Be lazy when starting Flymake checks

Don't start the check immediately if the buffer is not being
displayed.  Wait until it is, using window-configuration-change-hook.

This enables the user to batch-enable flymake-mode on many buffers and
not have that operation exhaust system resources for checking each
one.  Likewise, an editing or save operation in a currently
non-displayed buffer does not immediately start a check.

* lisp/progmodes/flymake.el (flymake-start-on-flymake-mode):
Rename from flymake-start-syntax-check-on-find-file.
(flymake-start-syntax-check-on-find-file): Obsolete alias for
flymake-start-on-flymake-mode.
(flymake-start): Redesign.  Affect the global post-command-hook
and local window-configuraiton-change-hook.
(flymake--schedule-timer-maybe)
(flymake-after-change-function, flymake-after-save-hook): Pass
t to flymake-start.

* test/lisp/progmodes/flymake-tests.el (flymake-tests--call-with-fixture)
(dummy-backends, recurrent-backend): Start flymake check
explicitly and immediately.

1 files changed, 9 insertions, 3 deletions

diff --git a/test/lisp/progmodes/flymake-tests.el b/test/lisp/progmodes/flymake-tests.el
index 5e042f2b082..0b29b6a9715 100644
--- a/test/lisp/progmodes/flymake-tests.el
+++ b/test/lisp/progmodes/flymake-tests.el
@@ -73,7 +73,9 @@ SEVERITY-PREDICATE is used to setup
             (when sev-pred-supplied-p
               (setq-local flymake-proc-diagnostic-type-pred severity-predicate))
             (goto-char (point-min))
-            (unless flymake-mode (flymake-mode 1))
+            (let ((flymake-start-on-flymake-mode nil))
+              (unless flymake-mode (flymake-mode 1)))
+            (flymake-start)
             (flymake-tests--wait-for-backends)
             (funcall fn)))
       (and buffer
@@ -230,7 +232,9 @@ SEVERITY-PREDICATE is used to setup
                    'crashing-backend
                    ))
             (flymake-wrap-around nil))
-        (flymake-mode)
+        (let ((flymake-start-on-flymake-mode nil))
+          (flymake-mode))
+        (flymake-start)
 
         (flymake-tests--assert-set (flymake-running-backends)
           (error-backend warning-backend panicking-backend)
@@ -299,7 +303,9 @@ SEVERITY-PREDICATE is used to setup
         (let ((flymake-diagnostic-functions
                (list 'eager-backend))
               (flymake-wrap-around nil))
-          (flymake-mode)
+          (let ((flymake-start-on-flymake-mode nil))
+            (flymake-mode))
+          (flymake-start)
           (flymake-tests--assert-set (flymake-running-backends)
             (eager-backend) ())
           (cl-loop until tick repeat 4 do (sleep-for 0.2))