Merge branch 'xw/am-empty'

"git am" learns "--empty=(stop|drop|keep)" option to tweak what is
done to a piece of e-mail without a patch in it.

* xw/am-empty:
  am: support --allow-empty to record specific empty patches
  am: support --empty=<option> to handle empty patches
  doc: git-format-patch: describe the option --always

6 files changed, 211 insertions, 16 deletions

diff --git a/Documentation/git-am.txt b/Documentation/git-am.txt
index 0a4a984dfd..09107fb106 100644
--- a/Documentation/git-am.txt
+++ b/Documentation/git-am.txt
@@ -16,8 +16,9 @@ SYNOPSIS
 	 [--exclude=<path>] [--include=<path>] [--reject] [-q | --quiet]
 	 [--[no-]scissors] [-S[<keyid>]] [--patch-format=<format>]
 	 [--quoted-cr=<action>]
+	 [--empty=(stop|drop|keep)]
 	 [(<mbox> | <Maildir>)...]
-'git am' (--continue | --skip | --abort | --quit | --show-current-patch[=(diff|raw)])
+'git am' (--continue | --skip | --abort | --quit | --show-current-patch[=(diff|raw)] | --allow-empty)
 
 DESCRIPTION
 -----------
@@ -63,6 +64,14 @@ OPTIONS
 --quoted-cr=<action>::
 	This flag will be passed down to 'git mailinfo' (see linkgit:git-mailinfo[1]).
 
+--empty=(stop|drop|keep)::
+	By default, or when the option is set to 'stop', the command
+	errors out on an input e-mail message lacking a patch
+	and stops into the middle of the current am session. When this
+	option is set to 'drop', skip such an e-mail message instead.
+	When this option is set to 'keep', create an empty commit,
+	recording the contents of the e-mail message as its log.
+
 -m::
 --message-id::
 	Pass the `-m` flag to 'git mailinfo' (see linkgit:git-mailinfo[1]),
@@ -191,6 +200,11 @@ default.   You can use `--no-utf8` to override this.
 	the e-mail message; if `diff`, show the diff portion only.
 	Defaults to `raw`.
 
+--allow-empty::
+	After a patch failure on an input e-mail message lacking a patch,
+	create an empty commit with the contents of the e-mail message
+	as its log message.
+
 DISCUSSION
 ----------
 
diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 113eabc107..be797d7a28 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -18,7 +18,7 @@ SYNOPSIS
 		   [-n | --numbered | -N | --no-numbered]
 		   [--start-number <n>] [--numbered-files]
 		   [--in-reply-to=<message id>] [--suffix=.<sfx>]
-		   [--ignore-if-in-upstream]
+		   [--ignore-if-in-upstream] [--always]
 		   [--cover-from-description=<mode>]
 		   [--rfc] [--subject-prefix=<subject prefix>]
 		   [(--reroll-count|-v) <n>]
@@ -192,6 +192,10 @@ will want to ensure that threading is disabled for `git send-email`.
 	patches being generated, and any patch that matches is
 	ignored.
 
+--always::
+	Include patches for commits that do not introduce any change,
+	which are omitted by default.
+
 --cover-from-description=<mode>::
 	Controls which parts of the cover letter will be automatically
 	populated using the branch's description.
diff --git a/builtin/am.c b/builtin/am.c
index 8677ea2348..f2e7e53388 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -87,6 +87,12 @@ enum show_patch_type {
 	SHOW_PATCH_DIFF = 1,
 };
 
+enum empty_action {
+	STOP_ON_EMPTY_COMMIT = 0,  /* output errors and stop in the middle of an am session */
+	DROP_EMPTY_COMMIT,         /* skip with a notice message, unless "--quiet" has been passed */
+	KEEP_EMPTY_COMMIT,         /* keep recording as empty commits */
+};
+
 struct am_state {
 	/* state directory path */
 	char *dir;
@@ -118,6 +124,7 @@ struct am_state {
 	int message_id;
 	int scissors; /* enum scissors_type */
 	int quoted_cr; /* enum quoted_cr_action */
+	int empty_type; /* enum empty_action */
 	struct strvec git_apply_opts;
 	const char *resolvemsg;
 	int committer_date_is_author_date;
@@ -178,6 +185,25 @@ static int am_option_parse_quoted_cr(const struct option *opt,
 	return 0;
 }
 
