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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 <kewillf@microsoft.com> Signed-off-by: Ben Peart <Ben.Peart@microsoft.com>
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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
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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
27+
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
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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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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
50+
note, that the process must not send any response before it received the
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final flush packet.
52+
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
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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
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
71+
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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77+
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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#!/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
11+
# 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!
41+
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)";
55+
}
56+
return ( 0, $buffer );
57+
}
58+
else {
59+
die "invalid packet size: $pkt_size";
60+
}
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}
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63+
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.";
67+
}
68+
return ( $res, $buf );
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}
70+
71+
sub packet_bin_write {
72+
my $buf = shift;
73+
print STDOUT sprintf( "%04x", length($buf) + 4 );
74+
print STDOUT $buf;
75+
STDOUT->flush();
76+
}
77+
78+
sub packet_txt_write {
79+
packet_bin_write( $_[0] . "\n" );
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}
81+
82+
sub packet_flush {
83+
print STDOUT sprintf( "%04x", 0 );
84+
STDOUT->flush();
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}
86+
87+
( packet_txt_read() eq ( 0, "git-read-object-client" ) ) || die "bad initialize";
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( packet_txt_read() eq ( 0, "version=1" ) ) || die "bad version";
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( packet_bin_read() eq ( 1, "" ) ) || die "bad version end";
90+
91+
packet_txt_write("git-read-object-server");
92+
packet_txt_write("version=1");
93+
packet_flush();
94+
95+
( packet_txt_read() eq ( 0, "capability=get" ) ) || die "bad capability";
96+
( packet_bin_read() eq ( 1, "" ) ) || die "bad capability end";
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98+
packet_txt_write("capability=get");
99+
packet_flush();
100+
101+
while (1) {
102+
my ($command) = packet_txt_read() =~ /^command=([^=]+)$/;
103+
104+
if ( $command eq "get" ) {
105+
my ($sha1) = packet_txt_read() =~ /^sha1=([0-9a-f]{40})$/;
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packet_bin_read();
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108+
system ('git --git-dir="' . $DIR . '" cat-file blob ' . $sha1 . ' | git -c core.virtualizeobjects=false hash-object -w --stdin >/dev/null 2>&1');
109+
packet_txt_write(($?) ? "status=error" : "status=success");
110+
packet_flush();
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} else {
112+
die "bad command '$command'";
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}
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}

‎object-file.c‎

Lines changed: 125 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -45,6 +45,9 @@
4545
#include "fsck.h"
4646
#include "trace.h"
4747
#include "hook.h"
48+
#include "sigchain.h"
49+
#include "sub-process.h"
50+
#include "pkt-line.h"
4851

4952
/* The maximum size for an object header. */
5053
#define MAX_HEADER_LEN 32
@@ -961,6 +964,115 @@ int has_alt_odb(struct repository *r)
961964
return !!r->objects->odb->next;
962965
}
963966

