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Hydrate missing loose objects in check_and_freshen()
Hydrate missing loose objects in check_and_freshen() when running virtualized. Add test cases to verify read-object hook works when running virtualized. This hook is called in check_and_freshen() rather than check_and_freshen_local() to make the hook work also with alternates. Helped-by: Kevin Willford <[email protected]> Signed-off-by: Ben Peart <[email protected]>
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‎Documentation/technical/read-object-protocol.txt

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Read Object Process
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The read-object process enables Git to read all missing blobs with a
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single process invocation for the entire life of a single Git command.
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This is achieved by using a packet format (pkt-line, see technical/
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protocol-common.txt) based protocol over standard input and standard
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output as follows. All packets, except for the "*CONTENT" packets and
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the "0000" flush packet, are considered text and therefore are
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terminated by a LF.
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Git starts the process when it encounters the first missing object that
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needs to be retrieved. After the process is started, Git sends a welcome
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message ("git-read-object-client"), a list of supported protocol version
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numbers, and a flush packet. Git expects to read a welcome response
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message ("git-read-object-server"), exactly one protocol version number
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from the previously sent list, and a flush packet. All further
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communication will be based on the selected version.
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The remaining protocol description below documents "version=1". Please
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note that "version=42" in the example below does not exist and is only
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there to illustrate how the protocol would look with more than one
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version.
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After the version negotiation Git sends a list of all capabilities that
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it supports and a flush packet. Git expects to read a list of desired
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capabilities, which must be a subset of the supported capabilities list,
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and a flush packet as response:
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------------------------
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packet: git> git-read-object-client
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packet: git> version=1
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packet: git> version=42
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packet: git> 0000
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packet: git< git-read-object-server
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packet: git< version=1
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packet: git< 0000
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packet: git> capability=get
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packet: git> capability=have
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packet: git> capability=put
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packet: git> capability=not-yet-invented
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packet: git> 0000
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packet: git< capability=get
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packet: git< 0000
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------------------------
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The only supported capability in version 1 is "get".
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Afterwards Git sends a list of "key=value" pairs terminated with a flush
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packet. The list will contain at least the command (based on the
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supported capabilities) and the sha1 of the object to retrieve. Please
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note, that the process must not send any response before it received the
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final flush packet.
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When the process receives the "get" command, it should make the requested
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object available in the git object store and then return success. Git will
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then check the object store again and this time find it and proceed.
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------------------------
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packet: git> command=get
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packet: git> sha1=0a214a649e1b3d5011e14a3dc227753f2bd2be05
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packet: git> 0000
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------------------------
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The process is expected to respond with a list of "key=value" pairs
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terminated with a flush packet. If the process does not experience
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problems then the list must contain a "success" status.
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------------------------
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packet: git< status=success
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packet: git< 0000
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------------------------
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In case the process cannot or does not want to process the content, it
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is expected to respond with an "error" status.
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------------------------
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packet: git< status=error
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packet: git< 0000
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------------------------
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In case the process cannot or does not want to process the content as
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well as any future content for the lifetime of the Git process, then it
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is expected to respond with an "abort" status at any point in the
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protocol.
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------------------------
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packet: git< status=abort
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packet: git< 0000
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------------------------
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Git neither stops nor restarts the process in case the "error"/"abort"
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status is set.
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If the process dies during the communication or does not adhere to the
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protocol then Git will stop the process and restart it with the next
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object that needs to be processed.
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After the read-object process has processed an object it is expected to
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wait for the next "key=value" list containing a command. Git will close
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the command pipe on exit. The process is expected to detect EOF and exit
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gracefully on its own. Git will wait until the process has stopped.
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A long running read-object process demo implementation can be found in
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`contrib/long-running-read-object/example.pl` located in the Git core
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repository. If you develop your own long running process then the
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`GIT_TRACE_PACKET` environment variables can be very helpful for
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debugging (see linkgit:git[1]).