+static int am_option_parse_empty(const struct option *opt,
+				     const char *arg, int unset)
+{
+	int *opt_value = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+
+	if (!strcmp(arg, "stop"))
+		*opt_value = STOP_ON_EMPTY_COMMIT;
+	else if (!strcmp(arg, "drop"))
+		*opt_value = DROP_EMPTY_COMMIT;
+	else if (!strcmp(arg, "keep"))
+		*opt_value = KEEP_EMPTY_COMMIT;
+	else
+		return error(_("Invalid value for --empty: %s"), arg);
+
+	return 0;
+}
+
 /**
  * Returns path relative to the am_state directory.
  */
@@ -1126,6 +1152,12 @@ static void NORETURN die_user_resolve(const struct am_state *state)
 
 		printf_ln(_("When you have resolved this problem, run \"%s --continue\"."), cmdline);
 		printf_ln(_("If you prefer to skip this patch, run \"%s --skip\" instead."), cmdline);
+
+		if (advice_enabled(ADVICE_AM_WORK_DIR) &&
+		    is_empty_or_missing_file(am_path(state, "patch")) &&
+		    !repo_index_has_changes(the_repository, NULL, NULL))
+			printf_ln(_("To record the empty patch as an empty commit, run \"%s --allow-empty\"."), cmdline);
+
 		printf_ln(_("To restore the original branch and stop patching, run \"%s --abort\"."), cmdline);
 	}
 
@@ -1248,11 +1280,6 @@ static int parse_mail(struct am_state *state, const char *mail)
 		goto finish;
 	}
 
-	if (is_empty_or_missing_file(am_path(state, "patch"))) {
-		printf_ln(_("Patch is empty."));
-		die_user_resolve(state);
-	}
-
 	strbuf_addstr(&msg, "\n\n");
 	strbuf_addbuf(&msg, &mi.log_message);
 	strbuf_stripspace(&msg, 0);
@@ -1763,6 +1790,7 @@ static void am_run(struct am_state *state, int resume)
 	while (state->cur <= state->last) {
 		const char *mail = am_path(state, msgnum(state));
 		int apply_status;
+		int to_keep;
 
 		reset_ident_date();
 
@@ -1792,8 +1820,29 @@ static void am_run(struct am_state *state, int resume)
 		if (state->interactive && do_interactive(state))
 			goto next;
 
+		to_keep = 0;
+		if (is_empty_or_missing_file(am_path(state, "patch"))) {
+			switch (state->empty_type) {
+			case DROP_EMPTY_COMMIT:
+				say(state, stdout, _("Skipping: %.*s"), linelen(state->msg), state->msg);
+				goto next;
+				break;
+			case KEEP_EMPTY_COMMIT:
+				to_keep = 1;
+				say(state, stdout, _("Creating an empty commit: %.*s"),
+					linelen(state->msg), state->msg);
+				break;
+			case STOP_ON_EMPTY_COMMIT:
+				printf_ln(_("Patch is empty."));
+				die_user_resolve(state);
+				break;
+			}
+		}
+
 		if (run_applypatch_msg_hook(state))
 			exit(1);
+		if (to_keep)
+			goto commit;
 
 		say(state, stdout, _("Applying: %.*s"), linelen(state->msg), state->msg);
 
@@ -1827,6 +1876,7 @@ static void am_run(struct am_state *state, int resume)
 			die_user_resolve(state);
 		}
 
+commit:
 		do_commit(state);
 
 next:
@@ -1856,19 +1906,24 @@ next:
 /**
  * Resume the current am session after patch application failure. The user did
  * all the hard work, and we do not have to do any patch application. Just
- * trust and commit what the user has in the index and working tree.
+ * trust and commit what the user has in the index and working tree. If `allow_empty`
+ * is true, commit as an empty commit when index has not changed and lacking a patch.
  */
-static void am_resolve(struct am_state *state)
+static void am_resolve(struct am_state *state, int allow_empty)
 {
 	validate_resume_state(state);
 
 	say(state, stdout, _("Applying: %.*s"), linelen(state->msg), state->msg);
 
 	if (!repo_index_has_changes(the_repository, NULL, NULL)) {
-		printf_ln(_("No changes - did you forget to use 'git add'?\n"
-			"If there is nothing left to stage, chances are that something else\n"
-			"already introduced the same changes; you might want to skip this patch."));
-		die_user_resolve(state);
+		if (allow_empty && is_empty_or_missing_file(am_path(state, "patch"))) {
+			printf_ln(_("No changes - recorded it as an empty commit."));
+		} else {
+			printf_ln(_("No changes - did you forget to use 'git add'?\n"
+				    "If there is nothing left to stage, chances are that something else\n"
+				    "already introduced the same changes; you might want to skip this patch."));
+			die_user_resolve(state);
+		}
 	}
 
