Assign "backend" type earlier during process start-up

Instead of assigning the backend type in the Main function of each
postmaster child, do it right after fork(), by which time it is already
known by postmaster_child_launch().  This reduces the time frame during
which MyBackendType is incorrect.

Before this commit, ProcessStartupPacket would overwrite MyBackendType
to B_BACKEND for dead-end backends, which is quite dubious.  Stop that.

We may now see MyBackendType == B_BG_WORKER before setting up
MyBgworkerEntry.  As far as I can see this is only a problem if we try
to log a message and %b is in log_line_prefix, so we now have a constant
string to cover that case.  Previously, it would print "unrecognized",
which seems strictly worse.

Author: Euler Taveira <euler@eulerto.com>
Discussion: https://postgr.es/m/e85c6671-1600-4112-8887-f97a8a5d07b2@app.fastmail.com

src/backend/postmaster/autovacuum.c

@@ -385,7 +385,6 @@ AutoVacLauncherMain(const void *startup_data, size_t startup_data_len)
 		PostmasterContext = NULL;
 	}
 
-	MyBackendType = B_AUTOVAC_LAUNCHER;
 	init_ps_display(NULL);
 
 	ereport(DEBUG1,
@@ -1398,7 +1397,6 @@ AutoVacWorkerMain(const void *startup_data, size_t startup_data_len)
 		PostmasterContext = NULL;
 	}
 
-	MyBackendType = B_AUTOVAC_WORKER;
 	init_ps_display(NULL);
 
 	Assert(GetProcessingMode() == InitProcessing);

src/backend/postmaster/bgworker.c

@@ -759,7 +759,6 @@ BackgroundWorkerMain(const void *startup_data, size_t startup_data_len)
 	}
 
 	MyBgworkerEntry = worker;
-	MyBackendType = B_BG_WORKER;
 	init_ps_display(worker->bgw_name);
 
 	Assert(GetProcessingMode() == InitProcessing);

src/backend/postmaster/bgwriter.c

@@ -94,7 +94,6 @@ BackgroundWriterMain(const void *startup_data, size_t startup_data_len)
 
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_BG_WRITER;
 	AuxiliaryProcessMainCommon();
 
 	/*

src/backend/postmaster/checkpointer.c

@@ -199,7 +199,6 @@ CheckpointerMain(const void *startup_data, size_t startup_data_len)
 
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_CHECKPOINTER;
 	AuxiliaryProcessMainCommon();
 
 	CheckpointerShmem->checkpointer_pid = MyProcPid;

src/backend/postmaster/launch_backend.c

@@ -223,6 +223,8 @@ postmaster_child_launch(BackendType child_type, int child_slot,
 	pid = fork_process();
 	if (pid == 0)				/* child */
 	{
+		MyBackendType = child_type;
+
 		/* Capture and transfer timings that may be needed for logging */
 		if (IsExternalConnectionBackend(child_type))
 		{
@@ -607,6 +609,7 @@ SubPostmasterMain(int argc, char *argv[])
 	child_type = (BackendType) atoi(child_kind);
 	if (child_type <= B_INVALID || child_type > BACKEND_NUM_TYPES - 1)
 		elog(ERROR, "unknown child kind %s", child_kind);
+	MyBackendType = child_type;
 
 	/* Read in the variables file */
 	read_backend_variables(argv[2], &startup_data, &startup_data_len);

src/backend/postmaster/pgarch.c

@@ -222,7 +222,6 @@ PgArchiverMain(const void *startup_data, size_t startup_data_len)
 {
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_ARCHIVER;
 	AuxiliaryProcessMainCommon();
 
 	/*

src/backend/postmaster/startup.c

@@ -217,7 +217,6 @@ StartupProcessMain(const void *startup_data, size_t startup_data_len)
 {
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_STARTUP;
 	AuxiliaryProcessMainCommon();
 
 	/* Arrange to clean up at startup process exit */

src/backend/postmaster/syslogger.c

@@ -206,7 +206,6 @@ SysLoggerMain(const void *startup_data, size_t startup_data_len)
 
 	now = MyStartTime;
 
-	MyBackendType = B_LOGGER;
 	init_ps_display(NULL);
 
 	/*

src/backend/postmaster/walsummarizer.c

@@ -234,7 +234,6 @@ WalSummarizerMain(const void *startup_data, size_t startup_data_len)
 
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_WAL_SUMMARIZER;
 	AuxiliaryProcessMainCommon();
 
 	ereport(DEBUG1,

src/backend/postmaster/walwriter.c

@@ -94,7 +94,6 @@ WalWriterMain(const void *startup_data, size_t startup_data_len)
 
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_WAL_WRITER;
 	AuxiliaryProcessMainCommon();
 
 	/*

src/backend/replication/logical/slotsync.c

@@ -1541,8 +1541,6 @@ ReplSlotSyncWorkerMain(const void *startup_data, size_t startup_data_len)
 
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_SLOTSYNC_WORKER;
-
 	init_ps_display(NULL);
 
 	Assert(GetProcessingMode() == InitProcessing);

src/backend/replication/walreceiver.c

@@ -169,7 +169,6 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
 
 	Assert(startup_data_len == 0);
 
-	MyBackendType = B_WAL_RECEIVER;
 	AuxiliaryProcessMainCommon();
 
 	/*

src/backend/storage/aio/method_worker.c

@@ -390,7 +390,6 @@ IoWorkerMain(const void *startup_data, size_t startup_data_len)
 	volatile int error_errno = 0;
 	char		cmd[128];
 
-	MyBackendType = B_IO_WORKER;
 	AuxiliaryProcessMainCommon();
 
 	pqsignal(SIGHUP, SignalHandlerForConfigReload);

src/backend/tcop/backend_startup.c

@@ -846,10 +846,9 @@ ProcessStartupPacket(Port *port, bool ssl_done, bool gss_done)
 	if (strlen(port->user_name) >= NAMEDATALEN)
 		port->user_name[NAMEDATALEN - 1] = '\0';
 
+	Assert(MyBackendType == B_BACKEND || MyBackendType == B_DEAD_END_BACKEND);
 	if (am_walsender)
 		MyBackendType = B_WAL_SENDER;
-	else
-		MyBackendType = B_BACKEND;
 
 	/*
 	 * Normal walsender backends, e.g. for streaming replication, are not

src/backend/utils/error/elog.c

@@ -2779,7 +2779,12 @@ get_backend_type_for_log(void)
 	if (MyProcPid == PostmasterPid)
 		backend_type_str = "postmaster";
 	else if (MyBackendType == B_BG_WORKER)
-		backend_type_str = MyBgworkerEntry->bgw_type;
+	{
+		if (MyBgworkerEntry)
+			backend_type_str = MyBgworkerEntry->bgw_type;
+		else
+			backend_type_str = "early bgworker";
+	}
 	else
 		backend_type_str = GetBackendTypeDesc(MyBackendType);