Improve Tramp cache for asynchronous processes

* lisp/net/tramp-adb.el (tramp-adb-handle-exec-path)
(tramp-adb-get-device):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handler-askquestion):
* lisp/net/tramp-sh.el (tramp-remote-selinux-p, tramp-remote-acl-p)
(tramp-open-connection-setup-interactive-shell)
(tramp-maybe-open-connection, tramp-get-remote-path)
(tramp-get-inline-compress, tramp-get-inline-coding):
* lisp/net/tramp-smb.el (tramp-smb-get-cifs-capabilities)
(tramp-smb-get-stat-capability):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-remote-acl-p)
(tramp-sudoedit-remote-selinux-p): Cache property in main process.

* lisp/net/tramp-cache.el (tramp-cache-undefined): New defconst.
(tramp-get-hash-table, tramp-connection-property-p): Use it.
(tramp-set-connection-property, tramp-flush-connection-property)
(tramp-flush-connection-properties): Add sanity checks.
(tramp-get-file-property, tramp-set-file-property)
(tramp-get-connection-property, tramp-set-connection-property)
(tramp-dump-connection-properties): Adapt docstring.

* lisp/net/tramp-cmds.el (tramp-cleanup-connection): Delete all
processes.

* lisp/net/tramp-gvfs.el (tramp-gvfs-unmount):
Use `tramp-cleanup-connection'.

* lisp/net/tramp-sh.el (tramp-sh-handle-vc-registered):
Use `bound-and-true-p'.

* lisp/net/tramp.el (tramp-get-process): New defun.

8 files changed, 130 insertions, 90 deletions

diff --git a/lisp/net/tramp-adb.el b/lisp/net/tramp-adb.el
index 4512179eb14..aae25d1dbf3 100644
--- a/lisp/net/tramp-adb.el
+++ b/lisp/net/tramp-adb.el
@@ -1097,7 +1097,7 @@ PRESERVE-UID-GID and PRESERVE-EXTENDED-ATTRIBUTES are completely ignored."
   "Like `exec-path' for Tramp files."
   (append
    (with-parsed-tramp-file-name default-directory nil
-     (with-tramp-connection-property v "remote-path"
+     (with-tramp-connection-property (tramp-get-process v) "remote-path"
        (tramp-adb-send-command v "echo \\\"$PATH\\\"")
        (split-string
         (with-current-buffer (tramp-get-connection-buffer v)
@@ -1112,11 +1112,7 @@ PRESERVE-UID-GID and PRESERVE-EXTENDED-ATTRIBUTES are completely ignored."
   "Return full host name from VEC to be used in shell execution.
 E.g. a host name \"192.168.1.1#5555\" returns \"192.168.1.1:5555\"
      a host name \"R38273882DE\" returns \"R38273882DE\"."
-  ;; Sometimes this is called before there is a connection process
-  ;; yet.  In order to work with the connection cache, we flush all
-  ;; unwanted entries first.
-  (tramp-flush-connection-properties nil)
-  (with-tramp-connection-property (tramp-get-connection-process vec) "device"
+  (with-tramp-connection-property (tramp-get-process vec) "device"
     (let* ((host (tramp-file-name-host vec))
            (port (tramp-file-name-port-or-default vec))
            (devices (mapcar #'cadr (tramp-adb-parse-device-names nil))))
diff --git a/lisp/net/tramp-cache.el b/lisp/net/tramp-cache.el
index 91ed5465695..93eeb16f547 100644
--- a/lisp/net/tramp-cache.el
+++ b/lisp/net/tramp-cache.el
@@ -31,13 +31,13 @@
 ;; a process, has a unique cache.  We distinguish 4 kind of caches,
 ;; depending on the key:
 ;;
-;; - localname is NIL.  This are reusable properties.  Examples:
+;; - localname is nil.  These are reusable properties.  Examples:
 ;;   "remote-shell" identifies the POSIX shell to be called on the
 ;;   remote host, or "perl" is the command to be called on the remote
 ;;   host when starting a Perl script.  These properties are saved in
 ;;   the file `tramp-persistency-file-name'.
 ;;
-;; - localname is a string.  This are temporary properties, which are
+;; - localname is a string.  These are temporary properties, which are
 ;;   related to the file localname is referring to.  Examples:
 ;;   "file-exists-p" is t or nil, depending on the file existence, or
 ;;   "file-attributes" caches the result of the function
@@ -45,21 +45,32 @@
 ;;   expire after `remote-file-name-inhibit-cache' seconds if this
 ;;   variable is set.
 ;;
-;; - The key is a process.  This are temporary properties related to
+;; - The key is a process.  These are temporary properties related to
 ;;   an open connection.  Examples: "scripts" keeps shell script
 ;;   definitions already sent to the remote shell, "last-cmd-time" is
 ;;   the time stamp a command has been sent to the remote process.
 ;;
-;; - The key is nil.  This are temporary properties related to the
+;; - The key is nil.  These are temporary properties related to the
 ;;   local machine.  Examples: "parse-passwd" and "parse-group" keep
 ;;   the results of parsing "/etc/passwd" and "/etc/group",
 ;;   "{uid,gid}-{integer,string}" are the local uid and gid, and
 ;;   "locale" is the used shell locale.
+;;
+;; - The key is `tramp-cache-undefined'.  All functions return the
+;;   expected values, but nothing is cached.
 