 	if (unmerged_cache()) {
@@ -2195,7 +2250,8 @@ enum resume_type {
 	RESUME_SKIP,
 	RESUME_ABORT,
 	RESUME_QUIT,
-	RESUME_SHOW_PATCH
+	RESUME_SHOW_PATCH,
+	RESUME_ALLOW_EMPTY,
 };
 
 struct resume_mode {
@@ -2348,6 +2404,9 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 		  N_("show the patch being applied"),
 		  PARSE_OPT_CMDMODE | PARSE_OPT_OPTARG | PARSE_OPT_NONEG | PARSE_OPT_LITERAL_ARGHELP,
 		  parse_opt_show_current_patch, RESUME_SHOW_PATCH },
+		OPT_CMDMODE(0, "allow-empty", &resume.mode,
+			N_("record the empty patch as an empty commit"),
+			RESUME_ALLOW_EMPTY),
 		OPT_BOOL(0, "committer-date-is-author-date",
 			&state.committer_date_is_author_date,
 			N_("lie about committer date")),
@@ -2357,6 +2416,9 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 		{ OPTION_STRING, 'S', "gpg-sign", &state.sign_commit, N_("key-id"),
 		  N_("GPG-sign commits"),
 		  PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
+		OPT_CALLBACK_F(STOP_ON_EMPTY_COMMIT, "empty", &state.empty_type, "{stop,drop,keep}",
+		  N_("how to handle empty patches"),
+		  PARSE_OPT_NONEG, am_option_parse_empty),
 		OPT_HIDDEN_BOOL(0, "rebasing", &state.rebasing,
 			N_("(internal use for git-rebase)")),
 		OPT_END()
@@ -2453,7 +2515,8 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 		am_run(&state, 1);
 		break;
 	case RESUME_RESOLVED:
-		am_resolve(&state);
+	case RESUME_ALLOW_EMPTY:
+		am_resolve(&state, resume.mode == RESUME_ALLOW_EMPTY ? 1 : 0);
 		break;
 	case RESUME_SKIP:
 		am_skip(&state);
diff --git a/t/t4150-am.sh b/t/t4150-am.sh
index 103cd39148..6caff0ca39 100755
--- a/t/t4150-am.sh
+++ b/t/t4150-am.sh
@@ -196,6 +196,12 @@ test_expect_success setup '
 
 	git format-patch -M --stdout lorem^ >rename-add.patch &&
 
+	git checkout -b empty-commit &&
+	git commit -m "empty commit" --allow-empty &&
+
+	: >empty.patch &&
+	git format-patch --always --stdout empty-commit^ >empty-commit.patch &&
+
 	# reset time
 	sane_unset test_tick &&
 	test_tick
@@ -1152,4 +1158,105 @@ test_expect_success 'apply binary blob in partial clone' '
 	git -C client am ../patch
 '
 
