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Fix consistency issues with replication slot copy

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Commit 9f06d79ef831's replication slot copying failed to
properly reserve the WAL that the slot is expecting to see
during DecodingContextFindStartpoint (to set the confirmed_flush
LSN), so concurrent activity could remove that WAL and cause the
copy process to error out.  But it doesn't actually *need* that
WAL anyway: instead of running decode to find confirmed_flush, it
can be copied from the source slot. Fix this by rearranging things
to avoid DecodingContextFindStartpoint() (leaving the target slot's
confirmed_flush_lsn to invalid), and set that up afterwards by copying
from the target slot's value.

Also ensure the source slot's confirmed_flush_lsn is valid.

Reported-by: Arseny Sher
Author: Masahiko Sawada, Arseny Sher
Discussion: https://postgr.es/m/871rr3ohbo.fsf@ars-thinkpad
This commit is contained in:
Alvaro Herrera 2020-03-17 16:13:18 -03:00
parent bcd460003a
commit e58e13e846
No known key found for this signature in database
GPG Key ID: 1C20ACB9D5C564AE
2 changed files with 41 additions and 7 deletions
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2
src/backend/replication/logical/logical.c
View File

@ -211,6 +211,8 @@ StartupDecodingContext(List *output_plugin_options,
*
* plugin -- contains the name of the output plugin
* output_plugin_options -- contains options passed to the output plugin
* need_full_snapshot -- if true, must obtain a snapshot able to read all
* tables; if false, one that can read only catalogs is acceptable.
* restart_lsn -- if given as invalid, it's this routine's responsibility to
* mark WAL as reserved by setting a convenient restart_lsn for the slot.
* Otherwise, we set for decoding to start from the given LSN without

46
src/backend/replication/slotfuncs.c
View File

@ -118,10 +118,14 @@ pg_create_physical_replication_slot(PG_FUNCTION_ARGS)
* Helper function for creating a new logical replication slot with
* given arguments. Note that this function doesn't release the created
* slot.
*
* When find_startpoint is false, the slot's confirmed_flush is not set; it's
* caller's responsibility to ensure it's set to something sensible.
*/
static void
create_logical_replication_slot(char *name, char *plugin,
bool temporary, XLogRecPtr restart_lsn)
bool temporary, XLogRecPtr restart_lsn,
bool find_startpoint)
{
LogicalDecodingContext *ctx = NULL;
@ -139,16 +143,24 @@ create_logical_replication_slot(char *name, char *plugin,
temporary ? RS_TEMPORARY : RS_EPHEMERAL);
/*
* Create logical decoding context, to build the initial snapshot.
* Create logical decoding context to find start point or, if we don't
* need it, to 1) bump slot's restart_lsn and xmin 2) check plugin sanity.
*
* Note: when !find_startpoint this is still important, because it's at
* this point that the output plugin is validated.
*/
ctx = CreateInitDecodingContext(plugin, NIL,
false, /* do not build snapshot */
false, /* just catalogs is OK */
restart_lsn,
logical_read_local_xlog_page, NULL, NULL,
NULL);
/* build initial snapshot, might take a while */
DecodingContextFindStartpoint(ctx);
/*
* If caller needs us to determine the decoding start point, do so now.
* This might take a while.
*/
if (find_startpoint)
DecodingContextFindStartpoint(ctx);
/* don't need the decoding context anymore */
FreeDecodingContext(ctx);
@ -179,7 +191,8 @@ pg_create_logical_replication_slot(PG_FUNCTION_ARGS)
create_logical_replication_slot(NameStr(*name),
NameStr(*plugin),
temporary,
InvalidXLogRecPtr);
InvalidXLogRecPtr,
true);
values[0] = NameGetDatum(&MyReplicationSlot->data.name);
values[1] = LSNGetDatum(MyReplicationSlot->data.confirmed_flush);
@ -691,10 +704,18 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
/* Create new slot and acquire it */
if (logical_slot)
{
/*
* We must not try to read WAL, since we haven't reserved it yet --
* hence pass find_startpoint false. confirmed_flush will be set
* below, by copying from the source slot.
*/
create_logical_replication_slot(NameStr(*dst_name),
plugin,
temporary,
src_restart_lsn);
src_restart_lsn,
false);
}
else
create_physical_replication_slot(NameStr(*dst_name),
true,
@ -711,6 +732,7 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
TransactionId copy_xmin;
TransactionId copy_catalog_xmin;
XLogRecPtr copy_restart_lsn;
XLogRecPtr copy_confirmed_flush;
bool copy_islogical;
char *copy_name;
@ -722,6 +744,7 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
copy_xmin = src->data.xmin;
copy_catalog_xmin = src->data.catalog_xmin;
copy_restart_lsn = src->data.restart_lsn;
copy_confirmed_flush = src->data.confirmed_flush;
/* for existence check */
copy_name = pstrdup(NameStr(src->data.name));
@ -746,6 +769,14 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
NameStr(*src_name)),
errdetail("The source replication slot was modified incompatibly during the copy operation.")));
/* The source slot must have a consistent snapshot */
if (src_islogical && XLogRecPtrIsInvalid(copy_confirmed_flush))
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("cannot copy unfinished logical replication slot \"%s\"",
NameStr(*src_name)),
errhint("Retry when the source replication slot's confirmed_flush_lsn is valid.")));
/* Install copied values again */
SpinLockAcquire(&MyReplicationSlot->mutex);
MyReplicationSlot->effective_xmin = copy_effective_xmin;
@ -754,6 +785,7 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
MyReplicationSlot->data.xmin = copy_xmin;
MyReplicationSlot->data.catalog_xmin = copy_catalog_xmin;
MyReplicationSlot->data.restart_lsn = copy_restart_lsn;
MyReplicationSlot->data.confirmed_flush = copy_confirmed_flush;
SpinLockRelease(&MyReplicationSlot->mutex);
ReplicationSlotMarkDirty();