 ;; Some properties are handled special:
 ;;
 ;; - "process-name", "process-buffer" and "first-password-request" are
-;;   not saved in the file `tramp-persistency-file-name'.
+;;   not saved in the file `tramp-persistency-file-name', although
+;;   being connection properties related to a `tramp-file-name'
+;;   structure.
+;;
+;; - Reusable properties, which should not be saved, are kept in the
+;;   process key retrieved by `tramp-get-process' (the main connection
+;;   process).  Other processes could reuse these properties, avoiding
+;;   recomputation when a new asynchronous process is created by
+;;   `make-process'.  Examples are "remote-path" or "device" (tramp-adb.el).
 
 ;;; Code:
 
@@ -96,25 +107,31 @@ details see the info pages."
 (defvar tramp-cache-data-changed nil
   "Whether persistent cache data have been changed.")
 
+;;;###tramp-autoload
+(defconst tramp-cache-undefined 'undef
+  "The symbol marking undefined hash keys and values.")
+
 (defun tramp-get-hash-table (key)
   "Return the hash table for KEY.
 If it doesn't exist yet, it is created and initialized with
-matching entries of `tramp-connection-properties'."
-  (or (gethash key tramp-cache-data)
-      (let ((hash
-             (puthash key (make-hash-table :test #'equal) tramp-cache-data)))
-        (when (tramp-file-name-p key)
-          (dolist (elt tramp-connection-properties)
-            (when (string-match-p
-                   (or (nth 0 elt) "")
-                   (tramp-make-tramp-file-name key 'noloc 'nohop))
-              (tramp-set-connection-property key (nth 1 elt) (nth 2 elt)))))
-        hash)))
+matching entries of `tramp-connection-properties'.
+If KEY is `tramp-cache-undefined', don't create anything, and return nil."
+  (unless (eq key tramp-cache-undefined)
+    (or (gethash key tramp-cache-data)
+        (let ((hash
+               (puthash key (make-hash-table :test #'equal) tramp-cache-data)))
+          (when (tramp-file-name-p key)
+            (dolist (elt tramp-connection-properties)
+              (when (string-match-p
+                     (or (nth 0 elt) "")
+                     (tramp-make-tramp-file-name key 'noloc 'nohop))
+                (tramp-set-connection-property key (nth 1 elt) (nth 2 elt)))))
+          hash))))
 