+test_expect_success 'an empty input file is error regardless of --empty option' '
+	test_when_finished "git am --abort || :" &&
+	test_must_fail git am --empty=drop empty.patch 2>actual &&
+	echo "Patch format detection failed." >expected &&
+	test_cmp expected actual
+'
+
+test_expect_success 'invalid when passing the --empty option alone' '
+	test_when_finished "git am --abort || :" &&
+	git checkout empty-commit^ &&
+	test_must_fail git am --empty empty-commit.patch 2>err &&
+	echo "error: Invalid value for --empty: empty-commit.patch" >expected &&
+	test_cmp expected err
+'
+
+test_expect_success 'a message without a patch is an error (default)' '
+	test_when_finished "git am --abort || :" &&
+	test_must_fail git am empty-commit.patch >err &&
+	grep "Patch is empty" err
+'
+
+test_expect_success 'a message without a patch is an error where an explicit "--empty=stop" is given' '
+	test_when_finished "git am --abort || :" &&
+	test_must_fail git am --empty=stop empty-commit.patch >err &&
+	grep "Patch is empty." err
+'
+
+test_expect_success 'a message without a patch will be skipped when "--empty=drop" is given' '
+	git am --empty=drop empty-commit.patch >output &&
+	git rev-parse empty-commit^ >expected &&
+	git rev-parse HEAD >actual &&
+	test_cmp expected actual &&
+	grep "Skipping: empty commit" output
+'
+
+test_expect_success 'record as an empty commit when meeting e-mail message that lacks a patch' '
+	git am --empty=keep empty-commit.patch >output &&
+	test_path_is_missing .git/rebase-apply &&
+	git show empty-commit --format="%B" >expected &&
+	git show HEAD --format="%B" >actual &&
+	grep -f actual expected &&
+	grep "Creating an empty commit: empty commit" output
+'
+
+test_expect_success 'skip an empty patch in the middle of an am session' '
+	git checkout empty-commit^ &&
+	test_must_fail git am empty-commit.patch >err &&
+	grep "Patch is empty." err &&
+	grep "To record the empty patch as an empty commit, run \"git am --allow-empty\"." err &&
+	git am --skip &&
+	test_path_is_missing .git/rebase-apply &&
+	git rev-parse empty-commit^ >expected &&
+	git rev-parse HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'record an empty patch as an empty commit in the middle of an am session' '
+	git checkout empty-commit^ &&
+	test_must_fail git am empty-commit.patch >err &&
+	grep "Patch is empty." err &&
+	grep "To record the empty patch as an empty commit, run \"git am --allow-empty\"." err &&
+	git am --allow-empty >output &&
+	grep "No changes - recorded it as an empty commit." output &&
+	test_path_is_missing .git/rebase-apply &&
+	git show empty-commit --format="%B" >expected &&
+	git show HEAD --format="%B" >actual &&
+	grep -f actual expected
+'
+
+test_expect_success 'create an non-empty commit when the index IS changed though "--allow-empty" is given' '
+	git checkout empty-commit^ &&
+	test_must_fail git am empty-commit.patch >err &&
+	: >empty-file &&
+	git add empty-file &&
+	git am --allow-empty &&
+	git show empty-commit --format="%B" >expected &&
+	git show HEAD --format="%B" >actual &&
+	grep -f actual expected &&
+	git diff HEAD^..HEAD --name-only
+'
+
+test_expect_success 'cannot create empty commits when there is a clean index due to merge conflicts' '
+	test_when_finished "git am --abort || :" &&
+	git rev-parse HEAD >expected &&
+	test_must_fail git am seq.patch &&
+	test_must_fail git am --allow-empty >err &&
+	! grep "To record the empty patch as an empty commit, run \"git am --allow-empty\"." err &&
+	git rev-parse HEAD >actual &&
+	test_cmp actual expected
+'
+
+test_expect_success 'cannot create empty commits when there is unmerged index due to merge conflicts' '
+	test_when_finished "git am --abort || :" &&
+	git rev-parse HEAD >expected &&
+	test_must_fail git am -3 seq.patch &&
+	test_must_fail git am --allow-empty >err &&
+	! grep "To record the empty patch as an empty commit, run \"git am --allow-empty\"." err &&
+	git rev-parse HEAD >actual &&
+	test_cmp actual expected
+'
+
 test_done
diff --git a/t/t7512-status-help.sh b/t/t7512-status-help.sh
index 7f2956d77a..2f16d5787e 100755
--- a/t/t7512-status-help.sh
+++ b/t/t7512-status-help.sh
@@ -659,6 +659,7 @@ On branch am_empty
 You are in the middle of an am session.
 The current patch is empty.
   (use "git am --skip" to skip this patch)
+  (use "git am --allow-empty" to record this patch as an empty commit)
   (use "git am --abort" to restore the original branch)
 
 nothing to commit (use -u to show untracked files)
diff --git a/wt-status.c b/wt-status.c
index 5d215f4e4f..335e723a71 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -1218,17 +1218,23 @@ static void show_merge_in_progress(struct wt_status *s,
 static void show_am_in_progress(struct wt_status *s,
 				const char *color)
 {
+	int am_empty_patch;
+
 	status_printf_ln(s, color,
 		_("You are in the middle of an am session."));
 	if (s->state.am_empty_patch)
 		status_printf_ln(s, color,
 			_("The current patch is empty."));
 	if (s->hints) {
-		if (!s->state.am_empty_patch)
+		am_empty_patch = s->state.am_empty_patch;
+		if (!am_empty_patch)
 			status_printf_ln(s, color,
 				_("  (fix conflicts and then run \"git am --continue\")"));
 		status_printf_ln(s, color,
 			_("  (use \"git am --skip\" to skip this patch)"));
+		if (am_empty_patch)
+			status_printf_ln(s, color,
+				_("  (use \"git am --allow-empty\" to record this patch as an empty commit)"));
 		status_printf_ln(s, color,
 			_("  (use \"git am --abort\" to restore the original branch)"));
 	}