Linux Kernel Summit 2013 decided on a commit message convention to
identify commits containing bugs fixed by a commit: a "Fixes:" line,
included in the standard commit footer (along with "Signed-off-by:" if
present), containing an abbreviated commit hash (at least 12 characters
to keep it valid for a long time) and the subject of the commit (for
human readers).  This helps people (or automated tools) determine how
far to backport a commit.

Add a command line option for git commit to automatically construct the
"Fixes:" line for a commit.  This avoids the need to manually construct
that line by copy-pasting the commit hash and subject.

Also works with --amend to modify an existing commit's message.  To add
a Fixes line to an earlier commit in a series, use rebase -i and add the
following line after the existing commit:
x git commit --amend --no-edit -f $commit_containing_bug

Generalize append_signoff to support appending arbitrary extra lines to
a commit in the signoff block; this avoids duplicating the logic to find
or construct that block.

Signed-off-by: Josh Triplett <j...@joshtriplett.org>
---
 Documentation/git-commit.txt | 12 ++++++++++--
 builtin/commit.c             | 29 +++++++++++++++++++++++++++--
 sequencer.c                  | 31 +++++++++++++++++++++++--------
 sequencer.h                  |  3 +++
 t/t7502-commit.sh            | 39 ++++++++++++++++++++++++++++++++++++++-
 5 files changed, 101 insertions(+), 13 deletions(-)

diff --git a/Documentation/git-commit.txt b/Documentation/git-commit.txt
index 1a7616c..fcc6ed2 100644
--- a/Documentation/git-commit.txt
+++ b/Documentation/git-commit.txt
@@ -8,8 +8,8 @@ git-commit - Record changes to the repository
 SYNOPSIS
 --------
 [verse]
-'git commit' [-a | --interactive | --patch] [-s] [-v] [-u<mode>] [--amend]
-          [--dry-run] [(-c | -C | --fixup | --squash) <commit>]
+'git commit' [-a | --interactive | --patch] [-s] [-f <commit>] [-v] [-u<mode>]
+          [--amend] [--dry-run] [(-c | -C | --fixup | --squash) <commit>]
           [-F <file> | -m <msg>] [--reset-author] [--allow-empty]
           [--allow-empty-message] [--no-verify] [-e] [--author=<author>]
           [--date=<date>] [--cleanup=<mode>] [--[no-]status]
@@ -156,6 +156,14 @@ OPTIONS
        Add Signed-off-by line by the committer at the end of the commit
        log message.
 
+-f <commit>::
+--fixes=<commit>::
+       Add Fixes line for the specified commit at the end of the commit
+       log message.  This line includes an abbreviated commit hash for
+       the specified commit; the `core.abbrev` option determines the
+       length of the abbreviated commit hash used, with a minimum length
+       of 12 hex digits.
+
 -n::
 --no-verify::
        This option bypasses the pre-commit and commit-msg hooks.
diff --git a/builtin/commit.c b/builtin/commit.c
index 6ab4605..9bbcd8a 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -123,6 +123,7 @@ static int use_editor = 1, include_status = 1;
 static int show_ignored_in_status, have_option_m;
 static const char *only_include_assumed;
 static struct strbuf message = STRBUF_INIT;
+static struct strbuf fixes = STRBUF_INIT;
 
 static enum status_format {
        STATUS_FORMAT_NONE = 0,
@@ -133,6 +134,28 @@ static enum status_format {
        STATUS_FORMAT_UNSPECIFIED
 } status_format = STATUS_FORMAT_UNSPECIFIED;
 
