log tailer: the rotated mail.log read until postlogd lets it go

postlogd opens maillog_file once per process (Postfix 3.7.11,
src/postlogd/postlogd.c, pre_jail_init()) and leaves it only when it
exits: at "postfix reload" master closes the service's status pipe
(src/master/master_service.c, master_restart_service() ->
master_status_cleanup()) and dgram_server_abort() exits on reading its end
(src/master/dgram_server.c). logrotate renames and creates, then runs the
reload (build/logrotate-mail.conf postrotate; logrotate 3.21.0,
logrotate.c, rotateLogSet(), after rotateSingleLog()), so lines go to the
renamed file in between, and the tailer, already on the new one, never
read them (assumptions A-33). follow() now keeps the renamed file open as a
rotatedTail and reads it on each tick for rotatedGrace (a minute, far
longer than the gap); its places are never saved as the resume point, so a
restart still reads the rotated file through readRotated. A truncated file
is not kept.

The RFC 3339 stamp goes (A-12): it was written for a maillog_file_format
said to arrive in Postfix 3.9; there is none in 3.7.11 nor in the 3.12
snapshot of 2026-09-25 (proto/postconf.proto; msg_logger.c,
msg_logger_print() keeps strftime("%b %d %H:%M:%S ")). Every fixture now
carries the stamp Postfix writes; logtail's TestMain fixes time.Local to
UTC, since the stamp is local time and this PC is UTC+3.

Tests: TestFollowReadsTheRotatedFileAfterTheSwitch (Linux: a line appended
to the renamed file after the switch reaches the store). Stand
(2026-10-14): go vet and go test ./... pass on the server (23 packages);
make e2e exit 0, delivery_log_survives_rotation included. The tested tree
was this commit plus the previous one; the CHANGELOG entry was written
after the run, and a comment's logrotate citation corrected
(rotateLogSet runs postrotate, not rotateSingleLog).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/logtail/inbound_test.go

@@ -54,10 +54,10 @@ var tailNow = time.Date(2026, 10, 11, 18, 0, 0, 0, time.UTC)
 
 // The mail.log lines in this file are composed by hand, after
 // src/smtpd/smtpd_check.c log_whatsup() and src/cleanup/cleanup_milter.c
-// cleanup_milter_apply() (Postfix 3.7.11). Their ISO stamp is not a format
-// Postfix writes (assumptions A-12); TestLineTime has the syslog one.
+// cleanup_milter_apply() (Postfix 3.7.11), with the stamp Postfix 3.7 writes
+// (src/util/msg_logger.c, "%b %d %H:%M:%S").
 const (
-	head       = "2026-10-11T12:00:05.123456+00:00 mail "
+	head       = "Oct 11 12:00:05 mail "
 	unknownRcp = head + "postfix/inbound/smtpd[41]: NOQUEUE: reject: RCPT from mx.example.net[203.0.113.9]: 550 5.1.1 <nobody@in.example.com>: Recipient address rejected: User unknown in relay recipient table; from=<a@example.net> to=<nobody@in.example.com> proto=ESMTP helo=<mx.example.net>"
 )
 
@@ -74,7 +74,7 @@ func TestInboundRejectByPostfix(t *testing.T) {
 		From: "a@example.net", To: "nobody@in.example.com",
 		Decision: store.SpamDecisionReject, Engine: store.SpamEnginePostfix,
 		Detail: "550 5.1.1 <nobody@in.example.com>: Recipient address rejected: User unknown in relay recipient table",
-		At:     time.Date(2026, 10, 11, 12, 0, 5, 123456000, time.UTC),
+		At:     time.Date(2026, 10, 11, 12, 0, 5, 0, time.UTC),
 	}
 	if st.rows[0] != want {
 		t.Errorf("row = %+v\nwant  %+v", st.rows[0], want)
@@ -280,7 +280,7 @@ func TestLineTime(t *testing.T) {
 		want time.Time
 		rest string
 	}{
-		{"2026-01-02T09:00:00.5+03:00 mail postfix/x: y", time.Date(2026, 1, 2, 6, 0, 0, 500000000, time.UTC), "mail postfix/x: y"},
+		{"Jan 02 09:00:00 mail postfix/x: y", time.Date(2026, 1, 2, 9, 0, 0, 0, time.Local).UTC(), "mail postfix/x: y"},
 		{"Jan  2 09:30:00 mail postfix/x: y", time.Date(2026, 1, 2, 9, 30, 0, 0, time.Local).UTC(), "mail postfix/x: y"},
 		// December read in January is last year's.
 		{"Dec 31 23:59:00 mail postfix/x: y", time.Date(2025, 12, 31, 23, 59, 0, 0, time.Local).UTC(), "mail postfix/x: y"},