‎contrib/long-running-read-object/example.pl

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#!/usr/bin/perl
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#
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# Example implementation for the Git read-object protocol version 1
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# See Documentation/technical/read-object-protocol.txt
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#
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# Allows you to test the ability for blobs to be pulled from a host git repo
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# "on demand." Called when git needs a blob it couldn't find locally due to
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# a lazy clone that only cloned the commits and trees.
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#
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# A lazy clone can be simulated via the following commands from the host repo
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# you wish to create a lazy clone of:
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#
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# cd /host_repo
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# git rev-parse HEAD
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# git init /guest_repo
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# git cat-file --batch-check --batch-all-objects | grep -v 'blob' |
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# cut -d' ' -f1 | git pack-objects /guest_repo/.git/objects/pack/noblobs
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# cd /guest_repo
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# git config core.virtualizeobjects true
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# git reset --hard <sha from rev-parse call above>
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#
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# Please note, this sample is a minimal skeleton. No proper error handling
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# was implemented.
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#
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use strict;
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use warnings;
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#
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# Point $DIR to the folder where your host git repo is located so we can pull
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# missing objects from it
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#
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my $DIR = "/host_repo/.git/";
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sub packet_bin_read {
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my $buffer;
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my $bytes_read = read STDIN, $buffer, 4;
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if ( $bytes_read == 0 ) {
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# EOF - Git stopped talking to us!
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exit();
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}
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elsif ( $bytes_read != 4 ) {
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die "invalid packet: '$buffer'";
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}
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my $pkt_size = hex($buffer);
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if ( $pkt_size == 0 ) {
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return ( 1, "" );
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}
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elsif ( $pkt_size > 4 ) {
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my $content_size = $pkt_size - 4;
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$bytes_read = read STDIN, $buffer, $content_size;
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if ( $bytes_read != $content_size ) {
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die "invalid packet ($content_size bytes expected; $bytes_read bytes read)";
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}
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return ( 0, $buffer );
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}
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else {
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die "invalid packet size: $pkt_size";
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}
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}
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sub packet_txt_read {
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my ( $res, $buf ) = packet_bin_read();
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unless ( $buf =~ s/\n$// ) {
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die "A non-binary line MUST be terminated by an LF.";
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}
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return ( $res, $buf );
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}
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sub packet_bin_write {
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my $buf = shift;
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print STDOUT sprintf( "%04x", length($buf) + 4 );
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print STDOUT $buf;
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STDOUT->flush();
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}
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sub packet_txt_write {
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packet_bin_write( $_[0] . "\n" );
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}
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sub packet_flush {
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print STDOUT sprintf( "%04x", 0 );
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STDOUT->flush();
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}
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( packet_txt_read() eq ( 0, "git-read-object-client" ) ) || die "bad initialize";
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( packet_txt_read() eq ( 0, "version=1" ) ) || die "bad version";
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( packet_bin_read() eq ( 1, "" ) ) || die "bad version end";
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packet_txt_write("git-read-object-server");
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packet_txt_write("version=1");
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packet_flush();
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( packet_txt_read() eq ( 0, "capability=get" ) ) || die "bad capability";
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( packet_bin_read() eq ( 1, "" ) ) || die "bad capability end";
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packet_txt_write("capability=get");
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packet_flush();
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while (1) {
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my ($command) = packet_txt_read() =~ /^command=([^=]+)$/;
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if ( $command eq "get" ) {
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my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
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packet_bin_read();
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system ('git --git-dir="' . $DIR . '" cat-file blob ' . $sha1 . ' | git -c core.virtualizeobjects=false hash-object -w --stdin >/dev/null 2>&1');
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packet_txt_write(($?) ? "status=error" : "status=success");
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packet_flush();
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} else {
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die "bad command '$command'";
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}
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}

‎object-file.c

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#include "environment.h"
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#include "fsck.h"
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#include "gettext.h"
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#include "gvfs.h"
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#include "hex.h"
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#include "loose.h"
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#include "object-file-convert.h"
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int freshen)
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{
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static struct strbuf path = STRBUF_INIT;
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int ret, tried_hook = 0;
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odb_loose_path(source, &path, oid);
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return check_and_freshen_file(path.buf, freshen);
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retry:
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ret = check_and_freshen_file(path.buf, freshen);
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if (!ret && gvfs_virtualize_objects(source->odb->repo) && !tried_hook) {
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tried_hook = 1;
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if (!read_object_process(source->odb->repo, oid))
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goto retry;
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}
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return ret;
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}
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int has_loose_object(struct odb_source *source,

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