 ;;;###tramp-autoload
 (defun tramp-get-file-property (key file property default)
   "Get the PROPERTY of FILE from the cache context of KEY.
-Returns DEFAULT if not set."
+Return DEFAULT if not set."
   ;; Unify localname.  Remove hop from `tramp-file-name' structure.
   (setq file (tramp-compat-file-name-unquote file)
         key (copy-tramp-file-name key))
@@ -152,7 +169,7 @@ Returns DEFAULT if not set."
 ;;;###tramp-autoload
 (defun tramp-set-file-property (key file property value)
   "Set the PROPERTY of FILE to VALUE, in the cache context of KEY.
-Returns VALUE."
+Return VALUE."
   ;; Unify localname.  Remove hop from `tramp-file-name' structure.
   (setq file (tramp-compat-file-name-unquote file)
         key (copy-tramp-file-name key))
@@ -283,8 +300,9 @@ This is suppressed for temporary buffers."
   "Get the named PROPERTY for the connection.
 KEY identifies the connection, it is either a process or a
 `tramp-file-name' structure.  A special case is nil, which is
-used to cache connection properties of the local machine.  If the
-value is not set for the connection, returns DEFAULT."
+used to cache connection properties of the local machine.
+If KEY is `tramp-cache-undefined', or if the value is not set for
+the connection, return DEFAULT."
   ;; Unify key by removing localname and hop from `tramp-file-name'
   ;; structure.  Work with a copy in order to avoid side effects.
   (when (tramp-file-name-p key)
@@ -308,19 +326,22 @@ value is not set for the connection, returns DEFAULT."
   "Set the named PROPERTY of a connection to VALUE.
 KEY identifies the connection, it is either a process or a
 `tramp-file-name' structure.  A special case is nil, which is
-used to cache connection properties of the local machine.
-PROPERTY is set persistent when KEY is a `tramp-file-name' structure."
+used to cache connection properties of the local machine.  If KEY
+is `tramp-cache-undefined', nothing is set.
+PROPERTY is set persistent when KEY is a `tramp-file-name' structure.
+Return VALUE."
   ;; Unify key by removing localname and hop from `tramp-file-name'
   ;; structure.  Work with a copy in order to avoid side effects.
   (when (tramp-file-name-p key)
     (setq key (copy-tramp-file-name key))
     (setf (tramp-file-name-localname key) nil
           (tramp-file-name-hop key) nil))
-  (let ((hash (tramp-get-hash-table key)))
-    (puthash property value hash)
-    (setq tramp-cache-data-changed t)
-    (tramp-message key 7 "%s %s" property value)
-    value))
+  (when-let ((hash (tramp-get-hash-table key)))
+    (puthash property value hash))
+  (setq tramp-cache-data-changed
+        (or tramp-cache-data-changed (tramp-tramp-file-p key)))
+  (tramp-message key 7 "%s %s" property value)
+  value)
 
 ;;;###tramp-autoload
 (defun tramp-connection-property-p (key property)
@@ -328,7 +349,8 @@ PROPERTY is set persistent when KEY is a `tramp-file-name' structure."
 KEY identifies the connection, it is either a process or a
 `tramp-file-name' structure.  A special case is nil, which is
 used to cache connection properties of the local machine."
-  (not (eq (tramp-get-connection-property key property 'undef) 'undef)))
+  (not (eq (tramp-get-connection-property key property tramp-cache-undefined)
+           tramp-cache-undefined)))
 
 ;;;###tramp-autoload
 (defun tramp-flush-connection-property (key property)
@@ -343,8 +365,10 @@ PROPERTY is set persistent when KEY is a `tramp-file-name' structure."
     (setq key (copy-tramp-file-name key))
     (setf (tramp-file-name-localname key) nil
           (tramp-file-name-hop key) nil))
-  (remhash property (tramp-get-hash-table key))
-  (setq tramp-cache-data-changed t)
+  (when-let ((hash (tramp-get-hash-table key)))
+    (remhash property hash))
+  (setq tramp-cache-data-changed
+        (or tramp-cache-data-changed (tramp-tramp-file-p key)))
   (tramp-message key 7 "%s" property))
 
 ;;;###tramp-autoload
@@ -361,9 +385,10 @@ used to cache connection properties of the local machine."
           (tramp-file-name-hop key) nil))
   (tramp-message
    key 7 "%s %s" key
-   (let ((hash (gethash key tramp-cache-data)))
-     (when (hash-table-p hash) (hash-table-keys hash))))
-  (setq tramp-cache-data-changed t)
+   (when-let ((hash (gethash key tramp-cache-data)))
+     (hash-table-keys hash)))
+  (setq tramp-cache-data-changed
+        (or tramp-cache-data-changed (tramp-tramp-file-p key)))
   (remhash key tramp-cache-data))
 
 ;;;###tramp-autoload
@@ -414,7 +439,8 @@ used to cache connection properties of the local machine."
                (hash-table-keys tramp-cache-data)))))
 
 (defun tramp-dump-connection-properties ()
-  "Write persistent connection properties into file `tramp-persistency-file-name'."
+  "Write persistent connection properties into file \
+`tramp-persistency-file-name'."
   ;; We shouldn't fail, otherwise Emacs might not be able to be closed.
   (ignore-errors
     (when (and (hash-table-p tramp-cache-data)
diff --git a/lisp/net/tramp-cmds.el b/lisp/net/tramp-cmds.el
index b4dca2321c1..7d353e262af 100644
--- a/lisp/net/tramp-cmds.el
+++ b/lisp/net/tramp-cmds.el
@@ -107,21 +107,19 @@ When called interactively, a Tramp connection has to be selected."
     ;; suppressed.
     (setq tramp-current-connection nil)
 
