Consider Git's exec path when searching for extensions

Commands that depend on optional third-party Git extensions use
executable-find to check whether the extension is available.  That
misses the legitimate case where an extension is installed in Git's
exec path rather than a directory in $PATH.

Update these spots to call a new helper that looks in Git's exec path
before calling executable-find.  Note that the order is chosen to
follow Git's precedence (though that doesn't matter for any of the
current callers because they don't use the return value).

Closes #4812.

4 files changed, 20 insertions, 3 deletions

diff --git a/docs/RelNotes/3.4.0.org b/docs/RelNotes/3.4.0.org
index 42ea3fa..9169af1 100644
--- a/docs/RelNotes/3.4.0.org
+++ b/docs/RelNotes/3.4.0.org
@@ -174,6 +174,10 @@ b1ad283941 #4526 magit-version: more compatible fix for #4511
 
 - A diff header added in a new Git release wasn't handled yet.  #4531
 
+- Commands that use optional third-party Git extensions didn't
+  consider that extension executables may be installed in Git's exec
+  path instead of a directory in ~exec-path~.  #4812
+
 - Fixed calculation of gravatar image size.  ecfaa325a3
 
 a14f847d97 magit-branch-checkout: Refresh after all configuration took place
diff --git a/lisp/magit-commit.el b/lisp/magit-commit.el
index c852065..03be34b 100644
--- a/lisp/magit-commit.el
+++ b/lisp/magit-commit.el
@@ -463,7 +463,7 @@ See `magit-commit-autofixup' for an alternative implementation."
                        (transient-args 'magit-commit-absorb))))
   (if (eq phase 'transient)
       (transient-setup 'magit-commit-absorb)
-    (unless (compat-executable-find "git-absorb" t)
+    (unless (magit-git-executable-find "git-absorb")
       (user-error "This command requires the git-absorb executable, which %s"
                   "is available from https://github.com/tummychow/git-absorb"))
     (unless (magit-anything-staged-p)
@@ -506,7 +506,7 @@ See `magit-commit-absorb' for an alternative implementation."
                        (transient-args 'magit-commit-autofixup))))
   (if (eq phase 'transient)
       (transient-setup 'magit-commit-autofixup)
-    (unless (compat-executable-find "git-autofixup" t)
+    (unless (magit-git-executable-find "git-autofixup")
       (user-error "This command requires the git-autofixup script, which %s"
                   "is available from https://github.com/torbiak/git-autofixup"))
     (unless (magit-anything-modified-p)
diff --git a/lisp/magit-git.el b/lisp/magit-git.el
index 641bbc3..502b5f0 100644
--- a/lisp/magit-git.el
+++ b/lisp/magit-git.el
@@ -582,6 +582,19 @@ call function WASHER with ARGS as its sole argument."
         (magit-cancel-section))
       (magit-maybe-make-margin-overlay))))
 
+(defun magit-git-executable-find (command)
+  "Search for COMMAND in Git's exec path, falling back to `exec-path'.
+Like `executable-find', return the absolute file name of the
+executable."
+  (or (locate-file command
+                   (list (concat
+                          (file-remote-p default-directory)
+                          (or (magit-git-string "--exec-path")
+                              (error "`git --exec-path' failed"))))
+                   exec-suffixes
+                   #'file-executable-p)
+      (compat-executable-find command t)))
+
 ;;; Git Version
 
 (defconst magit--git-version-regexp
diff --git a/lisp/magit-log.el b/lisp/magit-log.el
index 95290d5..eac0536 100644
--- a/lisp/magit-log.el
+++ b/lisp/magit-log.el
@@ -826,7 +826,7 @@ https://github.com/mhagger/git-when-merged."
              (list commit
                    (magit-read-other-branch "Merged into" commit)))
            (magit-log-arguments)))
-  (unless (compat-executable-find "git-when-merged" t)
+  (unless (magit-git-executable-find "git-when-merged")
     (user-error "This command requires git-when-merged (%s)"
                 "https://github.com/mhagger/git-when-merged"))
   (let (exit m)