Refactor postmaster child process launching

Introduce new postmaster_child_launch() function that deals with the
differences in EXEC_BACKEND mode.

Refactor the mechanism of passing information from the parent to child
process. Instead of using different command-line arguments when
launching the child process in EXEC_BACKEND mode, pass a
variable-length blob of startup data along with all the global
variables. The contents of that blob depend on the kind of child
process being launched. In !EXEC_BACKEND mode, we use the same blob,
but it's simply inherited from the parent to child process.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

Author: hlinnaka

src/backend/postmaster/autovacuum.c

@@ -85,7 +85,6 @@
 #include "nodes/makefuncs.h"
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
-#include "postmaster/fork_process.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/postmaster.h"
 #include "storage/bufmgr.h"
@@ -311,13 +310,6 @@ static WorkerInfo MyWorkerInfo = NULL;
 /* PID of launcher, valid only in worker while shutting down */
 int			AutovacuumLauncherPid = 0;
 
-#ifdef EXEC_BACKEND
-static pid_t avlauncher_forkexec(void);
-static pid_t avworker_forkexec(void);
-#endif
-NON_EXEC_STATIC void AutoVacWorkerMain(int argc, char *argv[]) pg_attribute_noreturn();
-NON_EXEC_STATIC void AutoVacLauncherMain(int argc, char *argv[]) pg_attribute_noreturn();
-
 static Oid	do_start_worker(void);
 static void HandleAutoVacLauncherInterrupts(void);
 static void AutoVacLauncherShutdown(void) pg_attribute_noreturn();
@@ -361,76 +353,23 @@ static void avl_sigusr2_handler(SIGNAL_ARGS);
  *					  AUTOVACUUM LAUNCHER CODE
  ********************************************************************/
 
-#ifdef EXEC_BACKEND
 /*
- * forkexec routine for the autovacuum launcher process.
- *
- * Format up the arglist, then fork and exec.
+ * Main entry point for the autovacuum launcher process.
  */
-static pid_t
-avlauncher_forkexec(void)
+void
+AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 {
-	char	   *av[10];
-	int			ac = 0;
-
-	av[ac++] = "postgres";
-	av[ac++] = "--forkavlauncher";
-	av[ac++] = NULL;			/* filled in by postmaster_forkexec */
-	av[ac] = NULL;
-
-	Assert(ac < lengthof(av));
-
-	return postmaster_forkexec(ac, av);
-}
-#endif
+	sigjmp_buf	local_sigjmp_buf;
 
-/*
- * Main entry point for autovacuum launcher process, to be called from the
- * postmaster.
- */
-int
-StartAutoVacLauncher(void)
-{
-	pid_t		AutoVacPID;
+	Assert(startup_data_len == 0);
 
-#ifdef EXEC_BACKEND
-	switch ((AutoVacPID = avlauncher_forkexec()))
-#else
-	switch ((AutoVacPID = fork_process()))
-#endif
+	/* Release postmaster's working memory context */
+	if (PostmasterContext)
 	{
-		case -1:
-			ereport(LOG,
-					(errmsg("could not fork autovacuum launcher process: %m")));
-			return 0;
-
-#ifndef EXEC_BACKEND
-		case 0:
-			/* in postmaster child ... */
-			InitPostmasterChild();
-
-			/* Close the postmaster's sockets */
-			ClosePostmasterPorts(false);
-
-			AutoVacLauncherMain(0, NULL);
-			break;
-#endif
-		default:
-			return (int) AutoVacPID;
+		MemoryContextDelete(PostmasterContext);
+		PostmasterContext = NULL;
 	}
 
-	/* shouldn't get here */
-	return 0;
-}
-
-/*
- * Main loop for the autovacuum launcher process.
- */
-NON_EXEC_STATIC void
-AutoVacLauncherMain(int argc, char *argv[])
-{
-	sigjmp_buf	local_sigjmp_buf;
-
 	MyBackendType = B_AUTOVAC_LAUNCHER;
 	init_ps_display(NULL);
 
