proc.c

PostgreSQL7.4.6 for Linux

/*-------------------------------------------------------------------------
 *
 * proc.c
 *	  routines to manage per-process shared memory data structure
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.136 2003/10/16 20:59:35 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
/*
 * Interface (a):
 *		ProcSleep(), ProcWakeup(),
 *		ProcQueueAlloc() -- create a shm queue for sleeping processes
 *		ProcQueueInit() -- create a queue without allocing memory
 *
 * Locking and waiting for buffers can cause the backend to be
 * put to sleep.  Whoever releases the lock, etc. wakes the
 * process up again (and gives it an error code so it knows
 * whether it was awoken on an error condition).
 *
 * Interface (b):
 *
 * ProcReleaseLocks -- frees the locks associated with current transaction
 *
 * ProcKill -- destroys the shared memory state (and locks)
 *		associated with the process.
 *
 * 5/15/91 -- removed the buffer pool based lock chain in favor
 *		of a shared memory lock chain.	The write-protection is
 *		more expensive if the lock chain is in the buffer pool.
 *		The only reason I kept the lock chain in the buffer pool
 *		in the first place was to allow the lock table to grow larger
 *		than available shared memory and that isn't going to work
 *		without a lot of unimplemented support anyway.
 */
#include "postgres.h"

#include <errno.h>
#include <signal.h>
#include <unistd.h>
#include <sys/time.h>

#include "miscadmin.h"
#include "access/xact.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/sinval.h"
#include "storage/spin.h"

/* GUC variables */
int			DeadlockTimeout = 1000;
int			StatementTimeout = 0;

/* Pointer to this process's PGPROC struct, if any */
PGPROC	   *MyProc = NULL;

/*
 * This spinlock protects the freelist of recycled PGPROC structures.
 * We cannot use an LWLock because the LWLock manager depends on already
 * having a PGPROC and a wait semaphore!  But these structures are touched
 * relatively infrequently (only at backend startup or shutdown) and not for
 * very long, so a spinlock is okay.
 */
static slock_t *ProcStructLock = NULL;

static PROC_HDR *ProcGlobal = NULL;

static PGPROC *DummyProc = NULL;

static bool waitingForLock = false;
static bool waitingForSignal = false;

/* Mark these volatile because they can be changed by signal handler */
static volatile bool statement_timeout_active = false;
static volatile bool deadlock_timeout_active = false;

/* statement_fin_time is valid only if statement_timeout_active is true */
static struct timeval statement_fin_time;


static void ProcKill(void);
static void DummyProcKill(void);
static bool CheckStatementTimeout(void);


/*
 * Report number of semaphores needed by InitProcGlobal.
 */
int
ProcGlobalSemas(int maxBackends)
{
	/* We need a sema per backend, plus one for the dummy process. */
	return maxBackends + 1;
}

/*
 * InitProcGlobal -
 *	  initializes the global process table. We put it here so that
 *	  the postmaster can do this initialization.
 *
 *	  We also create all the per-process semaphores we will need to support
 *	  the requested number of backends.  We used to allocate semaphores
 *	  only when backends were actually started up, but that is bad because
 *	  it lets Postgres fail under load --- a lot of Unix systems are
 *	  (mis)configured with small limits on the number of semaphores, and
 *	  running out when trying to start another backend is a common failure.
 *	  So, now we grab enough semaphores to support the desired max number
 *	  of backends immediately at initialization --- if the sysadmin has set
 *	  MaxBackends higher than his kernel will support, he'll find out sooner
 *	  rather than later.
 *
 *	  Another reason for creating semaphores here is that the semaphore
 *	  implementation typically requires us to create semaphores in the
 *	  postmaster, not in backends.
 */
void
InitProcGlobal(int maxBackends)
{
	bool		found = false;

	/* Create or attach to the ProcGlobal shared structure */
	ProcGlobal = (PROC_HDR *)
		ShmemInitStruct("Proc Header", sizeof(PROC_HDR), &found);

	/* --------------------
	 * We're the first - initialize.
	 * XXX if found should ever be true, it is a sign of impending doom ...
	 * ought to complain if so?
	 * --------------------
	 */
	if (!found)
	{
		int			i;

		ProcGlobal->freeProcs = INVALID_OFFSET;

