))
 
-(ert-deftest erc--get-inserted-msg-bounds/readonly ()
-  (erc-tests--assert-get-inserted-msg-readonly-with
-   #'erc-tests--assert-get-inserted-msg/basic
-   (lambda (arg)
-     (should (equal '(3 . 11) (erc--get-inserted-msg-bounds arg))))))
-
-(ert-deftest erc--get-inserted-msg-bounds/stamp ()
-  (erc-tests--assert-get-inserted-msg/stamp
-   (lambda (arg)
-     (should (equal '(3 . 19) (erc--get-inserted-msg-bounds arg))))))
-
-(ert-deftest erc--get-inserted-msg-bounds/readonly/stamp ()
-  (erc-tests--assert-get-inserted-msg-readonly-with
-   #'erc-tests--assert-get-inserted-msg/stamp
-   (lambda (arg)
-     (should (equal '(3 . 19) (erc--get-inserted-msg-bounds arg))))))
-
 (ert-deftest erc--delete-inserted-message ()
   (erc-mode)
   (erc--initialize-markers (point) nil)
@@ -2631,8 +2500,8 @@
     (should (equal (erc--format-speaker-input-message "oh my") expect))))
 
 (ert-deftest erc--route-insertion ()
-  (erc-tests--send-prep)
-  (erc-tests--set-fake-server-process "sleep" "1")
+  (erc-tests-common-prep-for-insertion)
+  (erc-tests-common-init-server-proc "sleep" "1")
   (setq erc-networks--id (erc-networks--id-create 'foonet))
 
   (let* ((erc-modules) ; for `erc--open-target'
@@ -3018,30 +2887,6 @@
                          (erc-server-connect-function
                           erc-open-network-stream))))))))
 
-(defun erc-tests--make-server-buf (name)
-  (with-current-buffer (get-buffer-create name)
-    (erc-mode)
-    (setq erc-server-process (start-process "sleep" (current-buffer)
-                                            "sleep" "1")
-          erc-session-server (concat "irc." name ".org")
-          erc-session-port 6667
-          erc-network (intern name))
-    (set-process-query-on-exit-flag erc-server-process nil)
-    (current-buffer)))
-
-(defun erc-tests--make-client-buf (server name)
-  (unless (bufferp server)
-    (setq server (get-buffer server)))
-  (with-current-buffer (get-buffer-create name)
-    (erc-mode)
-    (setq erc--target (erc--target-from-string name))
-    (dolist (v '(erc-server-process
-                 erc-session-server
-                 erc-session-port
-                 erc-network))
-      (set v (buffer-local-value v server)))
-    (current-buffer)))
-
 (ert-deftest erc-handle-irc-url ()
   (let* (calls
          rvbuf
@@ -3055,10 +2900,10 @@
     (cl-letf (((symbol-function 'erc-cmd-JOIN)
                (lambda (&rest r) (push r calls))))
 
-      (with-current-buffer (erc-tests--make-server-buf "foonet")
+      (with-current-buffer (erc-tests-common-make-server-buf "foonet")
         (setq rvbuf (current-buffer)))
-      (erc-tests--make-server-buf "barnet")
-      (erc-tests--make-server-buf "baznet")
+      (erc-tests-common-make-server-buf "barnet")
+      (erc-tests-common-make-server-buf "baznet")
 
       (ert-info ("Unknown network")
         (erc-handle-irc-url "irc.foonet.org" 6667 "#chan" nil nil "irc")
@@ -3082,7 +2927,8 @@
         (should-not calls))
 
       (ert-info ("Known network, existing chan with key")
-        (erc-tests--make-client-buf "foonet" "#chan")
+        (save-excursion
+          (with-current-buffer "foonet" (erc--open-target "#chan")))
         (erc-handle-irc-url "irc.foonet.org" nil "#chan?sec" nil nil "irc")
         (should (equal '("#chan" "sec") (pop calls)))
         (should-not calls))
@@ -3095,7 +2941,7 @@
       (ert-info ("Unknown network, connect, chan")
         (with-current-buffer "foonet"
           (should-not (local-variable-p 'erc-after-connect)))
-        (setq rvbuf (lambda () (erc-tests--make-server-buf "gnu")))
+        (setq rvbuf (lambda () (erc-tests-common-make-server-buf "gnu")))
         (erc-handle-irc-url "irc.gnu.org" nil "#spam" nil nil "irc")
         (should (equal '("irc" :server "irc.gnu.org") (pop calls)))
         (should-not calls)
@@ -3107,10 +2953,7 @@
         (should-not calls))))
 
