From: Lars Schneider <larsxschnei...@gmail.com>

Add a simple pass-thru filter as example implementation for the Git
filter protocol version 2. See Documentation/gitattributes.txt, section
"Filter Protocol" for more info.

Signed-off-by: Lars Schneider <larsxschnei...@gmail.com>
---
 Documentation/gitattributes.txt        |   4 +-
 contrib/long-running-filter/example.pl | 128 +++++++++++++++++++++++++++++++++
 2 files changed, 131 insertions(+), 1 deletion(-)
 create mode 100755 contrib/long-running-filter/example.pl

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 3f4d1ed..976243a 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -516,7 +516,9 @@ the command pipe on exit. The filter is expected to detect 
EOF
 and exit gracefully on its own. Git will wait until the filter
 process has stopped.
 
-If you develop your own long running filter
+A long running filter demo implementation can be found in
+`contrib/long-running-filter/example.pl` located in the Git
+core repository. If you develop your own long running filter
 process then the `GIT_TRACE_PACKET` environment variables can be
 very helpful for debugging (see linkgit:git[1]).
 
diff --git a/contrib/long-running-filter/example.pl 
b/contrib/long-running-filter/example.pl
new file mode 100755
index 0000000..3945705
--- /dev/null
+++ b/contrib/long-running-filter/example.pl
@@ -0,0 +1,128 @@
+#!/usr/bin/perl
+#
+# Example implementation for the Git filter protocol version 2
+# See Documentation/gitattributes.txt, section "Filter Protocol"
+#
+# Please note, this pass-thru filter is a minimal skeleton. No proper
+# error handling was implemented.
+#
+
+use strict;
+use warnings;
+
+my $MAX_PACKET_CONTENT_SIZE = 65516;
+
+sub packet_bin_read {
+       my $buffer;
+       my $bytes_read = read STDIN, $buffer, 4;
+       if ( $bytes_read == 0 ) {
+
+               # EOF - Git stopped talking to us!
+               exit();
+       }
+       elsif ( $bytes_read != 4 ) {
+               die "invalid packet: '$buffer'";
+       }
+       my $pkt_size = hex($buffer);
+       if ( $pkt_size == 0 ) {
+               return ( 1, "" );
+       }
+       elsif ( $pkt_size > 4 ) {
+               my $content_size = $pkt_size - 4;
+               $bytes_read = read STDIN, $buffer, $content_size;
+               if ( $bytes_read != $content_size ) {
+                       die "invalid packet ($content_size bytes expected; 
$bytes_read bytes read)";
+               }
+               return ( 0, $buffer );
+       }
+       else {
+               die "invalid packet size: $pkt_size";
+       }
+}
+
+sub packet_txt_read {
+       my ( $res, $buf ) = packet_bin_read();
+       unless ( $buf =~ s/\n$// ) {
+               die "A non-binary line MUST be terminated by an LF.";
+       }
+       return ( $res, $buf );
+}
+
+sub packet_bin_write {
+       my $buf = shift;
+       print STDOUT sprintf( "%04x", length($buf) + 4 );
+       print STDOUT $buf;
+       STDOUT->flush();
+}
+
+sub packet_txt_write {
+       packet_bin_write( $_[0] . "\n" );
+}
+
+sub packet_flush {
+       print STDOUT sprintf( "%04x", 0 );
+       STDOUT->flush();
+}
+
+( packet_txt_read() eq ( 0, "git-filter-client" ) ) || die "bad initialize";
+( packet_txt_read() eq ( 0, "version=2" ) )         || die "bad version";
+( packet_bin_read() eq ( 1, "" ) )                  || die "bad version end";
+
+packet_txt_write("git-filter-server");
+packet_txt_write("version=2");
+packet_flush();
+
+( packet_txt_read() eq ( 0, "capability=clean" ) )  || die "bad capability";
+( packet_txt_read() eq ( 0, "capability=smudge" ) ) || die "bad capability";
+( packet_bin_read() eq ( 1, "" ) )                  || die "bad capability 
end";
+
+packet_txt_write("capability=clean");
+packet_txt_write("capability=smudge");
+packet_flush();
+
+while (1) {
+       my ($command)  = packet_txt_read() =~ /^command=([^=]+)$/;
+       my ($pathname) = packet_txt_read() =~ /^pathname=([^=]+)$/;
+
+       packet_bin_read();
+
+       my $input = "";
+       {
+               binmode(STDIN);
+               my $buffer;
+               my $done = 0;
+               while ( !$done ) {
+                       ( $done, $buffer ) = packet_bin_read();
+                       $input .= $buffer;
+               }
+       }
+
+       my $output;
+       if ( $command eq "clean" ) {
+               ### Perform clean here ###
+               $output = $input;
+       }
+       elsif ( $command eq "smudge" ) {
+               ### Perform smudge here ###
+               $output = $input;
+       }
+       else {
+               die "bad command '$command'";
+       }
+
+       packet_txt_write("status=success");
+       packet_flush();
+       while ( length($output) > 0 ) {
+               my $packet = substr( $output, 0, $MAX_PACKET_CONTENT_SIZE );
+               packet_bin_write($packet);
+               if ( length($output) > $MAX_PACKET_CONTENT_SIZE ) {
+                       $output = substr( $output, $MAX_PACKET_CONTENT_SIZE );
+               }
+               else {
+                       $output = "";
+               }
+       }
+       packet_flush();    # flush content!
+       packet_flush();    # empty list, keep "status=success" unchanged!
+
+}
-- 
2.10.0

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