Pass MyPMChildSlot as an explicit argument to child process

All the other global variables passed from postmaster to child have
the same value in all the processes, while MyPMChildSlot is more like
a parameter to each child process.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

src/backend/postmaster/launch_backend.c

@@ -88,7 +88,6 @@ typedef int InheritableSocket;
 typedef struct
 {
 	char		DataDir[MAXPGPATH];
-	int			MyPMChildSlot;
 #ifndef WIN32
 	unsigned long UsedShmemSegID;
 #else
@@ -130,6 +129,8 @@ typedef struct
 	char		my_exec_path[MAXPGPATH];
 	char		pkglib_path[MAXPGPATH];
 
+	int			MyPMChildSlot;
+
 	/*
 	 * These are only used by backend processes, but are here because passing
 	 * a socket needs some special handling on Windows. 'client_sock' is an
@@ -151,13 +152,16 @@ typedef struct
 static void read_backend_variables(char *id, char **startup_data, size_t *startup_data_len);
 static void restore_backend_variables(BackendParameters *param);
 
-static bool save_backend_variables(BackendParameters *param, ClientSocket *client_sock,
+static bool save_backend_variables(BackendParameters *param, int child_slot,
+								   ClientSocket *client_sock,
 #ifdef WIN32
 								   HANDLE childProcess, pid_t childPid,
 #endif
 								   char *startup_data, size_t startup_data_len);
 
-static pid_t internal_forkexec(const char *child_kind, char *startup_data, size_t startup_data_len, ClientSocket *client_sock);
+static pid_t internal_forkexec(const char *child_kind, int child_slot,
+							   char *startup_data, size_t startup_data_len,
+							   ClientSocket *client_sock);
 
 #endif							/* EXEC_BACKEND */
 
@@ -215,11 +219,12 @@ PostmasterChildName(BackendType child_type)
  * appropriate, and fds and other resources that we've inherited from
  * postmaster that are not needed in a child process have been closed.
  *
- * 'startup_data' is an optional contiguous chunk of data that is passed to
- * the child process.
+ * 'child_slot' is the PMChildFlags array index reserved for the child
+ * process.  'startup_data' is an optional contiguous chunk of data that is
+ * passed to the child process.
  */
 pid_t
-postmaster_child_launch(BackendType child_type,
+postmaster_child_launch(BackendType child_type, int child_slot,
 						char *startup_data, size_t startup_data_len,
 						ClientSocket *client_sock)
 {
@@ -228,7 +233,7 @@ postmaster_child_launch(BackendType child_type,
 	Assert(IsPostmasterEnvironment && !IsUnderPostmaster);
 
 #ifdef EXEC_BACKEND
-	pid = internal_forkexec(child_process_kinds[child_type].name,
+	pid = internal_forkexec(child_process_kinds[child_type].name, child_slot,
 							startup_data, startup_data_len, client_sock);
 	/* the child process will arrive in SubPostmasterMain */
 #else							/* !EXEC_BACKEND */
@@ -256,6 +261,7 @@ postmaster_child_launch(BackendType child_type,
 		 */
 		MemoryContextSwitchTo(TopMemoryContext);
 
+		MyPMChildSlot = child_slot;
 		if (client_sock)
 		{
 			MyClientSocket = palloc(sizeof(ClientSocket));
@@ -282,7 +288,8 @@ postmaster_child_launch(BackendType child_type,
  * - fork():s, and then exec():s the child process
  */
 static pid_t
-internal_forkexec(const char *child_kind, char *startup_data, size_t startup_data_len, ClientSocket *client_sock)
+internal_forkexec(const char *child_kind, int child_slot,
+				  char *startup_data, size_t startup_data_len, ClientSocket *client_sock)
 {
 	static unsigned long tmpBackendFileNum = 0;
 	pid_t		pid;
@@ -302,7 +309,7 @@ internal_forkexec(const char *child_kind, char *startup_data, size_t startup_dat
 	 */
 	paramsz = SizeOfBackendParameters(startup_data_len);
 	param = palloc0(paramsz);
-	if (!save_backend_variables(param, client_sock, startup_data, startup_data_len))
+	if (!save_backend_variables(param, child_slot, client_sock, startup_data, startup_data_len))
 	{
 		pfree(param);
 		return -1;				/* log made by save_backend_variables */
@@ -391,7 +398,8 @@ internal_forkexec(const char *child_kind, char *startup_data, size_t startup_dat
  *	 file is complete.
  */
 static pid_t
-internal_forkexec(const char *child_kind, char *startup_data, size_t startup_data_len, ClientSocket *client_sock)
+internal_forkexec(const char *child_kind, int child_slot,
+				  char *startup_data, size_t startup_data_len, ClientSocket *client_sock)
 {
 	int			retry_count = 0;
 	STARTUPINFO si;
@@ -472,7 +480,9 @@ retry:
 		return -1;
 	}
 