   (when noninteractive
-    (kill-buffer "foonet")
-    (kill-buffer "barnet")
-    (kill-buffer "baznet")
-    (kill-buffer "#chan")))
+    (erc-tests-common-kill-buffers)))
 
 (ert-deftest erc-channel-user ()
   ;; Traditional and alternate constructor swapped for compatibility.
@@ -3201,31 +3044,7 @@
   (should (eq (erc--normalize-module-symbol 'nickserv) 'services)))
 
 (defun erc-tests--assert-printed-in-subprocess (code expected)
-  (let* ((package (if-let* ((found (getenv "ERC_PACKAGE_NAME"))
-                            ((string-prefix-p "erc-" found)))
-                      (intern found)
-                    'erc))
-         ;; This is for integrations testing with managed configs
-         ;; ("starter kits") that use a different package manager.
-         (init (and-let* ((found (getenv "ERC_TESTS_INIT"))
-                          (files (split-string found ",")))
-                 (mapcan (lambda (f) (list "-l" f)) files)))
-         (prog
-          `(progn
-             ,@(and (not init) (featurep 'compat)
-                    `((require 'package)
-                      (let ((package-load-list '((compat t) (,package t))))
-                        (package-initialize))))
-             (require 'erc)
-             (cl-assert (equal erc-version ,erc-version) t)
-             ,code))
-         (proc (apply #'start-process
-                      (symbol-name (ert-test-name (ert-running-test)))
-                      (current-buffer)
-                      (concat invocation-directory invocation-name)
-                      `("-batch" ,@(or init '("-Q"))
-                        "-eval" ,(format "%S" prog)))))
-    (set-process-query-on-exit-flag proc t)
+  (let ((proc (erc-tests-common-create-subprocess code '("-batch") nil)))
     (while (accept-process-output proc 10))
     (goto-char (point-min))
     (unless (equal (read (current-buffer)) expected)
@@ -3573,38 +3392,11 @@ connection."
                       (put 'erc-mname-enable 'definition-name 'mname)
                       (put 'erc-mname-disable 'definition-name 'mname))))))
 
-(defun erc-tests--string-to-propertized-parts (string)
-  "Return a sequence of `propertize' forms for generating STRING.
-Expect maintainers manipulating template catalogs to use this
-with `pp-eval-last-sexp' or similar to convert back and forth
-between literal strings."
-  `(concat
-    ,@(mapcar
-       (pcase-lambda (`(,beg ,end ,plist))
-         ;; At the time of writing, `propertize' produces a string
-         ;; with the order of the input plist reversed.
-         `(propertize ,(substring-no-properties string beg end)
-                      ,@(let (out)
-                          (while-let ((plist)
-                                      (k (pop plist))
-                                      (v (pop plist)))
-                            (push (if (or (consp v) (symbolp v)) `',v v) out)
-                            (push `',k out))
-                          out)))
-       (object-intervals string))))
-
-(defun erc-tests-pp-propertized-parts (arg)
-  "Convert literal string before point into a `propertize'd form.
-For simplicity, assume string evaluates to itself."
-  (interactive "P")
-  (let ((sexp (erc-tests--string-to-propertized-parts (pp-last-sexp))))
-    (if arg (insert (pp-to-string sexp)) (pp-eval-expression sexp))))
-
-(ert-deftest erc-tests--string-to-propertized-parts ()
+(ert-deftest erc-tests-common-string-to-propertized-parts ()
   :tags '(:unstable) ; only run this locally
   (unless (>= emacs-major-version 28) (ert-skip "Missing `object-intervals'"))
 