@@ -1412,78 +1351,24 @@ avl_sigusr2_handler(SIGNAL_ARGS)
  *					  AUTOVACUUM WORKER CODE
  ********************************************************************/
 
-#ifdef EXEC_BACKEND
 /*
- * forkexec routines for the autovacuum worker.
- *
- * Format up the arglist, then fork and exec.
+ * Main entry point for autovacuum worker processes.
  */
-static pid_t
-avworker_forkexec(void)
+void
+AutoVacWorkerMain(char *startup_data, size_t startup_data_len)
 {
-	char	   *av[10];
-	int			ac = 0;
-
-	av[ac++] = "postgres";
-	av[ac++] = "--forkavworker";
-	av[ac++] = NULL;			/* filled in by postmaster_forkexec */
-	av[ac] = NULL;
-
-	Assert(ac < lengthof(av));
-
-	return postmaster_forkexec(ac, av);
-}
-#endif
+	sigjmp_buf	local_sigjmp_buf;
+	Oid			dbid;
 
-/*
- * Main entry point for autovacuum worker process.
- *
- * This code is heavily based on pgarch.c, q.v.
- */
-int
-StartAutoVacWorker(void)
-{
-	pid_t		worker_pid;
+	Assert(startup_data_len == 0);
 
-#ifdef EXEC_BACKEND
-	switch ((worker_pid = avworker_forkexec()))
-#else
-	switch ((worker_pid = fork_process()))
-#endif
+	/* Release postmaster's working memory context */
+	if (PostmasterContext)
 	{
-		case -1:
-			ereport(LOG,
-					(errmsg("could not fork autovacuum worker process: %m")));
-			return 0;
-
-#ifndef EXEC_BACKEND
-		case 0:
-			/* in postmaster child ... */
-			InitPostmasterChild();
-
-			/* Close the postmaster's sockets */
-			ClosePostmasterPorts(false);
-
-			AutoVacWorkerMain(0, NULL);
-			break;
-#endif
-		default:
-			return (int) worker_pid;
+		MemoryContextDelete(PostmasterContext);
+		PostmasterContext = NULL;
 	}
 
-	/* shouldn't get here */
-	return 0;
-}
-
-/*
- * AutoVacWorkerMain
- */
-NON_EXEC_STATIC void
-AutoVacWorkerMain(int argc, char *argv[])
-{
-	sigjmp_buf	local_sigjmp_buf;
-	Oid			dbid;
-
 	MyBackendType = B_AUTOVAC_WORKER;
 	init_ps_display(NULL);
 

src/backend/postmaster/auxprocess.c

@@ -27,26 +27,30 @@
 #include "storage/ipc.h"
 #include "storage/proc.h"
 #include "storage/procsignal.h"
+#include "utils/memutils.h"
 #include "utils/ps_status.h"
 
 
 static void ShutdownAuxiliaryProcess(int code, Datum arg);
 
 
 /*
- *	 AuxiliaryProcessMain
+ *	 AuxiliaryProcessMainCommon
  *
- *	 The main entry point for auxiliary processes, such as the bgwriter,
- *	 walwriter, walreceiver, bootstrapper and the shared memory checker code.
- *
- *	 This code is here just because of historical reasons.
+ *	 Common initialization code for auxiliary processes, such as the bgwriter,
+ *	 walwriter, walreceiver, and the startup process.
  */
 void
-AuxiliaryProcessMain(BackendType auxtype)
+AuxiliaryProcessMainCommon(void)
 {
 	Assert(IsUnderPostmaster);
 
-	MyBackendType = auxtype;
+	/* Release postmaster's working memory context */
+	if (PostmasterContext)
+	{
+		MemoryContextDelete(PostmasterContext);
+		PostmasterContext = NULL;
+	}
 
 	init_ps_display(NULL);
 
@@ -84,41 +88,6 @@ AuxiliaryProcessMain(BackendType auxtype)
 	before_shmem_exit(ShutdownAuxiliaryProcess, 0);
 