		/*
		 * Pre-create the PGPROC structures and create a semaphore for
		 * each.
		 */
		for (i = 0; i < maxBackends; i++)
		{
			PGPROC	   *proc;

			proc = (PGPROC *) ShmemAlloc(sizeof(PGPROC));
			if (!proc)
				ereport(FATAL,
						(errcode(ERRCODE_OUT_OF_MEMORY),
						 errmsg("out of shared memory")));
			MemSet(proc, 0, sizeof(PGPROC));
			PGSemaphoreCreate(&proc->sem);
			proc->links.next = ProcGlobal->freeProcs;
			ProcGlobal->freeProcs = MAKE_OFFSET(proc);
		}

		/*
		 * Pre-allocate a PGPROC structure for dummy (checkpoint)
		 * processes, too.	This does not get linked into the freeProcs
		 * list.
		 */
		DummyProc = (PGPROC *) ShmemAlloc(sizeof(PGPROC));
		if (!DummyProc)
			ereport(FATAL,
					(errcode(ERRCODE_OUT_OF_MEMORY),
					 errmsg("out of shared memory")));
		MemSet(DummyProc, 0, sizeof(PGPROC));
		DummyProc->pid = 0;		/* marks DummyProc as not in use */
		PGSemaphoreCreate(&DummyProc->sem);

		/* Create ProcStructLock spinlock, too */
		ProcStructLock = (slock_t *) ShmemAlloc(sizeof(slock_t));
		SpinLockInit(ProcStructLock);
	}
}

/*
 * InitProcess -- initialize a per-process data structure for this backend
 */
void
InitProcess(void)
{
	SHMEM_OFFSET myOffset;

	/* use volatile pointer to prevent code rearrangement */
	volatile PROC_HDR *procglobal = ProcGlobal;

	/*
	 * ProcGlobal should be set by a previous call to InitProcGlobal (if
	 * we are a backend, we inherit this by fork() from the postmaster).
	 */
	if (procglobal == NULL)
		elog(PANIC, "proc header uninitialized");

	if (MyProc != NULL)
		elog(ERROR, "you already exist");

	/*
	 * Try to get a proc struct from the free list.  If this fails, we
	 * must be out of PGPROC structures (not to mention semaphores).
	 */
	SpinLockAcquire(ProcStructLock);

	myOffset = procglobal->freeProcs;

	if (myOffset != INVALID_OFFSET)
	{
		MyProc = (PGPROC *) MAKE_PTR(myOffset);
		procglobal->freeProcs = MyProc->links.next;
		SpinLockRelease(ProcStructLock);
	}
	else
	{
		/*
		 * If we reach here, all the PGPROCs are in use.  This is one of
		 * the possible places to detect "too many backends", so give the
		 * standard error message.
		 */
		SpinLockRelease(ProcStructLock);
		ereport(FATAL,
				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
				 errmsg("sorry, too many clients already")));
	}

	/*
	 * Initialize all fields of MyProc, except for the semaphore which was
	 * prepared for us by InitProcGlobal.
	 */
	SHMQueueElemInit(&(MyProc->links));
	MyProc->errType = STATUS_OK;
	MyProc->xid = InvalidTransactionId;
	MyProc->xmin = InvalidTransactionId;
	MyProc->pid = MyProcPid;
	MyProc->databaseId = MyDatabaseId;
	MyProc->logRec.xrecoff = 0;
	MyProc->lwWaiting = false;
	MyProc->lwExclusive = false;
	MyProc->lwWaitLink = NULL;
	MyProc->waitLock = NULL;
	MyProc->waitHolder = NULL;
	SHMQueueInit(&(MyProc->procHolders));

	/*
	 * Arrange to clean up at backend exit.
	 */
	on_shmem_exit(ProcKill, 0);

	/*
	 * We might be reusing a semaphore that belonged to a failed process.
	 * So be careful and reinitialize its value here.
	 */
	PGSemaphoreReset(&MyProc->sem);

	/*
	 * Now that we have a PGPROC, we could try to acquire locks, so
	 * initialize the deadlock checker.
	 */
	InitDeadLockChecking();
}

