From 59f71a0d0b56b2df48db4bf1738aece5551f7a47 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Wed, 14 Jan 2015 18:45:22 +0100
Subject: Add a default local latch for use in signal handlers.

To do so, move InitializeLatchSupport() into the new common process
initialization functions, and add a new global variable MyLatch.

MyLatch is usable as soon InitPostmasterChild() has been called
(i.e. very early during startup). Initially it points to a process
local latch that exists in all processes. InitProcess/InitAuxiliaryProcess
then replaces that local latch with PGPROC->procLatch. During shutdown
the reverse happens.

This is primarily advantageous for two reasons: For one it simplifies
dealing with the shared process latch, especially in signal handlers,
because instead of having to check for MyProc, MyLatch can be used
unconditionally. For another, a later patch that makes FEs/BE
communication use latches, now can rely on the existence of a latch,
even before having gone through InitProcess.

Discussion: 20140927191243.GD5423@alap3.anarazel.de
---
 src/backend/utils/init/miscinit.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

(limited to 'src/backend/utils/init/miscinit.c')

diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index 414b05e3b8c..4646e0938e9 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -39,6 +39,7 @@
 #include "postmaster/postmaster.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
+#include "storage/latch.h"
 #include "storage/pg_shmem.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
@@ -55,6 +56,7 @@ ProcessingMode Mode = InitProcessing;
 /* List of lock files to be removed at proc exit */
 static List *lock_files = NIL;
 
+static Latch LocalLatchData;
 
 /* ----------------------------------------------------------------
  *		ignoring system indexes support stuff
@@ -189,6 +191,11 @@ InitPostmasterChild(void)
 	/* We don't want the postmaster's proc_exit() handlers */
 	on_exit_reset();
 
+	/* Initialize process-local latch support */
+	InitializeLatchSupport();
+	MyLatch = &LocalLatchData;
+	InitLatch(MyLatch);
+
 	/*
 	 * If possible, make this process a group leader, so that the postmaster
 	 * can signal any child processes too. Not all processes will have
@@ -215,6 +222,11 @@ InitStandaloneProcess(const char *argv0)
 
 	MyStartTime = time(NULL);	/* set our start time in case we call elog */
 
+	/* Initialize process-local latch support */
+	InitializeLatchSupport();
+	MyLatch = &LocalLatchData;
+	InitLatch(MyLatch);
+
 	/* Compute paths, no postmaster to inherit from */
 	if (my_exec_path[0] == '\0')
 	{
@@ -227,6 +239,31 @@ InitStandaloneProcess(const char *argv0)
 		get_pkglib_path(my_exec_path, pkglib_path);
 }
 
+void
+SwitchToSharedLatch(void)
+{
+	Assert(MyLatch == &LocalLatchData);
+	Assert(MyProc != NULL);
+
+	MyLatch = &MyProc->procLatch;
+	/*
+	 * Set the shared latch as the local one might have been set. This
+	 * shouldn't normally be necessary as code is supposed to check the
+	 * condition before waiting for the latch, but a bit care can't hurt.
+	 */
+	SetLatch(MyLatch);
+}
+
+void
+SwitchBackToLocalLatch(void)
+{
+	Assert(MyLatch != &LocalLatchData);
+	Assert(MyProc != NULL && MyLatch == &MyProc->procLatch);
+
+	MyLatch = &LocalLatchData;
+	SetLatch(MyLatch);
+}
+
 /*
  * GetUserId - get the current effective user ID.
  *
-- 
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