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Remove MaxBackends variable in favor of GetMaxBackends() function.

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Previously, it was really easy to write code that accessed MaxBackends
before we'd actually initialized it, especially when coding up an
extension. To make this less error-prune, introduce a new function
GetMaxBackends() which should be used to obtain the correct value.
This will ERROR if called too early. Demote the global variable to
a file-level static, so that nobody can peak at it directly.

Nathan Bossart. Idea by Andres Freund. Review by Greg Sabino Mullane,
by Michael Paquier (who had doubts about the approach), and by me.

Discussion: http://postgr.es/m/20210802224204.bckcikl45uezv5e4@alap3.anarazel.de
This commit is contained in:
Robert Haas
2022-02-08 15:52:40 -05:00
parent 2da896182c
commit aa64f23b02
19 changed files with 218 additions and 133 deletions
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11
src/backend/commands/async.c
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@ -518,7 +518,7 @@ AsyncShmemSize(void)
Size size;
/* This had better match AsyncShmemInit */
size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
size = mul_size(GetMaxBackends() + 1, sizeof(QueueBackendStatus));
size = add_size(size, offsetof(AsyncQueueControl, backend));
size = add_size(size, SimpleLruShmemSize(NUM_NOTIFY_BUFFERS, 0));
@ -534,6 +534,7 @@ AsyncShmemInit(void)
{
bool found;
Size size;
int max_backends = GetMaxBackends();
/*
* Create or attach to the AsyncQueueControl structure.
@ -541,7 +542,7 @@ AsyncShmemInit(void)
* The used entries in the backend[] array run from 1 to MaxBackends; the
* zero'th entry is unused but must be allocated.
*/
size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
size = mul_size(max_backends + 1, sizeof(QueueBackendStatus));
size = add_size(size, offsetof(AsyncQueueControl, backend));
asyncQueueControl = (AsyncQueueControl *)
@ -556,7 +557,7 @@ AsyncShmemInit(void)
QUEUE_FIRST_LISTENER = InvalidBackendId;
asyncQueueControl->lastQueueFillWarn = 0;
/* zero'th entry won't be used, but let's initialize it anyway */
for (int i = 0; i <= MaxBackends; i++)
for (int i = 0; i <= max_backends; i++)
{
QUEUE_BACKEND_PID(i) = InvalidPid;
QUEUE_BACKEND_DBOID(i) = InvalidOid;
@ -1641,8 +1642,8 @@ SignalBackends(void)
* XXX in principle these pallocs could fail, which would be bad. Maybe
* preallocate the arrays? They're not that large, though.
*/
pids = (int32 *) palloc(MaxBackends * sizeof(int32));
ids = (BackendId *) palloc(MaxBackends * sizeof(BackendId));
pids = (int32 *) palloc(GetMaxBackends() * sizeof(int32));
ids = (BackendId *) palloc(GetMaxBackends() * sizeof(BackendId));
count = 0;
LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);