 	SetProcessingMode(NormalProcessing);
-
-	switch (MyBackendType)
-	{
-		case B_STARTUP:
-			StartupProcessMain();
-			proc_exit(1);
-
-		case B_ARCHIVER:
-			PgArchiverMain();
-			proc_exit(1);
-
-		case B_BG_WRITER:
-			BackgroundWriterMain();
-			proc_exit(1);
-
-		case B_CHECKPOINTER:
-			CheckpointerMain();
-			proc_exit(1);
-
-		case B_WAL_WRITER:
-			WalWriterMain();
-			proc_exit(1);
-
-		case B_WAL_RECEIVER:
-			WalReceiverMain();
-			proc_exit(1);
-
-		case B_WAL_SUMMARIZER:
-			WalSummarizerMain();
-			proc_exit(1);
-
-		default:
-			elog(PANIC, "unrecognized process type: %d", (int) MyBackendType);
-			proc_exit(1);
-	}
 }
 
 /*

src/backend/postmaster/bgworker.c

@@ -720,15 +720,29 @@ bgworker_die(SIGNAL_ARGS)
  * Main entry point for background worker processes.
  */
 void
-BackgroundWorkerMain(void)
+BackgroundWorkerMain(char *startup_data, size_t startup_data_len)
 {
 	sigjmp_buf	local_sigjmp_buf;
-	BackgroundWorker *worker = MyBgworkerEntry;
+	BackgroundWorker *worker;
 	bgworker_main_type entrypt;
 
-	if (worker == NULL)
+	if (startup_data == NULL)
 		elog(FATAL, "unable to find bgworker entry");
+	Assert(startup_data_len == sizeof(BackgroundWorker));
+	worker = MemoryContextAlloc(TopMemoryContext, sizeof(BackgroundWorker));
+	memcpy(worker, startup_data, sizeof(BackgroundWorker));
 
+	/*
+	 * Now that we're done reading the startup data, release postmaster's
+	 * working memory context.
+	 */
+	if (PostmasterContext)
+	{
+		MemoryContextDelete(PostmasterContext);
+		PostmasterContext = NULL;
+	}
+
+	MyBgworkerEntry = worker;
 	MyBackendType = B_BG_WORKER;
 	init_ps_display(worker->bgw_name);
 

src/backend/postmaster/bgwriter.c

@@ -35,6 +35,7 @@
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "postmaster/auxprocess.h"
 #include "postmaster/bgwriter.h"
 #include "postmaster/interrupt.h"
 #include "storage/buf_internals.h"
@@ -83,13 +84,18 @@ static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
  * basic execution environment, but not enabled signals yet.
  */
 void
-BackgroundWriterMain(void)
+BackgroundWriterMain(char *startup_data, size_t startup_data_len)
 {
 	sigjmp_buf	local_sigjmp_buf;
 	MemoryContext bgwriter_context;
 	bool		prev_hibernate;
 	WritebackContext wb_context;
 
+	Assert(startup_data_len == 0);
+
+	MyBackendType = B_BG_WRITER;
+	AuxiliaryProcessMainCommon();
+
 	/*
 	 * Properly accept or ignore signals that might be sent to us.
 	 */

src/backend/postmaster/checkpointer.c

@@ -42,6 +42,7 @@
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "postmaster/auxprocess.h"
 #include "postmaster/bgwriter.h"
 #include "postmaster/interrupt.h"
 #include "replication/syncrep.h"
@@ -169,11 +170,16 @@ static void ReqCheckpointHandler(SIGNAL_ARGS);
  * basic execution environment, but not enabled signals yet.
  */
 void
-CheckpointerMain(void)
+CheckpointerMain(char *startup_data, size_t startup_data_len)
 {
 	sigjmp_buf	local_sigjmp_buf;
 	MemoryContext checkpointer_context;
 
+	Assert(startup_data_len == 0);
+
+	MyBackendType = B_CHECKPOINTER;
+	AuxiliaryProcessMainCommon();
+
 	CheckpointerShmem->checkpointer_pid = MyProcPid;
 
 	/*