Fix IsUnderPostmaster/EXEC_BACKEND confusion

src/backend/bootstrap/bootstrap.c

@@ -359,6 +359,10 @@ AuxiliaryProcessMain(int argc, char *argv[])
 	SetProcessingMode(BootstrapProcessing);
 	IgnoreSystemIndexes = true;
 
+	/* Initialize MaxBackends (if under postmaster, was done already) */
+	if (!IsUnderPostmaster)
+		InitializeMaxBackends();
+
 	BaseInit();
 
 	/*

src/backend/tcop/postgres.c

@@ -3654,10 +3654,8 @@ PostgresMain(int argc, char *argv[], const char *username)
 		 */
 		CreateDataDirLockFile(false);
 
-		/* In EXEC_BACKEND, this was set via BackendParameters */
-#ifndef EXEC_BACKEND
+		/* Initialize MaxBackends (if under postmaster, was done already) */
 		InitializeMaxBackends();
-#endif
 	}
 
 	/* Early initialization */

src/backend/utils/init/postinit.c

@@ -427,6 +427,11 @@ pg_split_opts(char **argv, int *argcp, char *optstr)
  * This must be called after modules have had the chance to register background
  * workers in shared_preload_libraries, and before shared memory size is
  * determined.
+ *
+ * Note that in EXEC_BACKEND environment, the value is passed down from
+ * postmaster to subprocesses via BackendParameters in SubPostmasterMain; only
+ * postmaster itself and processes not under postmaster control should call
+ * this.
  */
 void
 InitializeMaxBackends(void)