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Fix race conditions in replication slot operations

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It is relatively easy to get a replication slot to look as still active
while one process is in the process of getting rid of it; when some
other process tries to "acquire" the slot, it would fail with an error
message of "replication slot XYZ is active for PID N".

The error message in itself is fine, except that when the intention is
to drop the slot, it is unhelpful: the useful behavior would be to wait
until the slot is no longer acquired, so that the drop can proceed.  To
implement this, we use a condition variable so that slot acquisition can
be told to wait on that condition variable if the slot is already
acquired, and we make any change in active_pid broadcast a signal on the
condition variable.  Thus, as soon as the slot is released, the drop
will proceed properly.

Reported by: Tom Lane
Discussion: https://postgr.es/m/11904.1499039688@sss.pgh.pa.us
Authors: Petr Jelínek, Álvaro Herrera
This commit is contained in:
Alvaro Herrera
2017-07-25 13:26:49 -04:00
parent 4132dbec69
commit 9915de6c1c
5 changed files with 117 additions and 55 deletions
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10
src/include/replication/slot.h
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@@ -12,6 +12,7 @@
#include "fmgr.h"
#include "access/xlog.h"
#include "access/xlogreader.h"
#include "storage/condition_variable.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "storage/spin.h"
@@ -19,7 +20,7 @@
/*
* Behaviour of replication slots, upon release or crash.
*
* Slots marked as PERSISTENT are crashsafe and will not be dropped when
* Slots marked as PERSISTENT are crash-safe and will not be dropped when
* released. Slots marked as EPHEMERAL will be dropped when released or after
* restarts.
*
@@ -117,6 +118,9 @@ typedef struct ReplicationSlot
/* is somebody performing io on this slot? */
LWLock io_in_progress_lock;
/* Condition variable signalled when active_pid changes */
ConditionVariable active_cv;
/* all the remaining data is only used for logical slots */
/*
@@ -162,9 +166,9 @@ extern void ReplicationSlotsShmemInit(void);
extern void ReplicationSlotCreate(const char *name, bool db_specific,
ReplicationSlotPersistency p);
extern void ReplicationSlotPersist(void);
extern void ReplicationSlotDrop(const char *name);
extern void ReplicationSlotDrop(const char *name, bool nowait);
extern void ReplicationSlotAcquire(const char *name);
extern void ReplicationSlotAcquire(const char *name, bool nowait);
extern void ReplicationSlotRelease(void);
extern void ReplicationSlotCleanup(void);
extern void ReplicationSlotSave(void);