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Commite58e13e

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Fix consistency issues with replication slot copy
Commit9f06d79's replication slot copying failed toproperly reserve the WAL that the slot is expecting to seeduring DecodingContextFindStartpoint (to set the confirmed_flushLSN), so concurrent activity could remove that WAL and cause thecopy process to error out. But it doesn't actually *need* thatWAL anyway: instead of running decode to find confirmed_flush, itcan be copied from the source slot. Fix this by rearranging thingsto avoid DecodingContextFindStartpoint() (leaving the target slot'sconfirmed_flush_lsn to invalid), and set that up afterwards by copyingfrom the target slot's value.Also ensure the source slot's confirmed_flush_lsn is valid.Reported-by: Arseny SherAuthor: Masahiko Sawada, Arseny SherDiscussion:https://postgr.es/m/871rr3ohbo.fsf@ars-thinkpad
1 parentbcd4600 commite58e13e

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2 files changed

+41
-7
lines changed

2 files changed

+41
-7
lines changed

‎src/backend/replication/logical/logical.c

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -211,6 +211,8 @@ StartupDecodingContext(List *output_plugin_options,
211211
*
212212
* plugin -- contains the name of the output plugin
213213
* output_plugin_options -- contains options passed to the output plugin
214+
* need_full_snapshot -- if true, must obtain a snapshot able to read all
215+
*tables; if false, one that can read only catalogs is acceptable.
214216
* restart_lsn -- if given as invalid, it's this routine's responsibility to
215217
*mark WAL as reserved by setting a convenient restart_lsn for the slot.
216218
*Otherwise, we set for decoding to start from the given LSN without

‎src/backend/replication/slotfuncs.c

Lines changed: 39 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -118,10 +118,14 @@ pg_create_physical_replication_slot(PG_FUNCTION_ARGS)
118118
* Helper function for creating a new logical replication slot with
119119
* given arguments. Note that this function doesn't release the created
120120
* slot.
121+
*
122+
* When find_startpoint is false, the slot's confirmed_flush is not set; it's
123+
* caller's responsibility to ensure it's set to something sensible.
121124
*/
122125
staticvoid
123126
create_logical_replication_slot(char*name,char*plugin,
124-
booltemporary,XLogRecPtrrestart_lsn)
127+
booltemporary,XLogRecPtrrestart_lsn,
128+
boolfind_startpoint)
125129
{
126130
LogicalDecodingContext*ctx=NULL;
127131

@@ -139,16 +143,24 @@ create_logical_replication_slot(char *name, char *plugin,
139143
temporary ?RS_TEMPORARY :RS_EPHEMERAL);
140144

141145
/*
142-
* Create logical decoding context, to build the initial snapshot.
146+
* Create logical decoding context to find start point or, if we don't
147+
* need it, to 1) bump slot's restart_lsn and xmin 2) check plugin sanity.
148+
*
149+
* Note: when !find_startpoint this is still important, because it's at
150+
* this point that the output plugin is validated.
143151
*/
144152
ctx=CreateInitDecodingContext(plugin,NIL,
145-
false,/*do not build snapshot */
153+
false,/*just catalogs is OK */
146154
restart_lsn,
147155
logical_read_local_xlog_page,NULL,NULL,
148156
NULL);
149157

150-
/* build initial snapshot, might take a while */
151-
DecodingContextFindStartpoint(ctx);
158+
/*
159+
* If caller needs us to determine the decoding start point, do so now.
160+
* This might take a while.
161+
*/
162+
if (find_startpoint)
163+
DecodingContextFindStartpoint(ctx);
152164

153165
/* don't need the decoding context anymore */
154166
FreeDecodingContext(ctx);
@@ -179,7 +191,8 @@ pg_create_logical_replication_slot(PG_FUNCTION_ARGS)
179191
create_logical_replication_slot(NameStr(*name),
180192
NameStr(*plugin),
181193
temporary,
182-
InvalidXLogRecPtr);
194+
InvalidXLogRecPtr,
195+
true);
183196

184197
values[0]=NameGetDatum(&MyReplicationSlot->data.name);
185198
values[1]=LSNGetDatum(MyReplicationSlot->data.confirmed_flush);
@@ -691,10 +704,18 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
691704

692705
/* Create new slot and acquire it */
693706
if (logical_slot)
707+
{
708+
/*
709+
* We must not try to read WAL, since we haven't reserved it yet --
710+
* hence pass find_startpoint false. confirmed_flush will be set
711+
* below, by copying from the source slot.
712+
*/
694713
create_logical_replication_slot(NameStr(*dst_name),
695714
plugin,
696715
temporary,
697-
src_restart_lsn);
716+
src_restart_lsn,
717+
false);
718+
}
698719
else
699720
create_physical_replication_slot(NameStr(*dst_name),
700721
true,
@@ -711,6 +732,7 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
711732
TransactionIdcopy_xmin;
712733
TransactionIdcopy_catalog_xmin;
713734
XLogRecPtrcopy_restart_lsn;
735+
XLogRecPtrcopy_confirmed_flush;
714736
boolcopy_islogical;
715737
char*copy_name;
716738

@@ -722,6 +744,7 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
722744
copy_xmin=src->data.xmin;
723745
copy_catalog_xmin=src->data.catalog_xmin;
724746
copy_restart_lsn=src->data.restart_lsn;
747+
copy_confirmed_flush=src->data.confirmed_flush;
725748

726749
/* for existence check */
727750
copy_name=pstrdup(NameStr(src->data.name));
@@ -746,6 +769,14 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
746769
NameStr(*src_name)),
747770
errdetail("The source replication slot was modified incompatibly during the copy operation.")));
748771

772+
/* The source slot must have a consistent snapshot */
773+
if (src_islogical&&XLogRecPtrIsInvalid(copy_confirmed_flush))
774+
ereport(ERROR,
775+
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
776+
errmsg("cannot copy unfinished logical replication slot \"%s\"",
777+
NameStr(*src_name)),
778+
errhint("Retry when the source replication slot's confirmed_flush_lsn is valid.")));
779+
749780
/* Install copied values again */
750781
SpinLockAcquire(&MyReplicationSlot->mutex);
751782
MyReplicationSlot->effective_xmin=copy_effective_xmin;
@@ -754,6 +785,7 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
754785
MyReplicationSlot->data.xmin=copy_xmin;
755786
MyReplicationSlot->data.catalog_xmin=copy_catalog_xmin;
756787
MyReplicationSlot->data.restart_lsn=copy_restart_lsn;
788+
MyReplicationSlot->data.confirmed_flush=copy_confirmed_flush;
757789
SpinLockRelease(&MyReplicationSlot->mutex);
758790

759791
ReplicationSlotMarkDirty();

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