-	if (!save_backend_variables(param, client_sock, pi.hProcess, pi.dwProcessId, startup_data, startup_data_len))
+	if (!save_backend_variables(param, child_slot, client_sock,
+								pi.hProcess, pi.dwProcessId,
+								startup_data, startup_data_len))
 	{
 		/*
 		 * log made by save_backend_variables, but we have to clean up the
@@ -684,7 +694,8 @@ static void read_inheritable_socket(SOCKET *dest, InheritableSocket *src);
 
 /* Save critical backend variables into the BackendParameters struct */
 static bool
-save_backend_variables(BackendParameters *param, ClientSocket *client_sock,
+save_backend_variables(BackendParameters *param,
+					   int child_slot, ClientSocket *client_sock,
 #ifdef WIN32
 					   HANDLE childProcess, pid_t childPid,
 #endif
@@ -701,7 +712,7 @@ save_backend_variables(BackendParameters *param, ClientSocket *client_sock,
 
 	strlcpy(param->DataDir, DataDir, MAXPGPATH);
 
-	param->MyPMChildSlot = MyPMChildSlot;
+	param->MyPMChildSlot = child_slot;
 
 #ifdef WIN32
 	param->ShmemProtectiveRegion = ShmemProtectiveRegion;

src/backend/postmaster/postmaster.c

@@ -3374,8 +3374,7 @@ BackendStartup(ClientSocket *client_sock)
 	/* Hasn't asked to be notified about any bgworkers yet */
 	bn->bgworker_notify = false;
 
-	MyPMChildSlot = bn->child_slot;
-	pid = postmaster_child_launch(bn->bkend_type,
+	pid = postmaster_child_launch(bn->bkend_type, bn->child_slot,
 								  (char *) &startup_data, sizeof(startup_data),
 								  client_sock);
 	if (pid < 0)
@@ -3700,8 +3699,7 @@ StartChildProcess(BackendType type)
 		return NULL;
 	}
 
-	MyPMChildSlot = pmchild->child_slot;
-	pid = postmaster_child_launch(type, NULL, 0, NULL);
+	pid = postmaster_child_launch(type, pmchild->child_slot, NULL, 0, NULL);
 	if (pid < 0)
 	{
 		/* in parent, fork failed */
@@ -3878,8 +3876,8 @@ StartBackgroundWorker(RegisteredBgWorker *rw)
 			(errmsg_internal("starting background worker process \"%s\"",
 							 rw->rw_worker.bgw_name)));
 
-	MyPMChildSlot = bn->child_slot;
-	worker_pid = postmaster_child_launch(B_BG_WORKER, (char *) &rw->rw_worker, sizeof(BackgroundWorker), NULL);
+	worker_pid = postmaster_child_launch(B_BG_WORKER, bn->child_slot,
+										 (char *) &rw->rw_worker, sizeof(BackgroundWorker), NULL);
 	if (worker_pid == -1)
 	{
 		/* in postmaster, fork failed ... */

src/backend/postmaster/syslogger.c

@@ -694,14 +694,15 @@ SysLogger_Start(int child_slot)
 		pfree(filename);
 	}
 
-	MyPMChildSlot = child_slot;
 #ifdef EXEC_BACKEND
 	startup_data.syslogFile = syslogger_fdget(syslogFile);
 	startup_data.csvlogFile = syslogger_fdget(csvlogFile);
 	startup_data.jsonlogFile = syslogger_fdget(jsonlogFile);
-	sysloggerPid = postmaster_child_launch(B_LOGGER, (char *) &startup_data, sizeof(startup_data), NULL);
+	sysloggerPid = postmaster_child_launch(B_LOGGER, child_slot,
+										   (char *) &startup_data, sizeof(startup_data), NULL);
 #else
-	sysloggerPid = postmaster_child_launch(B_LOGGER, NULL, 0, NULL);
+	sysloggerPid = postmaster_child_launch(B_LOGGER, child_slot,
+										   NULL, 0, NULL);
 #endif							/* EXEC_BACKEND */
 
 	if (sysloggerPid == -1)

src/include/postmaster/postmaster.h

@@ -108,6 +108,7 @@ extern PGDLLIMPORT struct ClientSocket *MyClientSocket;
 
 /* prototypes for functions in launch_backend.c */
 extern pid_t postmaster_child_launch(BackendType child_type,
+									 int child_slot,
 									 char *startup_data,
 									 size_t startup_data_len,
 									 struct ClientSocket *client_sock);