Subject: [PATCH v2 0/2] commit-msg.sample: reject messages that would confuse "git am"

From: Phillip Wood <[email-masked]>
Date: Fri, 13 Feb 2026 14:34:47 +0000

From: Phillip Wood <[email-masked]>

This series adds a check to the sample commit-msg hook to reject commit
messages where the body of the message contains lines starting with
"diff -" and "Index: ". Such lines confuse "git am".

Changes since V1:

 - Allow subjects to start with "diff -" as they end up in an email
   header and so do not confuse "git am"

 - Allow "---" lines as they are useful when preparing patches.

Base-Commit: b2826b52eb7caff9f4ed6e85ec45e338bf02ad09
Published-As: https://github.com/phillipwood/git/releases/tag/pw%2Fsample-commit-msg-reject-diff%2Fv2
View-Changes-At: https://github.com/phillipwood/git/compare/b2826b52e...494f4df68
Fetch-It-Via: git fetch https://github.com/phillipwood/git pw/sample-commit-msg-reject-diff/v2

Phillip Wood (2):
  templates: add .gitattributes entry for sample hooks
  templates: detect commit messages containing diffs

 .editorconfig                     |  2 +-
 .gitattributes                    |  1 +
 templates/hooks/commit-msg.sample | 54 +++++++++++++++++++++++++++++--
 3 files changed, 54 insertions(+), 3 deletions(-)

Range-diff against v1:
1:  5f5e3091435 = 1:  5f5e3091435 templates: add .gitattributes entry for sample hooks
2:  e75978b9591 ! 2:  494f4df6865 templates: detect commit messages containing diffs
    @@ Metadata
      ## Commit message ##
         templates: detect commit messages containing diffs
     
    -    If a commit message contains a diff that is not indented then "git
    -    am" will treat that diff as part of the patch rather than as part
    -    of the commit message. This allows it to apply email messages that
    -    were created by adding a commit message in front of a regular diff
    +    If the body of a commit message contains a diff that is not indented
    +    then "git am" will treat that diff as part of the patch rather than
    +    as part of the commit message. This allows it to apply email messages
    +    that were created by adding a commit message in front of a regular diff
         without adding the "---" separator used by "git format-patch". This
         often surprises users [1-4] so add a check to the sample "commit-msg"
    -    hook to reject messages that would confuse "git am".
    -
    -    Detecting if the message contains a diff is complicated by the hook
    -    being passed the message before it is cleaned up so we need to ignore
    -    any diffs below the scissors line. There are also two possible
    -    config keys to check to find the comment character at the start
    -    of the scissors line.
    +    hook to reject messages that would confuse "git am". Even if a project
    +    does not use an email based workflow it is not uncommon for people
    +    to generate patches from it and apply them with "git am". Therefore
    +    it is still worth discouraging the creation of commit messages that
    +    would not be applied correctly.
    +
    +    A further source of confusion when applying patches with "git am" is
    +    the "---" separator that is added by "git format patch". If a commit
    +    message body contains that line then it will be truncated by "git am".
    +    As this is often used by patch authors to add some commentary that
    +    they do not want to end up in the commit message when the patch is
    +    applied, the hook does not complain about the presence of "---" lines
    +    in the message.
    +
    +    Detecting if the message contains a diff is complicated by the
    +    hook being passed the message before it is cleaned up so we need to
    +    ignore any diffs below the scissors line. There are also two possible
    +    config keys to check to find the comment character at the start of
    +    the scissors line. The first paragraph of the commit message becomes
    +    the email subject header which beings "Subject: " and so does not
    +    need to be checked. The trailing ".*" when matching commented lines
    +    ensures that if the comment string ends with a "$" it is not treated
    +    as an anchor.
     
         [1] https://lore.kernel.org/git/bcqvh7ahjjgzpgxwnr4kh3hfkksfruf54refyry3ha7qk7dldf@fij5calmscvm
         [2] https://lore.kernel.org/git/[email-masked]/
    @@ templates/hooks/commit-msg.sample
     +			p
     +		}'
     +)"
    -+line="$(sed -n -e "/^${comment_re} -\{8,\} >8 -\{8,\}\$/q
    -+		   /^diff -/{p;q;}
    -+		   /^Index: /{p;q;}" "$1")"
    ++scissors_line="^${comment_re} -\{8,\} >8 -\{8,\}\$"
    ++comment_line="^${comment_re}.*"
    ++blank_line='^[ 	]*$'
    ++# Disallow lines starting with "diff -" or "Index: " in the body of the
    ++# message. Stop looking if we see a scissors line.
    ++line="$(sed -n -e "
    ++	# Skip comments and blank lines at the start of the file.
    ++	/${scissors_line}/q
    ++	/${comment_line}/d
    ++	/${blank_line}/d
    ++	# The first paragraph will become the subject header so
    ++	# does not need to be checked.
    ++	: subject
    ++	n
    ++	/${scissors_line}/q
    ++	/${blank_line}/!b subject
    ++	# Check the body of the message for problematic
    ++	# prefixes.
    ++	: body
    ++	n
    ++	/${scissors_line}/q
    ++	/${comment_line}/b body
    ++	/^diff -/{p;q;}
    ++	/^Index: /{p;q;}
    ++	b body
    ++	" "$1")"
     +if test -n "$line"
     +then
     +	echo >&2 "Message contains a diff that will confuse 'git am'."
3:  83c100a73ec < -:  ----------- templates: detect messages that contain a separator line
-- 
2.52.0.362.g884e03848a9