+static int opt_parse_f(const struct option *opt, const char *arg, int unset)
+{
+       struct strbuf *sb = opt->value;
+       if (unset) {
+               strbuf_setlen(sb, 0);
+       } else {
+               struct pretty_print_context ctx = {0};
+               struct commit *commit;
+
+               commit = lookup_commit_reference_by_name(arg);
+               if (!commit)
+                       die(_("could not lookup commit %s"), arg);
+               ctx.output_encoding = get_commit_output_encoding();
+               ctx.abbrev = DEFAULT_ABBREV;
+               if (ctx.abbrev < 12)
+                       ctx.abbrev = 12;
+               format_commit_message(commit, "Fixes: %h ('%s')\n", sb, &ctx);
+       }
+
+       return 0;
+}
+
 static int opt_parse_m(const struct option *opt, const char *arg, int unset)
 {
        struct strbuf *buf = opt->value;
@@ -718,7 +741,7 @@ static int prepare_to_commit(const char *index_file, const 
char *prefix,
        if (clean_message_contents)
                stripspace(&sb, 0);
 
-       if (signoff) {
+       if (signoff || fixes.len) {
                /*
                 * See if we have a Conflicts: block at the end. If yes, count
                 * its size, so we can ignore it.
@@ -742,7 +765,8 @@ static int prepare_to_commit(const char *index_file, const 
char *prefix,
                        previous = eol;
                }
 
-               append_signoff(&sb, ignore_footer, 0);
+               append_signoff_extra(&sb, ignore_footer,
+                                    signoff ? 0 : APPEND_EXTRA_ONLY, &fixes);
        }
 
        if (fwrite(sb.buf, 1, sb.len, s->fp) < sb.len)
@@ -1463,6 +1487,7 @@ int cmd_commit(int argc, const char **argv, const char 
*prefix)
                OPT_STRING(0, "squash", &squash_message, N_("commit"), N_("use 
autosquash formatted message to squash specified commit")),
                OPT_BOOL(0, "reset-author", &renew_authorship, N_("the commit 
is authored by me now (used with -C/-c/--amend)")),
                OPT_BOOL('s', "signoff", &signoff, N_("add Signed-off-by:")),
+               OPT_CALLBACK('f', "fixes", &fixes, N_("commit"), N_("add Fixes: 
for the specified commit"), opt_parse_f),
                OPT_FILENAME('t', "template", &template_file, N_("use specified 
template file")),
                OPT_BOOL('e', "edit", &edit_flag, N_("force edit of commit")),
                OPT_STRING(0, "cleanup", &cleanup_arg, N_("default"), N_("how 
to strip spaces and #comments from message")),
diff --git a/sequencer.c b/sequencer.c
index 06e52b4..f4cf0e1 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -1135,26 +1135,33 @@ int sequencer_pick_revisions(struct replay_opts *opts)
        return pick_commits(todo_list, opts);
 }
 
-void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag)
+void append_signoff_extra(struct strbuf *msgbuf, int ignore_footer,
+                         unsigned flag, struct strbuf *extrabuf)
 {
        unsigned no_dup_sob = flag & APPEND_SIGNOFF_DEDUP;
+       unsigned append_sob = !(flag & APPEND_EXTRA_ONLY);
        struct strbuf sob = STRBUF_INIT;
        int has_footer;
 
-       strbuf_addstr(&sob, sign_off_header);
-       strbuf_addstr(&sob, fmt_name(getenv("GIT_COMMITTER_NAME"),
-                               getenv("GIT_COMMITTER_EMAIL")));
-       strbuf_addch(&sob, '\n');
+       if (append_sob) {
+               strbuf_addstr(&sob, sign_off_header);
+               strbuf_addstr(&sob, fmt_name(getenv("GIT_COMMITTER_NAME"),
+                                       getenv("GIT_COMMITTER_EMAIL")));
+               strbuf_addch(&sob, '\n');
+       }
 
