/*
* PGSemaphoreCreate
*
* Allocate a PGSemaphore structure with initial count 1
*/
PGSemaphore
PGSemaphoreCreate(void)
{
PGSemaphore sema;
/* Can't do this in a backend, because static state is postmaster's */
Assert(!IsUnderPostmaster);
if (nextSemaNumber >= SEMAS_PER_SET)
{
/* Time to allocate another semaphore set */
if (numSemaSets >= maxSemaSets)
elog(PANIC, "too many semaphores created");
mySemaSets[numSemaSets] = IpcSemaphoreCreate(SEMAS_PER_SET);
numSemaSets++;
nextSemaNumber = 0;
}
/* Use the next shared PGSemaphoreData */
if (numSharedSemas >= maxSharedSemas)
elog(PANIC, "too many semaphores created");
sema = &sharedSemas[numSharedSemas++];
/* Assign the next free semaphore in the current set */
sema->semId = mySemaSets[numSemaSets - 1];
sema->semNum = nextSemaNumber++;
/* Initialize it to count 1 */
IpcSemaphoreInitialize(sema->semId, sema->semNum, 1);
return sema;
}
/*
* PGSemaphoreCreate
*
* Initialize a PGSemaphore structure to represent a sema with count 1
*/
void
PGSemaphoreCreate(PGSemaphore sema)
{
/* Can't do this in a backend, because static state is postmaster's */
Assert(!IsUnderPostmaster);
if (nextSemaNumber >= SEMAS_PER_SET)
{
/* Time to allocate another semaphore set */
if (numSemaSets >= maxSemaSets)
elog(PANIC, "too many semaphores created");
mySemaSets[numSemaSets] = IpcSemaphoreCreate(SEMAS_PER_SET);
numSemaSets++;
nextSemaNumber = 0;
}
/* Assign the next free semaphore in the current set */
sema->semId = mySemaSets[numSemaSets - 1];
sema->semNum = nextSemaNumber++;
/* Initialize it to count 1 */
IpcSemaphoreInitialize(sema->semId, sema->semNum, 1);
elog((Debug_print_semaphore_detail ? LOG : DEBUG5),
"created SYSV semaphore semId %d, semNum %d",
sema->semId, sema->semNum);
}
/*
* PGSemaphoreReset
*
* Reset a previously-initialized PGSemaphore to have count 0
*/
void
PGSemaphoreReset(PGSemaphore sema)
{
IpcSemaphoreInitialize(sema->semId, sema->semNum, 0);
}
/*
* Create a semaphore set with the given number of useful semaphores
* (an additional sema is actually allocated to serve as identifier).
* Dead Postgres sema sets are recycled if found, but we do not fail
* upon collision with non-Postgres sema sets.
*
* The idea here is to detect and re-use keys that may have been assigned
* by a crashed postmaster or backend.
*/
static IpcSemaphoreId
IpcSemaphoreCreate(int numSems)
{
IpcSemaphoreId semId;
union semun semun;
PGSemaphoreData mysema;
/* Loop till we find a free IPC key */
for (nextSemaKey++;; nextSemaKey++)
{
pid_t creatorPID;
/* Try to create new semaphore set */
semId = InternalIpcSemaphoreCreate(nextSemaKey, numSems + 1);
if (semId >= 0)
break; /* successful create */
/* See if it looks to be leftover from a dead Postgres process */
semId = semget(nextSemaKey, numSems + 1, 0);
if (semId < 0)
continue; /* failed: must be some other app's */
if (IpcSemaphoreGetValue(semId, numSems) != PGSemaMagic)
continue; /* sema belongs to a non-Postgres app */
/*
* If the creator PID is my own PID or does not belong to any extant
* process, it's safe to zap it.
*/
creatorPID = IpcSemaphoreGetLastPID(semId, numSems);
if (creatorPID <= 0)
continue; /* oops, GETPID failed */
if (creatorPID != getpid())
{
if (kill(creatorPID, 0) == 0 || errno != ESRCH)
continue; /* sema belongs to a live process */
}
/*
* The sema set appears to be from a dead Postgres process, or from a
* previous cycle of life in this same process. Zap it, if possible.
* This probably shouldn't fail, but if it does, assume the sema set
* belongs to someone else after all, and continue quietly.
*/
semun.val = 0; /* unused, but keep compiler quiet */
if (semctl(semId, 0, IPC_RMID, semun) < 0)
continue;
/*
* Now try again to create the sema set.
*/
semId = InternalIpcSemaphoreCreate(nextSemaKey, numSems + 1);
if (semId >= 0)
break; /* successful create */
/*
* Can only get here if some other process managed to create the same
* sema key before we did. Let him have that one, loop around to try
* next key.
*/
}
/*
* OK, we created a new sema set. Mark it as created by this process. We
* do this by setting the spare semaphore to PGSemaMagic-1 and then
* incrementing it with semop(). That leaves it with value PGSemaMagic
* and sempid referencing this process.
*/
IpcSemaphoreInitialize(semId, numSems, PGSemaMagic - 1);
mysema.semId = semId;
mysema.semNum = numSems;
PGSemaphoreUnlock(&mysema);
return semId;
}
