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db_include/storage/latch.h

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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 */