[PATCH 4/7] format-patch: make cover letter subject configurable

[Denton Liu <liu.denton@gmail.com>]

We used to populate the subject of the cover letter generated by
git-format-patch with "*** SUBJECT HERE ***". However, if a user submits
multiple patchsets, they may want to keep a consistent subject between
rerolls.

If git-format-patch is run on a branch that has
`branch.<name>.coverSubject` defined, make the cover letter's subject be
that value instead of the generic "*** SUBJECT HERE ***".

In addition, add the `--cover-subject` option to override this setting.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
---
 Documentation/config/branch.txt    |  4 ++++
 Documentation/git-format-patch.txt | 12 ++++++++++++
 branch.c                           |  5 +++++
 branch.h                           |  5 +++++
 builtin/log.c                      | 26 +++++++++++++++++++-------
 t/t4014-format-patch.sh            | 20 ++++++++++++++++++++
 t/t9902-completion.sh              |  5 ++++-
 7 files changed, 69 insertions(+), 8 deletions(-)

diff --git a/Documentation/config/branch.txt b/Documentation/config/branch.txt
index 019d60ede2..2bff738982 100644
--- a/Documentation/config/branch.txt
+++ b/Documentation/config/branch.txt
@@ -100,3 +100,7 @@ branch.<name>.description::
 	`git branch --edit-description`. Branch description is
 	automatically added in the format-patch cover letter or
 	request-pull summary.
+
+branch.<name>.coverSubject::
+	When format-patch generates a cover letter, use the specified
+	subject for the cover letter instead of the generic template.
diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 7b71d4e2ed..af7883acbe 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -19,6 +19,7 @@ SYNOPSIS
 		   [--start-number <n>] [--numbered-files]
 		   [--in-reply-to=<Message-Id>] [--suffix=.<sfx>]
 		   [--ignore-if-in-upstream]
+		   [--cover-subject=<subject>]
 		   [--rfc] [--subject-prefix=<Subject-Prefix>]
 		   [(--reroll-count|-v) <n>]
 		   [--to=<email>] [--cc=<email>]
@@ -170,6 +171,10 @@ will want to ensure that threading is disabled for `git send-email`.
 	patches being generated, and any patch that matches is
 	ignored.
 
+--cover-subject=<subject>::
+	Instead of using the default "*** SUBJECT HERE ***" subject for
+	the cover letter, use the given <subject>.
+
 --subject-prefix=<Subject-Prefix>::
 	Instead of the standard '[PATCH]' prefix in the subject
 	line, instead use '[<Subject-Prefix>]'. This
@@ -346,6 +351,13 @@ attachments, and sign off patches with configuration variables.
 	coverletter = auto
 ------------
 
+In addition, for a specific branch, you can specify a custom cover
+letter subject.
+
+------------
+[branch "branch-name"]
+	coverSubject = "subject for branch-name only"
+------------
 
 DISCUSSION
 ----------
diff --git a/branch.c b/branch.c
index 4b49976924..40d30b8fa7 100644
--- a/branch.c
+++ b/branch.c
@@ -182,6 +182,11 @@ int read_branch_desc(struct strbuf *buf, const char *branch_name)
 	return read_branch_config(buf, branch_name, "description");
 }
 
+int read_branch_subject(struct strbuf *buf, const char *branch_name)
+{
+	return read_branch_config(buf, branch_name, "coversubject");
+}
+
 /*
  * Check if 'name' can be a valid name for a branch; die otherwise.
  * Return 1 if the named branch already exists; return 0 otherwise.
diff --git a/branch.h b/branch.h
index 29c1afa4d0..6a8936bbc8 100644
--- a/branch.h
+++ b/branch.h
@@ -79,6 +79,11 @@ extern int install_branch_config(int flag, const char *local, const char *origin
  */
 extern int read_branch_desc(struct strbuf *, const char *branch_name);
 
+/*
+ * Read branch subject
+ */
+extern int read_branch_subject(struct strbuf *, const char *branch_name);
+
 /*
  * Check if a branch is checked out in the main worktree or any linked
  * worktree and die (with a message describing its checkout location) if
diff --git a/builtin/log.c b/builtin/log.c
index ab859f5904..6f19326aea 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1034,13 +1034,14 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 			      struct commit *origin,
 			      int nr, struct commit **list,
 			      const char *branch_name,
+			      const char *subject,
 			      int quiet)
 {
 	const char *committer;
-	const char *body = "*** SUBJECT HERE ***\n\n*** BLURB HERE ***\n";
-	const char *msg;
+	const char *body = "*** BLURB HERE ***";
 	struct shortlog log;
 	struct strbuf sb = STRBUF_INIT;
+	struct strbuf subject_sb = STRBUF_INIT;
 	int i;
 	const char *encoding = "UTF-8";
 	int need_8bit_cte = 0;
@@ -1068,17 +1069,24 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	if (!branch_name)
 		branch_name = find_branch_name(rev);
 
