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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
2+
^^^^^^^^^^^^^^^^^^^^^^^^^^^
3+
4+
The read-object process enables Git to read all missing blobs with a
5+
single process invocation for the entire life of a single Git command.
6+
This is achieved by using a packet format (pkt-line, see technical/
7+
protocol-common.txt) based protocol over standard input and standard
8+
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
10+
terminated by a LF.
11+
12+
Git starts the process when it encounters the first missing object that
13+
needs to be retrieved. After the process is started, Git sends a welcome
14+
message ("git-read-object-client"), a list of supported protocol version
15+
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
17+
from the previously sent list, and a flush packet. All further
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communication will be based on the selected version.
19+
20+
The remaining protocol description below documents "version=1". Please
21+
note that "version=42" in the example below does not exist and is only
22+
there to illustrate how the protocol would look with more than one
23+
version.
24+
25+
After the version negotiation Git sends a list of all capabilities that
26+
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,
28+
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
42+
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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47+
Afterwards Git sends a list of "key=value" pairs terminated with a flush
48+
packet. The list will contain at least the command (based on the
49+
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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53+
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
55+
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
64+
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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86+
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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1+
#!/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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26+
use strict;
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use warnings;
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29+
#
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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
32+
#
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my $DIR = "/host_repo/.git/";
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35+
sub packet_bin_read {
36+
my $buffer;
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my $bytes_read = read STDIN, $buffer, 4;
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if ( $bytes_read == 0 ) {
39+
40+
# EOF - Git stopped talking to us!
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exit();
42+
}
43+
elsif ( $bytes_read != 4 ) {
44+
die "invalid packet: '$buffer'";
45+
}
46+
my $pkt_size = hex($buffer);
47+
if ( $pkt_size == 0 ) {
48+
return ( 1, "" );
49+
}
50+
elsif ( $pkt_size > 4 ) {
51+
my $content_size = $pkt_size - 4;
52+
$bytes_read = read STDIN, $buffer, $content_size;
53+
if ( $bytes_read != $content_size ) {
54+
die "invalid packet ($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 {
59+
die "invalid packet size: $pkt_size";
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}
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}
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sub packet_txt_read {
64+
my ( $res, $buf ) = packet_bin_read();
65+
unless ( $buf =~ s/\n$// ) {
66+
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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}
70+
71+
sub packet_bin_write {
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my $buf = shift;
73+
print STDOUT sprintf( "%04x", length($buf) + 4 );
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print STDOUT $buf;
75+
STDOUT->flush();
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}
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78+
sub packet_txt_write {
79+
packet_bin_write( $_[0] . "\n" );
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}
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82+
sub packet_flush {
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print STDOUT sprintf( "%04x", 0 );
84+
STDOUT->flush();
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}
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87+
( packet_txt_read() eq ( 0, "git-read-object-client" ) ) || die "bad initialize";
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( packet_txt_read() eq ( 0, "version=1" ) ) || die "bad version";
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( packet_bin_read() eq ( 1, "" ) ) || die "bad version end";
90+
91+
packet_txt_write("git-read-object-server");
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packet_txt_write("version=1");
93+
packet_flush();
94+
95+
( packet_txt_read() eq ( 0, "capability=get" ) ) || die "bad capability";
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( packet_bin_read() eq ( 1, "" ) ) || die "bad capability end";
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packet_txt_write("capability=get");
99+
packet_flush();
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101+
while (1) {
102+
my ($command) = packet_txt_read() =~ /^command=([^=]+)$/;
103+
104+
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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@@ -42,6 +42,9 @@
4242
#include "object-file-convert.h"
4343
#include "trace.h"
4444
#include "hook.h"
45+
#include "sigchain.h"
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#include "sub-process.h"
47+
#include "pkt-line.h"
4548

4649
/* The maximum size for an object header. */
4750
#define MAX_HEADER_LEN 32
@@ -952,6 +955,115 @@ int has_alt_odb(struct repository *r)
952955
return !!r->objects->odb->next;
953956
}
954957