/*
 * InitDummyProcess -- create a dummy per-process data structure
 *
 * This is called by checkpoint processes so that they will have a MyProc
 * value that's real enough to let them wait for LWLocks.  The PGPROC and
 * sema that are assigned are the extra ones created during InitProcGlobal.
 */
void
InitDummyProcess(void)
{
	/*
	 * ProcGlobal should be set by a previous call to InitProcGlobal (we
	 * inherit this by fork() from the postmaster).
	 */
	if (ProcGlobal == NULL || DummyProc == NULL)
		elog(PANIC, "proc header uninitialized");

	if (MyProc != NULL)
		elog(ERROR, "you already exist");

	/*
	 * DummyProc should not presently be in use by anyone else
	 */
	if (DummyProc->pid != 0)
		elog(FATAL, "DummyProc is in use by PID %d", DummyProc->pid);
	MyProc = DummyProc;

	/*
	 * Initialize all fields of MyProc, except MyProc->sem which was set
	 * up by InitProcGlobal.
	 */
	MyProc->pid = MyProcPid;	/* marks DummyProc as in use by me */
	SHMQueueElemInit(&(MyProc->links));
	MyProc->errType = STATUS_OK;
	MyProc->xid = InvalidTransactionId;
	MyProc->xmin = InvalidTransactionId;
	MyProc->databaseId = MyDatabaseId;
	MyProc->logRec.xrecoff = 0;
	MyProc->lwWaiting = false;
	MyProc->lwExclusive = false;
	MyProc->lwWaitLink = NULL;
	MyProc->waitLock = NULL;
	MyProc->waitHolder = NULL;
	SHMQueueInit(&(MyProc->procHolders));

	/*
	 * Arrange to clean up at process exit.
	 */
	on_shmem_exit(DummyProcKill, 0);

	/*
	 * We might be reusing a semaphore that belonged to a failed process.
	 * So be careful and reinitialize its value here.
	 */
	PGSemaphoreReset(&MyProc->sem);
}

/*
 * Cancel any pending wait for lock, when aborting a transaction.
 *
 * Returns true if we had been waiting for a lock, else false.
 *
 * (Normally, this would only happen if we accept a cancel/die
 * interrupt while waiting; but an ereport(ERROR) while waiting is
 * within the realm of possibility, too.)
 */
bool
LockWaitCancel(void)
{
	/* Nothing to do if we weren't waiting for a lock */
	if (!waitingForLock)
		return false;

	waitingForLock = false;

	/* Turn off the deadlock timer, if it's still running (see ProcSleep) */
	disable_sig_alarm(false);

	/* Unlink myself from the wait queue, if on it (might not be anymore!) */
	LWLockAcquire(LockMgrLock, LW_EXCLUSIVE);
	if (MyProc->links.next != INVALID_OFFSET)
		RemoveFromWaitQueue(MyProc);
	LWLockRelease(LockMgrLock);

	/*
	 * Reset the proc wait semaphore to zero.  This is necessary in the
	 * scenario where someone else granted us the lock we wanted before we
	 * were able to remove ourselves from the wait-list.  The semaphore
	 * will have been bumped to 1 by the would-be grantor, and since we
	 * are no longer going to wait on the sema, we have to force it back
	 * to zero. Otherwise, our next attempt to wait for a lock will fall
	 * through prematurely.
	 */
	PGSemaphoreReset(&MyProc->sem);

	/*
	 * Return true even if we were kicked off the lock before we were able
	 * to remove ourselves.
	 */
	return true;
}


/*
 * ProcReleaseLocks() -- release locks associated with current transaction
 *			at transaction commit or abort
 *
 * At commit, we release only locks tagged with the current transaction's XID,
 * leaving those marked with XID 0 (ie, session locks) undisturbed.  At abort,
 * we release all locks including XID 0, because we need to clean up after
 * a failure.  This logic will need extension if we ever support nested
 * transactions.
 *
 * Note that user locks are not released in either case.
 */
void
ProcReleaseLocks(bool isCommit)
{
	if (!MyProc)
		return;
	/* If waiting, get off wait queue (should only be needed after error) */
	LockWaitCancel();
	/* Release locks */
	LockReleaseAll(DEFAULT_LOCKMETHOD, MyProc,
				   !isCommit, GetCurrentTransactionId());
}