-  (should (equal (erc-tests--string-to-propertized-parts
+  (should (equal (erc-tests-common-string-to-propertized-parts
                   #("abc"
                     0 1 (face default foo 1)
                     1 3 (face (default italic) bar "2")))
diff --git a/test/lisp/erc/resources/erc-tests-common.el b/test/lisp/erc/resources/erc-tests-common.el
new file mode 100644
index 00000000000..9d9cc4294bb
--- /dev/null
+++ b/test/lisp/erc/resources/erc-tests-common.el
@@ -0,0 +1,287 @@
+;;; erc-tests-common.el --- Common helpers for ERC tests -*- lexical-binding: t -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file must *not* contain any `ert-deftest' definitions.  See
+;; top of test/lisp/erc/erc-tests.el for loading example.
+;;
+;; Environment variables:
+;;
+;;  `ERC_PACKAGE_NAME': Name of the installed ERC package currently
+;;   running.  ERC needs this in order to load the same package in
+;;   tests that run in a subprocess.  Necessary even when the package
+;;   name is `erc' and not something like `erc-49860'.
+;;
+;;  `ERC_TESTS_INIT': The name of an alternate init file.  Mainly for
+;;   integrations tests involving starter kits.
+;;
+;;  `ERC_TESTS_SNAPSHOT_SAVE': When set, ERC saves the current test's
+;;   snapshots to disk.
+;;
+
+;;; Code:
+(require 'ert-x)
+(require 'erc)
+
+;; Caller should probably shadow `erc-insert-modify-hook' or populate
+;; user tables for erc-button.
+;; FIXME explain this comment ^ in more detail or delete.
+(defun erc-tests-common-prep-for-insertion ()
+  "Initialize current buffer with essentials for message insertion.
+Assume caller intends to use `erc-display-message'."
+  (erc-mode)
+  (erc--initialize-markers (point) nil)
+  (should (= (point) erc-input-marker)))
+
+(defun erc-tests-common-init-server-proc (&rest args)
+  "Create a process with `start-process' from ARGS.
+Assign the result to `erc-server-process' in the current buffer."
+  (setq erc-server-process
+        (apply #'start-process (car args) (current-buffer) args))
+  (set-process-query-on-exit-flag erc-server-process nil)
+  erc-server-process)
+
+;; After dropping support for Emacs 27, callers can use
+;; `get-buffer-create' with INHIBIT-BUFFER-HOOKS.
+(defun erc-tests-common-kill-buffers (&rest extra-buffers)
+  "Kill all ERC buffers and possibly EXTRA-BUFFERS."
+  (let (erc-kill-channel-hook erc-kill-server-hook erc-kill-buffer-hook)
+    (dolist (buf (erc-buffer-list))
+      (kill-buffer buf))
+    (named-let doit ((buffers extra-buffers))
+      (dolist (buf buffers)
+        (if (consp buf) (doit buf) (kill-buffer buf))))))
+
+(defun erc-tests-common-with-process-input-spy (test-fn)
+  "Mock `erc-process-input-line' and call TEST-FN.
+Shadow `erc--input-review-functions' and `erc-pre-send-functions'
+with `erc-add-to-input-ring' removed.  Shadow other relevant
+variables as nil, and bind `erc-last-input-time' to 0.  Also mock
+`erc-server-buffer' to return the current buffer.  Call TEST-FN
+with a utility function that returns the set of arguments most
+recently passed to the mocked `erc-process-input-line'.  Make
+`inhibit-message' non-nil unless running interactively."
+  (with-current-buffer (get-buffer-create "FakeNet")
+    (let* ((erc--input-review-functions
+            (remove 'erc-add-to-input-ring erc--input-review-functions))
+           (erc-pre-send-functions
+            (remove 'erc-add-to-input-ring erc-pre-send-functions)) ; for now
+           (inhibit-message noninteractive)
+           (erc-server-current-nick "tester")
+           (erc-last-input-time 0)
+           erc-accidental-paste-threshold-seconds
+           erc-send-modify-hook
+           ;;
+           calls)
+      (cl-letf (((symbol-function 'erc-process-input-line)
+                 (lambda (&rest r) (push r calls)))
+                ((symbol-function 'erc-server-buffer)
+                 (lambda () (current-buffer))))
+        (erc-tests-common-prep-for-insertion)
+        (funcall test-fn (lambda () (pop calls)))))
+    (when noninteractive (kill-buffer))))
+
+(defun erc-tests-common-make-server-buf (name)
+  "Return a server buffer named NAME, creating it if necessary.
+Use NAME for the network and the session server as well."
+  (with-current-buffer (get-buffer-create name)
+    (erc-tests-common-prep-for-insertion)