        /*
         * If the whole message buffer is equal to the sob, pretend that we
         * found a conforming footer with a matching sob
         */
-       if (msgbuf->len - ignore_footer == sob.len &&
+       if (append_sob &&
+           msgbuf->len - ignore_footer == sob.len &&
            !strncmp(msgbuf->buf, sob.buf, sob.len))
                has_footer = 3;
        else
-               has_footer = has_conforming_footer(msgbuf, &sob, ignore_footer);
+               has_footer = has_conforming_footer(msgbuf,
+                                                  append_sob ? &sob : NULL,
+                                                  ignore_footer);
 
        if (!has_footer) {
                const char *append_newlines = NULL;
@@ -1193,9 +1200,17 @@ void append_signoff(struct strbuf *msgbuf, int 
ignore_footer, unsigned flag)
                                append_newlines, strlen(append_newlines));
        }
 
-       if (has_footer != 3 && (!no_dup_sob || has_footer != 2))
+       if (append_sob && has_footer != 3 && (!no_dup_sob || has_footer != 2))
                strbuf_splice(msgbuf, msgbuf->len - ignore_footer, 0,
                                sob.buf, sob.len);
+       if (extrabuf)
+               strbuf_insert(msgbuf, msgbuf->len - ignore_footer,
+                               extrabuf->buf, extrabuf->len);
 
        strbuf_release(&sob);
 }
+
+void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag)
+{
+       append_signoff_extra(msgbuf, ignore_footer, flag, NULL);
+}
diff --git a/sequencer.h b/sequencer.h
index 1fc22dc..8716ad0 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -7,6 +7,7 @@
 #define SEQ_OPTS_FILE  "sequencer/opts"
 
 #define APPEND_SIGNOFF_DEDUP (1u << 0)
+#define APPEND_EXTRA_ONLY (1u << 1)
 
 enum replay_action {
        REPLAY_REVERT,
@@ -51,5 +52,7 @@ int sequencer_pick_revisions(struct replay_opts *opts);
 extern const char sign_off_header[];
 
 void append_signoff(struct strbuf *msgbuf, int ignore_footer, unsigned flag);
+void append_signoff_extra(struct strbuf *msgbuf, int ignore_footer,
+                         unsigned flag, struct strbuf *extrabuf);
 
 #endif
diff --git a/t/t7502-commit.sh b/t/t7502-commit.sh
index 6313da2..12b123a 100755
--- a/t/t7502-commit.sh
+++ b/t/t7502-commit.sh
@@ -137,13 +137,50 @@ test_expect_success 'partial removal' '
 
 '
 
+signoff_ident () {
+       git var GIT_COMMITTER_IDENT | sed -e "s/>.*/>/"
+}
+
 test_expect_success 'sign off' '
 
        >positive &&
        git add positive &&
        git commit -s -m "thank you" &&
        actual=$(git cat-file commit HEAD | sed -ne "s/Signed-off-by: //p") &&
-       expected=$(git var GIT_COMMITTER_IDENT | sed -e "s/>.*/>/") &&
+       expected=$(signoff_ident) &&
+       test "z$actual" = "z$expected"
+
+'
+
+fixes_for_commits () {
+       for commit in "$@"; do
+               git -c core.abbrev=12 log -1 --pretty=format:"Fixes: %h 
('%s')%n" "$commit"
+       done
+}
+
+test_expect_success '--fixes' '
+
+       echo >>positive &&
+       git add positive &&
+       git commit -f HEAD -m "fix bug" &&
+       actual=$(git cat-file commit HEAD | sed -e "1,/^\$/d") &&
+       expected=$(echo fix bug; echo; fixes_for_commits HEAD^) &&
+       test "z$actual" = "z$expected"
+
+'
+
+test_expect_success 'multiple --fixes with signoff' '
+
+       echo >>positive &&
+       git add positive &&
+       git commit -f HEAD^ -f HEAD -s -m "signed bugfix" &&
+       actual=$(git cat-file commit HEAD | sed -e "1,/^\$/d") &&
+       expected=$(
+               echo signed bugfix
+               echo
+               echo "Signed-off-by: $(signoff_ident)"
+               fixes_for_commits HEAD^^ HEAD^
+       ) &&
        test "z$actual" = "z$expected"
 
 '
-- 
1.8.4.rc3

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