-    ;; Flush file cache.
-    (tramp-flush-directory-properties vec "")
-
-    ;; Flush connection cache.
-    (when (processp (tramp-get-connection-process vec))
-      (tramp-flush-connection-properties (tramp-get-connection-process vec))
-      (delete-process (tramp-get-connection-process vec)))
-    (tramp-flush-connection-properties vec)
-
     ;; Cancel timer.
     (dolist (timer timer-list)
       (when (and (eq (timer--function timer) 'tramp-timeout-session)
                  (tramp-file-name-equal-p vec (car (timer--args timer))))
         (cancel-timer timer)))
 
+    ;; Delete processes.
+    (dolist (key (hash-table-keys tramp-cache-data))
+      (when (and (processp key)
+                 (tramp-file-name-equal-p (process-get key 'vector) vec))
+        (tramp-flush-connection-properties key)
+        (delete-process key)))
+
     ;; Remove buffers.
     (dolist
         (buf (list (get-buffer (tramp-buffer-name vec))
@@ -130,6 +128,12 @@ When called interactively, a Tramp connection has to be selected."
                    (tramp-get-connection-property vec "process-buffer" nil)))
       (when (bufferp buf) (kill-buffer buf)))
 
+    ;; Flush file cache.
+    (tramp-flush-directory-properties vec "")
+
+    ;; Flush connection cache.
+    (tramp-flush-connection-properties vec)
+
     ;; The end.
     (run-hook-with-args 'tramp-cleanup-connection-hook vec)))
 
diff --git a/lisp/net/tramp-gvfs.el b/lisp/net/tramp-gvfs.el
index 85f28076168..526c564ee33 100644
--- a/lisp/net/tramp-gvfs.el
+++ b/lisp/net/tramp-gvfs.el
@@ -1731,8 +1731,7 @@ a downcased host name only."
                   (list
                    t ;; handled.
                    nil ;; no abort of D-Bus.
-                   (with-tramp-connection-property
-                       (tramp-get-connection-process v) message
+                   (with-tramp-connection-property (tramp-get-process v) message
                      ;; In theory, there can be several choices.
                      ;; Until now, there is only the question whether
                      ;; to accept an unknown host signature or certificate.
@@ -1946,8 +1945,7 @@ a downcased host name only."
      (tramp-gvfs-url-file-name (tramp-make-tramp-file-name vec))))
   (while (tramp-gvfs-connection-mounted-p vec)
     (read-event nil nil 0.1))
-  (tramp-flush-connection-properties vec)
-  (tramp-flush-connection-properties (tramp-get-connection-process vec)))
+  (tramp-cleanup-connection vec 'keep-debug 'keep-password))
 
 (defun tramp-gvfs-mount-spec-entry (key value)
   "Construct a mount-spec entry to be used in a mount_spec.
diff --git a/lisp/net/tramp-sh.el b/lisp/net/tramp-sh.el
index 06dca312275..c770e3ce400 100644
--- a/lisp/net/tramp-sh.el
+++ b/lisp/net/tramp-sh.el
@@ -1539,7 +1539,7 @@ of."
 
 (defun tramp-remote-selinux-p (vec)
   "Check, whether SELINUX is enabled on the remote host."
-  (with-tramp-connection-property (tramp-get-connection-process vec) "selinux-p"
+  (with-tramp-connection-property (tramp-get-process vec) "selinux-p"
     (tramp-send-command-and-check vec "selinuxenabled")))
 
 (defun tramp-sh-handle-file-selinux-context (filename)
@@ -1588,7 +1588,7 @@ of."
 
 (defun tramp-remote-acl-p (vec)
   "Check, whether ACL is enabled on the remote host."
-  (with-tramp-connection-property (tramp-get-connection-process vec) "acl-p"
+  (with-tramp-connection-property (tramp-get-process vec) "acl-p"
     (tramp-send-command-and-check vec "getfacl /")))
 
