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/*-------------------------------------------------------------------------
*
* latch.h
* Routines for interprocess latches
*
* A latch is a boolean variable, with operations that let processes sleep
* until it is set. A latch can be set from another process, or a signal
* handler within the same process.
*
* The latch interface is a reliable replacement for the common pattern of
* using pg_usleep() or select() to wait until a signal arrives, where the
* signal handler sets a flag variable. Because on some platforms an
* incoming signal doesn't interrupt sleep, and even on platforms where it
* does there is a race condition if the signal arrives just before
* entering the sleep, the common pattern must periodically wake up and
* poll the flag variable. The pselect() system call was invented to solve
* this problem, but it is not portable enough. Latches are designed to
* overcome these limitations, allowing you to sleep without polling and
* ensuring quick response to signals from other processes.
*
* There are two kinds of latches: local and shared. A local latch is
* initialized by InitLatch, and can only be set from the same process.
* A local latch can be used to wait for a signal to arrive, by calling
* SetLatch in the signal handler. A shared latch resides in shared memory,
* and must be initialized at postmaster startup by InitSharedLatch. Before
* a shared latch can be waited on, it must be associated with a process
* with OwnLatch. Only the process owning the latch can wait on it, but any
* process can set it.
*
* There are three basic operations on a latch:
*
* SetLatch - Sets the latch
* ResetLatch - Clears the latch, allowing it to be set again
* WaitLatch - Waits for the latch to become set
*
* WaitLatch includes a provision for timeouts (which should be avoided
* when possible, as they incur extra overhead) and a provision for
* postmaster child processes to wake up immediately on postmaster death.
* See latch.c for detailed specifications for the exported functions.
*
* The correct pattern to wait for event(s) is:
*
* for (;;)
* {
* ResetLatch();
* if (work to do)
* Do Stuff();
* WaitLatch();
* }
*
* It's important to reset the latch *before* checking if there's work to
* do. Otherwise, if someone sets the latch between the check and the
* ResetLatch call, you will miss it and Wait will incorrectly block.
*
* Another valid coding pattern looks like:
*
* for (;;)
* {
* if (work to do)
* Do Stuff(); // in particular, exit loop if some condition satisfied
* WaitLatch();
* ResetLatch();
* }
*
* This is useful to reduce latch traffic if it's expected that the loop's
* termination condition will often be satisfied in the first iteration;
* the cost is an extra loop iteration before blocking when it is not.
* What must be avoided is placing any checks for asynchronous events after
* WaitLatch and before ResetLatch, as that creates a race condition.
*
* To wake up the waiter, you must first set a global flag or something
* else that the wait loop tests in the "if (work to do)" part, and call
* SetLatch *after* that. SetLatch is designed to return quickly if the
* latch is already set.
*
* On some platforms, signals will not interrupt the latch wait primitive
* by themselves. Therefore, it is critical that any signal handler that
* is meant to terminate a WaitLatch wait calls SetLatch.
*
* Note that use of the process latch (PGPROC.procLatch) is generally better
* than an ad-hoc shared latch for signaling auxiliary processes. This is
* because generic signal handlers will call SetLatch on the process latch
* only, so using any latch other than the process latch effectively precludes
* use of any generic handler.
*
*
* WaitEventSets allow to wait for latches being set and additional events -
* postmaster dying and socket readiness of several sockets currently - at the
* same time. On many platforms using a long lived event set is more
* efficient than using WaitLatch or WaitLatchOrSocket.
*
*
* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/storage/latch.h
*
*-------------------------------------------------------------------------
*/
#ifndef LATCH_H
#define LATCH_H
#include <signal.h>
/*
* Latch structure should be treated as opaque and only accessed through
* the public functions. It is defined here to allow embedding Latches as
* part of bigger structs.
*/
typedef struct Latch
{
sig_atomic_t is_set;
sig_atomic_t maybe_sleeping;
bool is_shared;
int owner_pid;
#ifdef WIN32
HANDLE event;
#endif
} Latch;
/*
* Bitmasks for events that may wake-up WaitLatch(), WaitLatchOrSocket(), or
* WaitEventSetWait().
*/
#define WL_LATCH_SET (1 << 0)
#define WL_SOCKET_READABLE (1 << 1)
#define WL_SOCKET_WRITEABLE (1 << 2)
#define WL_TIMEOUT (1 << 3) /* not for WaitEventSetWait() */
#define WL_POSTMASTER_DEATH (1 << 4)
#define WL_EXIT_ON_PM_DEATH (1 << 5)
#ifdef WIN32
#define WL_SOCKET_CONNECTED (1 << 6)
#else
/* avoid having to deal with case on platforms not requiring it */
#define WL_SOCKET_CONNECTED WL_SOCKET_WRITEABLE
#endif
#define WL_SOCKET_MASK (WL_SOCKET_READABLE | \
WL_SOCKET_WRITEABLE | \
WL_SOCKET_CONNECTED)
typedef struct WaitEvent
{
int pos; /* position in the event data structure */
uint32 events; /* triggered events */
pgsocket fd; /* socket fd associated with event */
void *user_data; /* pointer provided in AddWaitEventToSet */
#ifdef WIN32
bool reset; /* Is reset of the event required? */
#endif
} WaitEvent;
/* forward declaration to avoid exposing latch.c implementation details */
typedef struct WaitEventSet WaitEventSet;
/*
* prototypes for functions in latch.c
*/
extern void InitializeLatchSupport(void);
extern void InitLatch(Latch *latch);
extern void InitSharedLatch(Latch *latch);
extern void OwnLatch(Latch *latch);
extern void DisownLatch(Latch *latch);
extern void SetLatch(Latch *latch);
extern void ResetLatch(Latch *latch);
extern void ShutdownLatchSupport(void);
extern WaitEventSet *CreateWaitEventSet(MemoryContext context, int nevents);
extern void FreeWaitEventSet(WaitEventSet *set);
extern int AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd,
Latch *latch, void *user_data);
extern void ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch);
extern int WaitEventSetWait(WaitEventSet *set, long timeout,
WaitEvent *occurred_events, int nevents,
uint32 wait_event_info);
extern int WaitLatch(Latch *latch, int wakeEvents, long timeout,
uint32 wait_event_info);
extern int WaitLatchOrSocket(Latch *latch, int wakeEvents,
pgsocket sock, long timeout, uint32 wait_event_info);
extern void InitializeLatchWaitSet(void);
extern int GetNumRegisteredWaitEvents(WaitEventSet *set);
#endif /* LATCH_H */
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