Remove duplicate InitPostmasterChild() call while starting a bgworker.

This is apparently harmless on Windows, but on Unix it results in an
assertion failure.  We'd not noticed because this code doesn't get
used on Unix unless you build with -DEXEC_BACKEND.  Bug was evidently
introduced by sloppy refactoring in commit 31c453165.

Thomas Munro

Discussion: <CAEepm=1VOnbVx4wsgQFvj94hu9jVt2nVabCr7QiooUSvPJXkgQ@mail.gmail.com>

src/backend/postmaster/postmaster.c

@@ -4806,8 +4806,6 @@ SubPostmasterMain(int argc, char *argv[])
 		/* do this as early as possible; in particular, before InitProcess() */
 		IsBackgroundWorker = true;
 
-		InitPostmasterChild();
-
 		/* Close the postmaster's sockets */
 		ClosePostmasterPorts(false);
 
@@ -4820,8 +4818,10 @@ SubPostmasterMain(int argc, char *argv[])
 		/* Attach process to shared data structures */
 		CreateSharedMemoryAndSemaphores(false, 0);
 
+		/* Fetch MyBgworkerEntry from shared memory */
 		shmem_slot = atoi(argv[1] + 15);
 		MyBgworkerEntry = BackgroundWorkerEntry(shmem_slot);
+
 		StartBackgroundWorker();
 	}
 	if (strcmp(argv[1], "--forkarch") == 0)