send-email: make message-id generation a bit more robust

Earlier code took Unix time and appended a few random digits.
If you are firing off many messages within a second, you could
issue the same id to different messages, which is a no-no.  If
you send out 31 messages within a single second, with random
integer taken out of rand(4200), you have about 10% chance of
producing the same message ID.

This fixes the problem by uses a prefix string which is
constant-per-invocation (time and pid), with a serial number for
each message generated by the process appended at the end.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-send-email.perl b/git-send-email.perl
index 195fe6f70a3a13c17b67ad8d067e8e0ff4cc9195..9547cc37a1c4fd326876e26b081bc6d6a5141ce6 100755 (executable)
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -437,10 +437,17 @@ sub extract_valid_address {
 
 # We'll setup a template for the message id, using the "from" address:
 
+my ($message_id_stamp, $message_id_serial);
 sub make_message_id
 {
-       my $date = time;
-       my $pseudo_rand = int (rand(4200));
+       my $uniq;
+       if (!defined $message_id_stamp) {
+               $message_id_stamp = sprintf("%s-%s", time, $$);
+               $message_id_serial = 0;
+       }
+       $message_id_serial++;
+       $uniq = "$message_id_stamp-$message_id_serial";
+
        my $du_part;
        for ($sender, $repocommitter, $repoauthor) {
                $du_part = extract_valid_address(sanitize_address($_));
@@ -450,8 +457,8 @@ sub make_message_id
                use Sys::Hostname qw();
                $du_part = 'user@' . Sys::Hostname::hostname();
        }
-       my $message_id_template = "<%s-git-send-email-$du_part>";
-       $message_id = sprintf $message_id_template, "$date$pseudo_rand";
+       my $message_id_template = "<%s-git-send-email-%s>";
+       $message_id = sprintf($message_id_template, $uniq, $du_part);
        #print "new message id = $message_id\n"; # Was useful for debugging
 }