/*------------------------------------------------------------------------- * * 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) 1996-2016, 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 #include #include /* * 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) 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 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); #endif /* _LATCH_H_ */