-	msg = body;
+	if (!subject) {
+		if (branch_name && *branch_name && !read_branch_subject(&subject_sb, branch_name))
+			subject = subject_sb.buf;
+		else
+			subject = "*** SUBJECT HERE ***";
+	}
+
 	pp.fmt = CMIT_FMT_EMAIL;
 	pp.date_mode.type = DATE_RFC2822;
 	pp.rev = rev;
 	pp.print_email_subject = 1;
 	pp_user_info(&pp, NULL, &sb, committer, encoding);
-	pp_title_line(&pp, &msg, &sb, encoding, need_8bit_cte);
-	pp_remainder(&pp, &msg, &sb, 0);
+	pp_title_line(&pp, &subject, &sb, encoding, need_8bit_cte);
+	pp_remainder(&pp, &body, &sb, 0);
 	add_branch_description(&sb, branch_name);
 	fprintf(rev->diffopt.file, "%s\n", sb.buf);
 
+	strbuf_release(&subject_sb);
 	strbuf_release(&sb);
 
 	shortlog_init(&log);
@@ -1512,6 +1520,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	int no_binary_diff = 0;
 	int zero_commit = 0;
 	struct commit *origin = NULL;
+	const char *cover_subject = NULL;
 	const char *in_reply_to = NULL;
 	struct patch_ids ids;
 	struct strbuf buf = STRBUF_INIT;
@@ -1554,6 +1563,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		{ OPTION_CALLBACK, 0, "rfc", &rev, NULL,
 			    N_("Use [RFC PATCH] instead of [PATCH]"),
 			    PARSE_OPT_NOARG | PARSE_OPT_NONEG, rfc_callback },
+		OPT_STRING(0, "cover-subject", &cover_subject, N_("subject"),
+			    N_("the subject for the cover letter")),
 		{ OPTION_CALLBACK, 0, "subject-prefix", &rev, N_("prefix"),
 			    N_("Use [<prefix>] instead of [PATCH]"),
 			    PARSE_OPT_NONEG, subject_prefix_callback },
@@ -1617,8 +1628,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	extra_to.strdup_strings = 1;
 	extra_cc.strdup_strings = 1;
 	init_log_defaults();
-	git_config(git_format_config, NULL);
 	repo_init_revisions(the_repository, &rev, prefix);
+
+	git_config(git_format_config, NULL);
 	rev.commit_format = CMIT_FMT_EMAIL;
 	rev.expand_tabs_in_log_default = 0;
 	rev.verbose_header = 1;
@@ -1893,7 +1905,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		if (thread)
 			gen_message_id(&rev, "cover");
 		make_cover_letter(&rev, use_stdout,
-				  origin, nr, list, branch_name, quiet);
+				  origin, nr, list, branch_name, cover_subject, quiet);
 		print_bases(&bases, rev.diffopt.file);
 		print_signature(rev.diffopt.file);
 		total++;
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index d05cd256c7..1cf4dfbce2 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -1463,6 +1463,26 @@ test_expect_success 'format patch ignores color.ui' '
 	test_cmp expect actual
 '
 
+test_expect_success 'cover letter with config subject' '
+	test_config branch.rebuild-1.coverSubject "config subject" &&
+	git checkout rebuild-1 &&
+	git format-patch --stdout --cover-letter master >actual &&
+	grep "Subject: \[PATCH 0/2\] config subject" actual
+'
+
+test_expect_success 'cover letter with command-line subject' '
+	git checkout rebuild-1 &&
+	git format-patch --stdout --cover-letter --cover-subject "command-line subject" master >actual &&
+	grep "Subject: \[PATCH 0/2\] command-line subject" actual
+'
+
+test_expect_success 'cover letter with command-line subject overrides config' '
+	test_config branch.rebuild-1.coverSubject "config subject" &&
+	git checkout rebuild-1 &&
+	git format-patch --stdout --cover-letter --cover-subject "command-line subject" master >actual &&
+	grep "Subject: \[PATCH 0/2\] command-line subject" actual
+'
+
 test_expect_success 'cover letter using branch description (1)' '
 	git checkout rebuild-1 &&
 	test_config branch.rebuild-1.description hello &&
diff --git a/t/t9902-completion.sh b/t/t9902-completion.sh
index f5e21bf970..0da92179da 100755
--- a/t/t9902-completion.sh
+++ b/t/t9902-completion.sh
@@ -1542,7 +1542,10 @@ test_expect_success 'complete tree filename with metacharacters' '
 '
 
 test_expect_success PERL 'send-email' '
-	test_completion "git send-email --cov" "--cover-letter " &&
+	test_completion "git send-email --cov" <<-\EOF &&
+	--cover-letter Z
+	--cover-subject=Z
+	EOF
 	test_completion "git send-email ma" "master "
 '
 
-- 
2.21.0.1049.geb646f7864