git-svn: reinstate old rev_db optimization in new rev_map

This reinstates an old optimization in .rev_db which
stored the highest revision number we scanned, allowing
us to avoid scanning the SVN log for those revisions
again in a subsequent invocation.

This means the last 24-byte record in a .rev_map file
can be a 4-byte SVN revision number with 20-bytes of
zeroes representing a non-existent commit.  This record
can and will be overwritten when a new commit iff
the commit is all zeroes.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/git-svn.perl b/git-svn.perl
index b45c7e3266176336a4d8c6deeaa051c6fd0e9be6..55484f70e26886f9de9cd2d576f58beae8cc334b 100755 (executable)
--- a/git-svn.perl
+++ b/git-svn.perl
@@ -1904,7 +1904,7 @@ sub last_rev_commit {
                ($self->{last_rev}, $self->{last_commit}) = (undef, undef);
                return (undef, undef);
        }
-       my ($rev, $commit) = $self->rev_map_max;
+       my ($rev, $commit) = $self->rev_map_max(1);
        ($self->{last_rev}, $self->{last_commit}) = ($rev, $commit);
        return ($rev, $commit);
 }
@@ -2534,11 +2534,16 @@ sub rebuild {
 #     * 4 bytes for the integer representing an SVN revision number
 #     * 20 bytes representing the sha1 of a git commit
 #   - No empty padding records like the old format
+#     (except the last record, which can be overwritten)
 #   - new records are written append-only since SVN revision numbers
 #     increase monotonically
 #   - lookups on SVN revision number are done via a binary search
-#   - Piping the file to xxd(1) -c24 is a good way of dumping it for
-#     viewing or editing, should the need ever arise.
+#   - Piping the file to xxd -c24 is a good way of dumping it for
+#     viewing or editing (piped back through xxd -r), should the need
+#     ever arise.
+#   - The last record can be padding revision with an all-zero sha1
+#     This is used to optimize fetch performance when using multiple
+#     "fetch" directives in .git/config
 #
 # These files are disposable unless noMetadata or useSvmProps is set
 
@@ -2548,16 +2553,32 @@ sub _rev_map_set {
        my $size = (stat($fh))[7];
        ($size % 24) == 0 or croak "inconsistent size: $size";
 
+       my $wr_offset = 0;
        if ($size > 0) {
                sysseek($fh, -24, SEEK_END) or croak "seek: $!";
                my $read = sysread($fh, my $buf, 24) or croak "read: $!";
                $read == 24 or croak "read only $read bytes (!= 24)";
                my ($last_rev, $last_commit) = unpack(rev_map_fmt, $buf);
-               if ($last_rev >= $rev) {
-                       croak "last_rev is higher!: $last_rev >= $rev";
+               if ($last_commit eq ('0' x40)) {
+                       if ($size >= 48) {
+                               sysseek($fh, -48, SEEK_END) or croak "seek: $!";
+                               $read = sysread($fh, $buf, 24) or
+                                   croak "read: $!";
+                               $read == 24 or
+                                   croak "read only $read bytes (!= 24)";
+                               ($last_rev, $last_commit) =
+                                   unpack(rev_map_fmt, $buf);
+                               if ($last_commit eq ('0' x40)) {
+                                       croak "inconsistent .rev_map\n";
+                               }
+                       }
+                       if ($last_rev >= $rev) {
+                               croak "last_rev is higher!: $last_rev >= $rev";
+                       }
+                       $wr_offset = -24;
                }
        }
-       sysseek($fh, 0, SEEK_END) or croak "seek: $!";
+       sysseek($fh, $wr_offset, SEEK_END) or croak "seek: $!";
        syswrite($fh, pack(rev_map_fmt, $rev, $commit), 24) == 24 or
          croak "write: $!";
 }
@@ -2599,7 +2620,7 @@ sub rev_map_set {
                                            "$db => $db_lock ($!)\n";
        }
 
-       sysopen(my $fh, $db_lock, O_RDWR | O_APPEND | O_CREAT)
+       sysopen(my $fh, $db_lock, O_RDWR | O_CREAT)
             or croak "Couldn't open $db_lock: $!\n";
        _rev_map_set($fh, $rev, $commit);
        if ($sync) {
@@ -2622,25 +2643,40 @@ sub rev_map_set {
        }
 }
 
+# If want_commit, this will return an array of (rev, commit) where
+# commit _must_ be a valid commit in the archive.
+# Otherwise, it'll return the max revision (whether or not the
+# commit is valid or just a 0x40 placeholder).
 sub rev_map_max {
-       my ($self) = @_;
+       my ($self, $want_commit) = @_;
        $self->rebuild;
        my $map_path = $self->map_path;
-       stat $map_path or return wantarray ? (0, undef) : 0;
+       stat $map_path or return $want_commit ? (0, undef) : 0;
        sysopen(my $fh, $map_path, O_RDONLY) or croak "open: $!";
        my $size = (stat($fh))[7];
        ($size % 24) == 0 or croak "inconsistent size: $size";
 
        if ($size == 0) {
                close $fh or croak "close: $!";
-               return wantarray ? (0, undef) : 0;
+               return $want_commit ? (0, undef) : 0;
        }
 
-       sysseek($fh, -24, SEEK_END);
+       sysseek($fh, -24, SEEK_END) or croak "seek: $!";
        sysread($fh, my $buf, 24) == 24 or croak "read: $!";
-       close $fh or croak "close: $!";
        my ($r, $c) = unpack(rev_map_fmt, $buf);
-       wantarray ? ($r, $c) : $r;
+       if ($want_commit && $c eq ('0' x40)) {
+               if ($size < 48) {
+                       return $want_commit ? (0, undef) : 0;
+               }
+               sysseek($fh, -48, SEEK_END) or croak "seek: $!";
+               sysread($fh, $buf, 24) == 24 or croak "read: $!";
+               ($r, $c) = unpack(rev_map_fmt, $buf);
+               if ($c eq ('0'x40)) {
+                       croak "Penultimate record is all-zeroes in $map_path";
+               }
+       }
+       close $fh or croak "close: $!";
+       $want_commit ? ($r, $c) : $r;
 }
 
 sub rev_map_get {
@@ -2672,7 +2708,7 @@ sub rev_map_get {
                        $u = $i - 24;
                } else { # $r == $rev
                        close($fh) or croak "close: $!";
-                       return $c;
+                       return $c eq ('0' x 40) ? undef : $c;
                }
        }
        close($fh) or croak "close: $!";
@@ -3863,6 +3899,13 @@ sub gs_fetch_loop_common {
                                $ra_invalid = undef;
                        }
                }
+               # pre-fill the .rev_db since it'll eventually get filled in
+               # with '0' x40 if something new gets committed
+               foreach my $gs (@$gsv) {
+                       next if $gs->rev_map_max >= $max;
+                       next if defined $gs->rev_map_get($max);
+                       $gs->rev_map_set($max, 0 x40);
+               }
                foreach my $g (@$globs) {
                        my $k = "svn-remote.$g->{remote}.$g->{t}-maxRev";
                        Git::SVN::tmp_config($k, $max);