 (defun tramp-sh-handle-file-acl (filename)
@@ -3580,23 +3580,29 @@ STDERR can also be a file name."
                   remote-file-name-inhibit-cache process-file-side-effects)
               ;; Reduce `vc-handled-backends' in order to minimize
               ;; process calls.
-              (when (and (memq 'Bzr vc-handled-backends)
-                         (boundp 'vc-bzr-program)
-                         (not (with-tramp-connection-property v vc-bzr-program
-                                (tramp-find-executable
-                                 v vc-bzr-program (tramp-get-remote-path v)))))
+              (when (and
+                     (memq 'Bzr vc-handled-backends)
+                     (not (and
+                           (bound-and-true-p vc-bzr-program)
+                           (with-tramp-connection-property v vc-bzr-program
+                             (tramp-find-executable
+                              v vc-bzr-program (tramp-get-remote-path v))))))
                 (setq vc-handled-backends (remq 'Bzr vc-handled-backends)))
-              (when (and (memq 'Git vc-handled-backends)
-                         (boundp 'vc-git-program)
-                         (not (with-tramp-connection-property v vc-git-program
-                                (tramp-find-executable
-                                 v vc-git-program (tramp-get-remote-path v)))))
+              (when (and
+                     (memq 'Git vc-handled-backends)
+                     (not (and
+                           (bound-and-true-p vc-git-program)
+                           (with-tramp-connection-property v vc-git-program
+                             (tramp-find-executable
+                              v vc-git-program (tramp-get-remote-path v))))))
                 (setq vc-handled-backends (remq 'Git vc-handled-backends)))
-              (when (and (memq 'Hg vc-handled-backends)
-                         (boundp 'vc-hg-program)
-                         (not (with-tramp-connection-property v vc-hg-program
-                                (tramp-find-executable
-                                 v vc-hg-program (tramp-get-remote-path v)))))
+              (when (and
+                     (memq 'Hg vc-handled-backends)
+                     (not (and
+                           (bound-and-true-p vc-hg-program)
+                           (with-tramp-connection-property v vc-hg-program
+                             (tramp-find-executable
+                              v vc-hg-program (tramp-get-remote-path v))))))
                 (setq vc-handled-backends (remq 'Hg vc-handled-backends)))
               ;; Run.
               (tramp-with-demoted-errors
@@ -4290,11 +4296,15 @@ process to set up.  VEC specifies the connection."
   ;; connection properties.  We start again with
   ;; `tramp-maybe-open-connection', it will be caught there.
   (tramp-message vec 5 "Checking system information")
-  (let ((old-uname (tramp-get-connection-property vec "uname" nil))
-        (uname
-         (tramp-set-connection-property
-          vec "uname"
-          (tramp-send-command-and-read vec "echo \\\"`uname -sr`\\\""))))
+  (let* ((old-uname (tramp-get-connection-property vec "uname" nil))
+         (uname
+          ;; If we are in `make-process', we don't need to recompute.
+          (if (and old-uname
+                   (tramp-get-connection-property vec "process-name" nil))
+              old-uname
+            (tramp-set-connection-property
+             vec "uname"
+             (tramp-send-command-and-read vec "echo \\\"`uname -sr`\\\"")))))
     (when (and (stringp old-uname) (not (string-equal old-uname uname)))
       (tramp-message
        vec 3
@@ -5053,7 +5063,7 @@ connection if a previous connection has died for some reason."
                          ;; we cannot use `tramp-get-connection-process'.
                          (tmpfile
                           (with-tramp-connection-property
-                              (get-process (tramp-buffer-name vec)) "temp-file"
+                              (tramp-get-process vec) "temp-file"
                             (make-temp-name
                              (expand-file-name
                               tramp-temp-name-prefix
@@ -5426,7 +5436,7 @@ Nonexistent directories are removed from spec."
         ;; cache the result for the session only.  Otherwise, the
         ;; result is cached persistently.
         (if (memq 'tramp-own-remote-path tramp-remote-path)
-            (tramp-get-connection-process vec)
+            (tramp-get-process vec)
           vec)
         "remote-path"
       (let* ((remote-path (copy-tree tramp-remote-path))
@@ -5945,10 +5955,9 @@ the length of the file to be compressed.
 If no corresponding command is found, nil is returned."
   (when (and (integerp tramp-inline-compress-start-size)
              (> size tramp-inline-compress-start-size))
-    (with-tramp-connection-property (tramp-get-connection-process vec) prop
+    (with-tramp-connection-property (tramp-get-process vec) prop
       (tramp-find-inline-compress vec)
-      (tramp-get-connection-property
-       (tramp-get-connection-process vec) prop nil))))
+      (tramp-get-connection-property (tramp-get-process vec) prop nil))))
 
