Fix possible crashes due to using elog/ereport too early in startup.

Per reports from Andres Freund and Luke Campbell, a server failure during
set_pglocale_pgservice results in a segfault rather than a useful error
message, because the infrastructure needed to use ereport hasn't been
initialized; specifically, MemoryContextInit hasn't been called.
One known cause of this is starting the server in a directory it
doesn't have permission to read.

We could try to prevent set_pglocale_pgservice from using anything that
depends on palloc or elog, but that would be messy, and the odds of future
breakage seem high.  Moreover there are other things being called in main.c
that look likely to use palloc or elog too --- perhaps those things
shouldn't be there, but they are there today.  The best solution seems to
be to move the call of MemoryContextInit to very early in the backend's
real main() function.  I've verified that an elog or ereport occurring
immediately after that is now capable of sending something useful to
stderr.

I also added code to elog.c to print something intelligible rather than
just crashing if MemoryContextInit hasn't created the ErrorContext.
This could happen if MemoryContextInit itself fails (due to malloc
failure), and provides some future-proofing against someone trying to
sneak in new code even earlier in server startup.

Back-patch to all supported branches.  Since we've only heard reports of
this type of failure recently, it may be that some recent change has made
it more likely to see a crash of this kind; but it sure looks like it's
broken all the way back.

src/backend/bootstrap/bootstrap.c

@@ -202,14 +202,6 @@ AuxiliaryProcessMain(int argc, char *argv[])
 
 	MyStartTime = time(NULL);
 
-	/*
-	 * Fire up essential subsystems: error and memory management
-	 *
-	 * If we are running under the postmaster, this is done already.
-	 */
-	if (!IsUnderPostmaster)
-		MemoryContextInit();
-
 	/* Compute paths, if we didn't inherit them from postmaster */
 	if (my_exec_path[0] == '\0')
 	{

src/backend/main/main.c

@@ -39,6 +39,7 @@
 #include "postmaster/postmaster.h"
 #include "tcop/tcopprot.h"
 #include "utils/help_config.h"
+#include "utils/memutils.h"
 #include "utils/pg_locale.h"
 #include "utils/ps_status.h"
 
@@ -85,6 +86,15 @@ main(int argc, char *argv[])
 	pgwin32_install_crashdump_handler();
 #endif
 
+	/*
+	 * Fire up essential subsystems: error and memory management
+	 *
+	 * Code after this point is allowed to use elog/ereport, though
+	 * localization of messages may not work right away, and messages won't go
+	 * anywhere but stderr until GUC settings get loaded.
+	 */
+	MemoryContextInit();
+
 	/*
 	 * Set up locale information from environment.	Note that LC_CTYPE and
 	 * LC_COLLATE will be overridden later from pg_control if we are in an

src/backend/postmaster/postmaster.c

@@ -560,11 +560,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	umask(S_IRWXG | S_IRWXO);
 
-	/*
-	 * Fire up essential subsystems: memory management
-	 */
-	MemoryContextInit();
-
 	/*
 	 * By default, palloc() requests in the postmaster will be allocated in
 	 * the PostmasterContext, which is space that can be recycled by backends.
@@ -4468,7 +4463,6 @@ SubPostmasterMain(int argc, char *argv[])
 	whereToSendOutput = DestNone;
 
 	/* Setup essential subsystems (to ensure elog() behaves sanely) */
-	MemoryContextInit();
 	InitializeGUCOptions();
 
 	/* Read in the variables file */

src/backend/tcop/postgres.c

@@ -3548,14 +3548,6 @@ PostgresMain(int argc, char *argv[],
 		MyStartTime = time(NULL);
 	}
 
-	/*
-	 * Fire up essential subsystems: error and memory management
-	 *
-	 * If we are running under the postmaster, this is done already.
-	 */
-	if (!IsUnderPostmaster)
-		MemoryContextInit();
-
 	SetProcessingMode(InitProcessing);
 
 	/* Compute paths, if we didn't inherit them from postmaster */

src/backend/utils/error/elog.c

@@ -310,6 +310,18 @@ errstart(int elevel, const char *filename, int lineno,
 	if (elevel < ERROR && !output_to_server && !output_to_client)
 		return false;
 
+	/*
+	 * We need to do some actual work.  Make sure that memory context
+	 * initialization has finished, else we can't do anything useful.
+	 */
+	if (ErrorContext == NULL)
+	{
+		/* Ooops, hard crash time; very little we can do safely here */
+		write_stderr("error occurred at %s:%d before error message processing is available\n",
+					 filename ? filename : "(unknown file)", lineno);
+		exit(2);
+	}
+
 	/*
 	 * Okay, crank up a stack entry to store the info in.
 	 */
@@ -1238,6 +1250,15 @@ elog_start(const char *filename, int lineno, const char *funcname)
 {
 	ErrorData  *edata;
 
+	/* Make sure that memory context initialization has finished */
+	if (ErrorContext == NULL)
+	{
+		/* Ooops, hard crash time; very little we can do safely here */
+		write_stderr("error occurred at %s:%d before error message processing is available\n",
+					 filename ? filename : "(unknown file)", lineno);
+		exit(2);
+	}
+
 	if (++errordata_stack_depth >= ERRORDATA_STACK_SIZE)
 	{
 		/*

src/backend/utils/mmgr/mcxt.c

@@ -72,7 +72,8 @@ static void MemoryContextStatsInternal(MemoryContext context, int level);
  * In normal multi-backend operation, this is called once during
  * postmaster startup, and not at all by individual backend startup
  * (since the backends inherit an already-initialized context subsystem
- * by virtue of being forked off the postmaster).
+ * by virtue of being forked off the postmaster).  But in an EXEC_BACKEND
+ * build, each process must do this for itself.
  *
  * In a standalone backend this must be called during backend startup.
  */
@@ -106,6 +107,9 @@ MemoryContextInit(void)
 	 * where retained memory in a context is *essential* --- we want to be
 	 * sure ErrorContext still has some memory even if we've run out
 	 * elsewhere!
+	 *
+	 * This should be the last step in this function, as elog.c assumes memory
+	 * management works once ErrorContext is non-null.
 	 */
 	ErrorContext = AllocSetContextCreate(TopMemoryContext,
 										 "ErrorContext",