Fix processing of PGC_BACKEND GUC parameters on Windows.

EXEC_BACKEND builds (i.e., Windows) failed to absorb values of PGC_BACKEND
parameters if they'd been changed post-startup via the config file.  This
for example prevented log_connections from working if it were turned on
post-startup.  The mechanism for handling this case has always been a bit
of a kluge, and it wasn't revisited when we implemented EXEC_BACKEND.
While in a normal forking environment new backends will inherit the
postmaster's value of such settings, EXEC_BACKEND backends have to read
the settings from the CONFIG_EXEC_PARAMS file, and they were mistakenly
rejecting them.  So this case has always been broken in the Windows port;
so back-patch to all supported branches.

Amit Kapila

src/backend/utils/misc/guc.c

@@ -5669,9 +5669,23 @@ set_config_option(const char *name, const char *value,
 				 * ignore it in existing backends.	This is a tad klugy, but
 				 * necessary because we don't re-read the config file during
 				 * backend start.
+				 *
+				 * In EXEC_BACKEND builds, this works differently: we load all
+				 * nondefault settings from the CONFIG_EXEC_PARAMS file during
+				 * backend start.  In that case we must accept PGC_SIGHUP
+				 * settings, so as to have the same value as if we'd forked
+				 * from the postmaster.  We detect this situation by checking
+				 * IsInitProcessingMode, which is a bit ugly, but it doesn't
+				 * seem worth passing down an explicit flag saying we're doing
+				 * read_nondefault_variables().
 				 */
+#ifdef EXEC_BACKEND
+				if (IsUnderPostmaster && !IsInitProcessingMode())
+					return -1;
+#else
 				if (IsUnderPostmaster)
 					return -1;
+#endif
 			}
 			else if (context != PGC_POSTMASTER && context != PGC_BACKEND &&
 					 source != PGC_S_CLIENT)
@@ -8310,6 +8324,12 @@ read_nondefault_variables(void)
 	GucSource	varsource;
 	GucContext	varscontext;
 
+	/*
+	 * Assert that PGC_BACKEND case in set_config_option() will do the right
+	 * thing.
+	 */
+	Assert(IsInitProcessingMode());
+
 	/*
 	 * Open file
 	 */