958+
#define CAP_GET (1u<<0)
959+
960+
static int subprocess_map_initialized;
961+
static struct hashmap subprocess_map;
962+
963+
struct read_object_process {
964+
struct subprocess_entry subprocess;
965+
unsigned int supported_capabilities;
966+
};
967+
968+
static int start_read_object_fn(struct subprocess_entry *subprocess)
969+
{
970+
struct read_object_process *entry = (struct read_object_process *)subprocess;
971+
static int versions[] = {1, 0};
972+
static struct subprocess_capability capabilities[] = {
973+
{ "get", CAP_GET },
974+
{ NULL, 0 }
975+
};
976+
977+
return subprocess_handshake(subprocess, "git-read-object", versions,
978+
NULL, capabilities,
979+
&entry->supported_capabilities);
980+
}
981+
982+
static int read_object_process(const struct object_id *oid)
983+
{
984+
int err;
985+
struct read_object_process *entry;
986+
struct child_process *process;
987+
struct strbuf status = STRBUF_INIT;
988+
const char *cmd = find_hook("read-object");
989+
uint64_t start;
990+
991+
start = getnanotime();
992+
993+
if (!subprocess_map_initialized) {
994+
subprocess_map_initialized = 1;
995+
hashmap_init(&subprocess_map, (hashmap_cmp_fn)cmd2process_cmp,
996+
NULL, 0);
997+
entry = NULL;
998+
} else {
999+
entry = (struct read_object_process *) subprocess_find_entry(&subprocess_map, cmd);
1000+
}
1001+
1002+
if (!entry) {
1003+
entry = xmalloc(sizeof(*entry));
1004+
entry->supported_capabilities = 0;
1005+
1006+
if (subprocess_start(&subprocess_map, &entry->subprocess, cmd,
1007+
start_read_object_fn)) {
1008+
free(entry);
1009+
return -1;
1010+
}
1011+
}
1012+
process = &entry->subprocess.process;
1013+
1014+
if (!(CAP_GET & entry->supported_capabilities))
1015+
return -1;
1016+
1017+
sigchain_push(SIGPIPE, SIG_IGN);
1018+
1019+
err = packet_write_fmt_gently(process->in, "command=get\n");
1020+
if (err)
1021+
goto done;
1022+
1023+
err = packet_write_fmt_gently(process->in, "sha1=%s\n", oid_to_hex(oid));
1024+
if (err)
1025+
goto done;
1026+
1027+
err = packet_flush_gently(process->in);
1028+
if (err)
1029+
goto done;
1030+
1031+
err = subprocess_read_status(process->out, &status);
1032+
err = err ? err : strcmp(status.buf, "success");
1033+
1034+
done:
1035+
sigchain_pop(SIGPIPE);
1036+
1037+
if (err || errno == EPIPE) {
1038+
err = err ? err : errno;
1039+
if (!strcmp(status.buf, "error")) {
1040+
/* The process signaled a problem with the file. */
1041+
}
1042+
else if (!strcmp(status.buf, "abort")) {
1043+
/*
1044+
* The process signaled a permanent problem. Don't try to read
1045+
* objects with the same command for the lifetime of the current
1046+
* Git process.
1047+
*/
1048+
entry->supported_capabilities &= ~CAP_GET;
1049+
}
1050+
else {
1051+
/*
1052+
* Something went wrong with the read-object process.
1053+
* Force shutdown and restart if needed.
1054+
*/
1055+
error("external process '%s' failed", cmd);
1056+
subprocess_stop(&subprocess_map,
1057+
(struct subprocess_entry *)entry);
1058+
free(entry);
1059+
}
1060+
}
1061+
1062+
trace_performance_since(start, "read_object_process");
1063+
1064+
return err;
1065+
}
1066+
9551067
/* Returns 1 if we have successfully freshened the file, 0 otherwise. */
9561068
static int freshen_file(const char *fn)
9571069
{
@@ -1002,8 +1114,19 @@ static int check_and_freshen_nonlocal(const struct object_id *oid, int freshen)
10021114

10031115
static int check_and_freshen(const struct object_id *oid, int freshen)
10041116
{
1005-
return check_and_freshen_local(oid, freshen) ||
1117+
int ret;
1118+
int tried_hook = 0;
1119+
1120+
retry:
1121+
ret = check_and_freshen_local(oid, freshen) ||
10061122
check_and_freshen_nonlocal(oid, freshen);
1123+
if (!ret && core_virtualize_objects && !tried_hook) {
1124+
tried_hook = 1;
1125+
if (!read_object_process(oid))
1126+
goto retry;
1127+
}
1128+
1129+
return ret;
10071130
}
10081131

10091132
int has_loose_object_nonlocal(const struct object_id *oid)
@@ -1547,20 +1670,6 @@ void disable_obj_read_lock(void)
15471670
pthread_mutex_destroy(&obj_read_mutex);
15481671
}
15491672

1550-
static int run_read_object_hook(const struct object_id *oid)
1551-
{
1552-
struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
1553-
int ret;
1554-
uint64_t start;
1555-
1556-
start = getnanotime();
1557-
strvec_push(&opt.args, oid_to_hex(oid));
1558-
ret = run_hooks_opt("read-object", &opt);
1559-
trace_performance_since(start, "run_read_object_hook");
1560-
1561-
return ret;
1562-
}
1563-
15641673
int fetch_if_missing = 1;
15651674

15661675
static int do_oid_object_info_extended(struct repository *r,
@@ -1619,7 +1728,7 @@ static int do_oid_object_info_extended(struct repository *r,
16191728
break;
16201729
if (core_virtualize_objects && !tried_hook) {
16211730
tried_hook = 1;
1622-
if (!run_read_object_hook(oid))
1731+
if (!read_object_process(oid))
16231732
goto retry;
16241733
}
16251734
}

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