Merge branch 'ac/log-use-mailmap-by-default-transition'

The "git log" command learns to issue a warning when log.mailmap
configuration is not set and --[no-]mailmap option is not used, to
prepare users for future versions of Git that uses the mailmap by
default.

* ac/log-use-mailmap-by-default-transition:
  tests: defang pager tests by explicitly disabling the log.mailmap warning
  documentation: mention --no-use-mailmap and log.mailmap false setting
  log: add warning for unspecified log.mailmap setting

Documentation/config/log.txt

@@ -40,4 +40,5 @@ log.showSignature::
 
 log.mailmap::
 	If true, makes linkgit:git-log[1], linkgit:git-show[1], and
-	linkgit:git-whatchanged[1] assume `--use-mailmap`.
+	linkgit:git-whatchanged[1] assume `--use-mailmap`, otherwise
+	assume `--no-use-mailmap`. False by default.

Documentation/git-log.txt

@@ -49,7 +49,7 @@ OPTIONS
 	Print out the ref name given on the command line by which each
 	commit was reached.
 
---use-mailmap::
+--[no-]use-mailmap::
 	Use mailmap file to map author and committer names and email
 	addresses to canonical real names and email addresses. See
 	linkgit:git-shortlog[1].

builtin/log.c

@@ -47,7 +47,7 @@ static int default_follow;
 static int default_show_signature;
 static int decoration_style;
 static int decoration_given;
-static int use_mailmap_config;
+static int use_mailmap_config = -1;
 static const char *fmt_patch_subject_prefix = "PATCH";
 static const char *fmt_pretty;
 
@@ -63,9 +63,14 @@ struct line_opt_callback_data {
 	struct string_list args;
 };
 
+static int session_is_interactive(void)
+{
+	return isatty(1) || pager_in_use();
+}
+
 static int auto_decoration_style(void)
 {
-	return (isatty(1) || pager_in_use()) ? DECORATE_SHORT_REFS : 0;
+	return session_is_interactive() ? DECORATE_SHORT_REFS : 0;
 }
 
 static int parse_decoration_style(const char *value)
@@ -151,6 +156,16 @@ static void cmd_log_init_defaults(struct rev_info *rev)
 		parse_date_format(default_date_mode, &rev->date_mode);
 }
 
+static char warn_unspecified_mailmap_msg[] =
+N_("log.mailmap is not set; its implicit value will change in an\n"
+   "upcoming release. To squelch this message and preserve current\n"
+   "behaviour, set the log.mailmap configuration value to false.\n"
+   "\n"
+   "To squelch this message and adopt the new behaviour now, set the\n"
+   "log.mailmap configuration value to true.\n"
+   "\n"
+   "See 'git help config' and search for 'log.mailmap' for further information.");
+
 static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
 			 struct rev_info *rev, struct setup_revision_opt *opt)
 {
@@ -199,6 +214,13 @@ static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
 	memset(&w, 0, sizeof(w));
 	userformat_find_requirements(NULL, &w);
 
+	if (mailmap < 0) {
+		if (session_is_interactive() && !rev->pretty_given)
+			warning("%s\n", _(warn_unspecified_mailmap_msg));
+
+		mailmap = 0;
+	}
+
 	if (!rev->show_notes_given && (!rev->pretty_given || w.notes))
 		rev->show_notes = 1;
 	if (rev->show_notes)

t/t7006-pager.sh

@@ -7,6 +7,8 @@ test_description='Test automatic use of a pager.'
 . "$TEST_DIRECTORY"/lib-terminal.sh
 
 test_expect_success 'setup' '
+	: squelch advice messages during the transition &&
+	git config --global log.mailmap false &&
 	sane_unset GIT_PAGER GIT_PAGER_IN_USE &&
 	test_unconfig core.pager &&