prepare send-email for smoother change of --chain-reply-to default

Give a warning message when send-email uses chain-reply-to to thread the
messages because of the current default, not because the user explicitly
asked to, either from the command line or from the configuration.

This way, by the time 1.7.0 switches the default, everybody will be ready.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -187,9 +187,11 @@ sub do_edit {
 my ($validate, $confirm);
 my (@suppress_cc);
 
+my $not_set_by_user = "true but not set by the user";
+
 my %config_bool_settings = (
     "thread" => [\$thread, 1],
-    "chainreplyto" => [\$chain_reply_to, 1],
+    "chainreplyto" => [\$chain_reply_to, $not_set_by_user],
     "suppressfrom" => [\$suppress_from, undef],
     "signedoffbycc" => [\$signed_off_by_cc, undef],
     "signedoffcc" => [\$signed_off_by_cc, undef],      # Deprecated
@@ -214,6 +216,19 @@ sub do_edit {
     "from" => \$sender,
 );
 
+# Help users prepare for 1.7.0
+sub chain_reply_to {
+	if (defined $chain_reply_to &&
+	    $chain_reply_to eq $not_set_by_user) {
+		print STDERR
+		    "In git 1.7.0, the default will be changed to --no-chain-reply-to\n" .
+		    "Set sendemail.chainreplyto configuration variable to true if\n" .
+		    "you want to keep --chain-reply-to as your default.\n";
+		$chain_reply_to = 1;
+	}
+	return $chain_reply_to;
+}
+
 # Handle Uncouth Termination
 sub signal_handler {
 
@@ -1157,7 +1172,7 @@ sub send_message
 
 	# set up for the next message
 	if ($thread && $message_was_sent &&
-		($chain_reply_to || !defined $reply_to || length($reply_to) == 0)) {
+		(chain_reply_to() || !defined $reply_to || length($reply_to) == 0)) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
 			$references .= "\n $message_id";

t/t9001-send-email.sh

@@ -769,4 +769,53 @@ test_expect_success 'threading but no chain-reply-to' '
 	grep "In-Reply-To: " stdout
 '
 
+test_expect_success 'warning with an implicit --chain-reply-to' '
+	git send-email \
+	--dry-run \
+	--from="Example <nobody@example.com>" \
+	--to=nobody@example.com \
+	outdir/000?-*.patch 2>errors >out &&
+	grep "no-chain-reply-to" errors
+'
+
+test_expect_success 'no warning with an explicit --chain-reply-to' '
+	git send-email \
+	--dry-run \
+	--from="Example <nobody@example.com>" \
+	--to=nobody@example.com \
+	--chain-reply-to \
+	outdir/000?-*.patch 2>errors >out &&
+	! grep "no-chain-reply-to" errors
+'
+
+test_expect_success 'no warning with an explicit --no-chain-reply-to' '
+	git send-email \
+	--dry-run \
+	--from="Example <nobody@example.com>" \
+	--to=nobody@example.com \
+	--no-chain-reply-to \
+	outdir/000?-*.patch 2>errors >out &&
+	! grep "no-chain-reply-to" errors
+'
+
+test_expect_success 'no warning with sendemail.chainreplyto = false' '
+	git config sendemail.chainreplyto false &&
+	git send-email \
+	--dry-run \
+	--from="Example <nobody@example.com>" \
+	--to=nobody@example.com \
+	outdir/000?-*.patch 2>errors >out &&
+	! grep "no-chain-reply-to" errors
+'
+
+test_expect_success 'no warning with sendemail.chainreplyto = true' '
+	git config sendemail.chainreplyto true &&
+	git send-email \
+	--dry-run \
+	--from="Example <nobody@example.com>" \
+	--to=nobody@example.com \
+	outdir/000?-*.patch 2>errors >out &&
+	! grep "no-chain-reply-to" errors
+'
+
 test_done