+    (erc-tests-common-init-server-proc "sleep" "1")
+    (setq erc-session-server (concat "irc." name ".org")
+          erc-server-announced-name (concat "west." name ".org")
+          erc-session-port 6667
+          erc-network (intern name)
+          erc-networks--id (erc-networks--id-create nil))
+    (current-buffer)))
+
+(defun erc-tests-common-string-to-propertized-parts (string)
+  "Return a sequence of `propertize' forms for generating STRING.
+Expect maintainers manipulating template catalogs to use this
+with `pp-eval-last-sexp' or similar to convert back and forth
+between literal strings."
+  `(concat
+    ,@(mapcar
+       (pcase-lambda (`(,beg ,end ,plist))
+         ;; At the time of writing, `propertize' produces a string
+         ;; with the order of the input plist reversed.
+         `(propertize ,(substring-no-properties string beg end)
+                      ,@(let (out)
+                          (while-let ((plist)
+                                      (k (pop plist))
+                                      (v (pop plist)))
+                            (push (if (or (consp v) (symbolp v)) `',v v) out)
+                            (push `',k out))
+                          out)))
+       (object-intervals string))))
+
+(defun erc-tests-common-pp-propertized-parts (arg)
+  "Convert literal string before point into a `propertize'd form.
+For simplicity, assume string evaluates to itself."
+  (interactive "P")
+  (let ((sexp (erc-tests-common-string-to-propertized-parts (pp-last-sexp))))
+    (if arg (insert (pp-to-string sexp)) (pp-eval-expression sexp))))
+
+;; The following utilities are meant to help prepare tests for
+;; `erc--get-inserted-msg-bounds' and friends.
+(defun erc-tests-common-get-inserted-msg-setup ()
+  (erc-tests-common-prep-for-insertion)
+  (let ((parsed (make-erc-response :unparsed ":bob PRIVMSG #chan :hi"
+                                   :sender "bob"
+                                   :command "PRIVMSG"
+                                   :command-args (list "#chan" "hi")
+                                   :contents "hi"))
+        (erc--msg-prop-overrides '((erc--ts . 0))))
+    (erc-display-message parsed nil (current-buffer)
+                         (erc-format-privmessage "bob" "hi" nil t)))
+  (goto-char 3)
+  (should (looking-at "<bob> hi")))
+
+;; All these bounds-finding functions take an optional POINT argument.
+;; So run each case with and without it at each pos in the message.
+(defun erc-tests-common-assert-get-inserted-msg (from to assert-fn)
+  (dolist (pt-arg '(nil t))
+    (dolist (i (number-sequence from to))
+      (goto-char i)
+      (ert-info ((format "At %d (%c) %s param" i (char-after i)
+                         (if pt-arg "with" "")))
+        (funcall assert-fn (and pt-arg i))))))
+
+(defun erc-tests-common-assert-get-inserted-msg/basic (test-fn)
+  (erc-tests-common-get-inserted-msg-setup)
+  (goto-char 11)
+  (should (looking-back "<bob> hi"))
+  (erc-tests-common-assert-get-inserted-msg 3 11 test-fn))
+
+;; This is a "mixin" and requires a base assertion function, like
+;; `erc-tests-common-assert-get-inserted-msg/basic', to work.
+(defun erc-tests-common-assert-get-inserted-msg-readonly-with
+    (assert-fn test-fn)
+  (defvar erc-readonly-mode)
+  (defvar erc-readonly-mode-hook)
+  (let ((erc-readonly-mode nil)
+        (erc-readonly-mode-hook nil)
+        (erc-send-post-hook erc-send-post-hook)
+        (erc-insert-post-hook erc-insert-post-hook))
+    (erc-readonly-mode +1)
+    (funcall assert-fn test-fn)))
+
+
+;;;; Buffer snapshots
+
+;; Use this variable to generate new snapshots after carefully
+;; reviewing the output of *each* snapshot (not just first and last).
+;; Obviously, only run one test at a time.
+(defvar erc-tests-common-snapshot-save-p (getenv "ERC_TESTS_SNAPSHOT_SAVE"))
+
+(defun erc-tests-common-snapshot-compare (name dir trans-fn buf-init-fn)
+  "Compare `buffer-string' to snapshot NAME.eld in DIR, if present.
+When non-nil, run TRANS-FN to fiter the current buffer string,
+and expect a similar string in return.  Call BUF-INIT-FN, when
+non-nil, in the preview buffer after inserting the filtered
+string."
+  (let* ((expect-file (file-name-with-extension (expand-file-name name dir)
+                                                "eld"))
