git-svn: replace .rev_db with a more space-efficient .rev_map format

Migrations are done automatically on an as-needed basis when new
revisions are to be fetched.  Stale remote branches do not get
migrated, yet.

However, unless you set noMetadata or useSvkProps it's safe to
just do:

  find $GIT_DIR/svn -name '.rev_db*' -print0 | xargs rm -f

to purge all the old .rev_db files.

The new format is a one-way migration and is NOT compatible with
old versions of git-svn.

This is the replacement for the rev_db format, which was too big
and inefficient for large repositories with a lot of sparse history
(mainly tags).

The format is this:

  - 24 bytes for every record,
    * 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

  - 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.

As with .rev_db, these files are disposable unless noMetadata or
useSvmProps is set.

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

1 files changed, 26 insertions, 0 deletions

diff --git a/t/lib-git-svn.sh b/t/lib-git-svn.sh
index 8d4a447..9ee35e7 100644
--- a/t/lib-git-svn.sh
+++ b/t/lib-git-svn.sh
@@ -82,3 +82,29 @@ stop_httpd () {
 	test -z "$SVN_HTTPD_PORT" && return
 	"$SVN_HTTPD_PATH" -f "$GIT_DIR"/httpd.conf -k stop
 }
+
+convert_to_rev_db () {
+	perl -w -- - "$@" <<\EOF
+use strict;
+@ARGV == 2 or die "Usage: convert_to_rev_db <input> <output>";
+open my $wr, '+>', $ARGV[1] or die "$!: couldn't open: $ARGV[1]";
+open my $rd, '<', $ARGV[0] or die "$!: couldn't open: $ARGV[0]";
+my $size = (stat($rd))[7];
+($size % 24) == 0 or die "Inconsistent size: $size";
+while (sysread($rd, my $buf, 24) == 24) {
+	my ($r, $c) = unpack('NH40', $buf);
+	my $offset = $r * 41;
+	seek $wr, 0, 2 or die $!;
+	my $pos = tell $wr;
+	if ($pos < $offset) {
+		for (1 .. (($offset - $pos) / 41)) {
+			print $wr (('0' x 40),"\n") or die $!;
+		}
+	}
+	seek $wr, $offset, 0 or die $!;
+	print $wr $c,"\n" or die $!;
+}
+close $wr or die $!;
+close $rd or die $!;
+EOF
+}