 (defun tramp-get-inline-coding (vec prop size)
   "Return the coding command related to PROP.
@@ -5966,11 +5975,9 @@ function cell is returned to be applied on a buffer."
   ;; no inline coding is found.
   (ignore-errors
     (let ((coding
-           (with-tramp-connection-property
-               (tramp-get-connection-process vec) prop
+           (with-tramp-connection-property (tramp-get-process vec) prop
              (tramp-find-inline-encoding vec)
-             (tramp-get-connection-property
-              (tramp-get-connection-process vec) prop nil)))
+             (tramp-get-connection-property (tramp-get-process vec) prop nil)))
           (prop1 (if (string-match-p "encoding" prop)
                      "inline-compress" "inline-decompress"))
           compress)
diff --git a/lisp/net/tramp-smb.el b/lisp/net/tramp-smb.el
index 100ddfaa681..d361db483a1 100644
--- a/lisp/net/tramp-smb.el
+++ b/lisp/net/tramp-smb.el
@@ -1845,7 +1845,7 @@ are listed.  Result is the list (LOCALNAME MODE SIZE MTIME)."
   (if (and (process-live-p (tramp-get-connection-process vec))
            (tramp-get-connection-property vec "posix" t))
       (with-tramp-connection-property
-          (tramp-get-connection-process vec) "cifs-capabilities"
+          (tramp-get-process vec) "cifs-capabilities"
         (save-match-data
           (when (tramp-smb-send-command vec "posix")
             (with-current-buffer (tramp-get-connection-buffer vec)
@@ -1862,8 +1862,7 @@ are listed.  Result is the list (LOCALNAME MODE SIZE MTIME)."
   ;; When we are not logged in yet, we return nil.
   (if (and (tramp-smb-get-share vec)
            (process-live-p (tramp-get-connection-process vec)))
-      (with-tramp-connection-property
-          (tramp-get-connection-process vec) "stat-capability"
+      (with-tramp-connection-property (tramp-get-process vec) "stat-capability"
         (tramp-smb-send-command vec "stat \"/\""))))
 
 
diff --git a/lisp/net/tramp-sudoedit.el b/lisp/net/tramp-sudoedit.el
index b6861ba7882..68e68a242c9 100644
--- a/lisp/net/tramp-sudoedit.el
+++ b/lisp/net/tramp-sudoedit.el
@@ -373,7 +373,7 @@ the result will be a local, non-Tramp, file name."
 
 (defun tramp-sudoedit-remote-acl-p (vec)
   "Check, whether ACL is enabled on the remote host."
-  (with-tramp-connection-property (tramp-get-connection-process vec) "acl-p"
+  (with-tramp-connection-property (tramp-get-process vec) "acl-p"
     (zerop (tramp-call-process vec "getfacl" nil nil nil "/"))))
 
 (defun tramp-sudoedit-handle-file-acl (filename)
@@ -478,7 +478,7 @@ the result will be a local, non-Tramp, file name."
 
 (defun tramp-sudoedit-remote-selinux-p (vec)
   "Check, whether SELINUX is enabled on the remote host."
-  (with-tramp-connection-property (tramp-get-connection-process vec) "selinux-p"
+  (with-tramp-connection-property (tramp-get-process vec) "selinux-p"
     (zerop (tramp-call-process vec "selinuxenabled"))))
 
 (defun tramp-sudoedit-handle-file-selinux-context (filename)
diff --git a/lisp/net/tramp.el b/lisp/net/tramp.el
index 3ce2225cb84..e30f27fd338 100644
--- a/lisp/net/tramp.el
+++ b/lisp/net/tramp.el
@@ -37,7 +37,7 @@
 ;; For more detailed instructions, please see the info file.
 ;;
 ;; Notes:
-;; -----
+;; ------
 ;;
 ;; Also see the todo list at the bottom of this file.
 ;;
@@ -46,6 +46,7 @@
 ;;
 ;; There's a mailing list for this, as well.  Its name is:
 ;;            tramp-devel@gnu.org
+
 ;; You can use the Web to subscribe, under the following URL:
 ;;            https://lists.gnu.org/mailman/listinfo/tramp-devel
 ;;
@@ -1631,6 +1632,15 @@ from the default one."
   (or (tramp-get-connection-property vec "process-name" nil)
       (tramp-buffer-name vec)))
 
+(defun tramp-get-process (vec-or-proc)
+  "Get the default connection process to be used for VEC-OR-PROC.
+Return `tramp-cache-undefined' in case it doesn't exist."
+  (or (and (tramp-file-name-p vec-or-proc)
+           (get-buffer-process (tramp-buffer-name vec-or-proc)))
+      (and (processp vec-or-proc)
+           (tramp-get-process (process-get vec-or-proc 'vector)))
+      tramp-cache-undefined))
+
 (defun tramp-get-connection-process (vec)
   "Get the connection process to be used for VEC.
 In case a second asynchronous communication has been started, it is different