967+
#define CAP_GET (1u<<0)
968+
969+
static int subprocess_map_initialized;
970+
static struct hashmap subprocess_map;
971+
972+
struct read_object_process {
973+
struct subprocess_entry subprocess;
974+
unsigned int supported_capabilities;
975+
};
976+
977+
static int start_read_object_fn(struct subprocess_entry *subprocess)
978+
{
979+
struct read_object_process *entry = (struct read_object_process *)subprocess;
980+
static int versions[] = {1, 0};
981+
static struct subprocess_capability capabilities[] = {
982+
{ "get", CAP_GET },
983+
{ NULL, 0 }
984+
};
985+
986+
return subprocess_handshake(subprocess, "git-read-object", versions,
987+
NULL, capabilities,
988+
&entry->supported_capabilities);
989+
}
990+
991+
static int read_object_process(const struct object_id *oid)
992+
{
993+
int err;
994+
struct read_object_process *entry;
995+
struct child_process *process;
996+
struct strbuf status = STRBUF_INIT;
997+
const char *cmd = find_hook("read-object");
998+
uint64_t start;
999+
1000+
start = getnanotime();
1001+
1002+
if (!subprocess_map_initialized) {
1003+
subprocess_map_initialized = 1;
1004+
hashmap_init(&subprocess_map, (hashmap_cmp_fn)cmd2process_cmp,
1005+
NULL, 0);
1006+
entry = NULL;
1007+
} else {
1008+
entry = (struct read_object_process *) subprocess_find_entry(&subprocess_map, cmd);
1009+
}
1010+
1011+
if (!entry) {
1012+
entry = xmalloc(sizeof(*entry));
1013+
entry->supported_capabilities = 0;
1014+
1015+
if (subprocess_start(&subprocess_map, &entry->subprocess, cmd,
1016+
start_read_object_fn)) {
1017+
free(entry);
1018+
return -1;
1019+
}
1020+
}
1021+
process = &entry->subprocess.process;
1022+
1023+
if (!(CAP_GET & entry->supported_capabilities))
1024+
return -1;
1025+
1026+
sigchain_push(SIGPIPE, SIG_IGN);
1027+
1028+
err = packet_write_fmt_gently(process->in, "command=get\n");
1029+
if (err)
1030+
goto done;
1031+
1032+
err = packet_write_fmt_gently(process->in, "sha1=%s\n", oid_to_hex(oid));
1033+
if (err)
1034+
goto done;
1035+
1036+
err = packet_flush_gently(process->in);
1037+
if (err)
1038+
goto done;
1039+
1040+
err = subprocess_read_status(process->out, &status);
1041+
err = err ? err : strcmp(status.buf, "success");
1042+
1043+
done:
1044+
sigchain_pop(SIGPIPE);
1045+
1046+
if (err || errno == EPIPE) {
1047+
err = err ? err : errno;
1048+
if (!strcmp(status.buf, "error")) {
1049+
/* The process signaled a problem with the file. */
1050+
}
1051+
else if (!strcmp(status.buf, "abort")) {
1052+
/*
1053+
* The process signaled a permanent problem. Don't try to read
1054+
* objects with the same command for the lifetime of the current
1055+
* Git process.
1056+
*/
1057+
entry->supported_capabilities &= ~CAP_GET;
1058+
}
1059+
else {
1060+
/*
1061+
* Something went wrong with the read-object process.
1062+
* Force shutdown and restart if needed.
1063+
*/
1064+
error("external process '%s' failed", cmd);
1065+
subprocess_stop(&subprocess_map,
1066+
(struct subprocess_entry *)entry);
1067+
free(entry);
1068+
}
1069+
}
1070+
1071+
trace_performance_since(start, "read_object_process");
1072+
1073+
return err;
1074+
}
1075+
9641076
/* Returns 1 if we have successfully freshened the file, 0 otherwise. */
9651077
static int freshen_file(const char *fn)
9661078
{
@@ -1011,8 +1123,19 @@ static int check_and_freshen_nonlocal(const struct object_id *oid, int freshen)
10111123

10121124
static int check_and_freshen(const struct object_id *oid, int freshen)
10131125
{
1014-
return check_and_freshen_local(oid, freshen) ||
1126+
int ret;
1127+
int tried_hook = 0;
1128+
1129+
retry:
1130+
ret = check_and_freshen_local(oid, freshen) ||
10151131
check_and_freshen_nonlocal(oid, freshen);
1132+
if (!ret && core_virtualize_objects && !tried_hook) {
1133+
tried_hook = 1;
1134+
if (!read_object_process(oid))
1135+
goto retry;
1136+
}
1137+
1138+
return ret;
10161139
}
10171140

10181141
int has_loose_object_nonlocal(const struct object_id *oid)
@@ -1554,20 +1677,6 @@ void disable_obj_read_lock(void)
15541677
pthread_mutex_destroy(&obj_read_mutex);
15551678
}
15561679

1557-
static int run_read_object_hook(const struct object_id *oid)
1558-
{
1559-
struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
1560-
int ret;
1561-
uint64_t start;
1562-
1563-
start = getnanotime();
1564-
strvec_push(&opt.args, oid_to_hex(oid));
1565-
ret = run_hooks_opt("read-object", &opt);
1566-
trace_performance_since(start, "run_read_object_hook");
1567-
1568-
return ret;
1569-
}
1570-
15711680
int fetch_if_missing = 1;
15721681

15731682
static int do_oid_object_info_extended(struct repository *r,
@@ -1626,7 +1735,7 @@ static int do_oid_object_info_extended(struct repository *r,
16261735
break;
16271736
if (core_virtualize_objects && !tried_hook) {
16281737
tried_hook = 1;
1629-
if (!run_read_object_hook(oid))
1738+
if (!read_object_process(oid))
16301739
goto retry;
16311740
}
16321741
}

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