+         (erc--own-property-names
+          (seq-difference `(font-lock-face ,@erc--own-property-names)
+                          `(field display wrap-prefix line-prefix
+                                  erc--msg erc--cmd erc--spkr erc--ts erc--ctcp
+                                  erc--ephemeral)
+                          #'eq))
+         (print-circle t)
+         (print-escape-newlines t)
+         (print-escape-nonascii t)
+         (got (erc--remove-text-properties
+               (buffer-substring (point-min) erc-insert-marker)))
+         (repr (funcall (or trans-fn #'identity) (prin1-to-string got))))
+    (with-current-buffer (generate-new-buffer name)
+      (with-silent-modifications
+        (insert (setq got (read repr))))
+      (when buf-init-fn (funcall buf-init-fn))
+      (erc-mode))
+    ;; LHS is a string, RHS is a symbol.
+    (if (string= erc-tests-common-snapshot-save-p
+                 (ert-test-name (ert-running-test)))
+        (let (inhibit-message)
+          (with-temp-file expect-file
+            (insert repr))
+          ;; Limit writing snapshots to one test at a time.
+          (message "erc-tests-common-snapshot-compare: wrote %S" expect-file))
+      (if (file-exists-p expect-file)
+          ;; Ensure string-valued properties, like timestamps, aren't
+          ;; recursive (signals `max-lisp-eval-depth' exceeded).
+          (named-let assert-equal
+              ((latest (read repr))
+               (expect (read (with-temp-buffer
+                               (insert-file-contents-literally expect-file)
+                               (buffer-string)))))
+            (pcase latest
+              ((or "" 'nil) t)
+              ((pred stringp)
+               (should (equal-including-properties latest expect))
+               (let ((latest-intervals (object-intervals latest))
+                     (expect-intervals (object-intervals expect)))
+                 (while-let ((l-iv (pop latest-intervals))
+                             (x-iv (pop expect-intervals))
+                             (l-tab (map-into (nth 2 l-iv) 'hash-table))
+                             (x-tab (map-into (nth 2 x-iv) 'hash-table)))
+                   (pcase-dolist (`(,l-k . ,l-v) (map-pairs l-tab))
+                     (assert-equal l-v (gethash l-k x-tab))
+                     (remhash l-k x-tab))
+                   (should (zerop (hash-table-count x-tab))))))
+              ((pred sequencep)
+               (assert-equal (seq-first latest) (seq-first expect))
+               (assert-equal (seq-rest latest) (seq-rest expect)))
+              (_ (should (equal latest expect)))))
+        (message "Snapshot file missing: %S" expect-file)))))
+
+(defun erc-tests-common-create-subprocess (code switches libs)
+  "Return subprocess for running CODE in an inferior Emacs.
+Include SWITCHES, like \"-batch\", as well as libs, after
+interspersing \"-l\" between members."
+  (let* ((package (if-let ((found (getenv "ERC_PACKAGE_NAME"))
+                           ((string-prefix-p "erc-" found)))
+                      (intern found)
+                    'erc))
+         ;; For integrations testing with managed configs that use a
+         ;; different package manager.
+         (init (and-let* ((found (getenv "ERC_TESTS_INIT"))
+                          (files (split-string found ",")))
+                 (mapcan (lambda (f) (list "-l" f)) files)))
+         (prog
+          `(progn
+             ,@(and (not init) (featurep 'compat)
+                    `((require 'package)
+                      (let ((package-load-list '((compat t) (,package t))))
+                        (package-initialize))))
+             (require 'erc)
+             (cl-assert (equal erc-version ,erc-version) t)
+             ,code))
+         (proc (apply #'start-process
+                      (symbol-name (ert-test-name (ert-running-test)))
+                      (current-buffer)
+                      (concat invocation-directory invocation-name)
+                      `(,@(or init '("-Q"))
+                        ,@switches
+                        ,@(mapcan (lambda (f) (list "-l" f)) libs)
+                        "-eval" ,(format "%S" prog)))))
+    (set-process-query-on-exit-flag proc t)
+    proc))
+
+(provide 'erc-tests-common)