Remember the source GucContext for each GUC parameter.

We used to just remember the GucSource, but saving GucContext too provides
a little more information --- notably, whether a SET was done by a
superuser or regular user.  This allows us to rip out the fairly dodgy code
that define_custom_variable used to use to try to infer the context to
re-install a pre-existing setting with.  In particular, it now works for
a superuser to SET a extension's SUSET custom variable before loading the
associated extension, because GUC can remember whether the SET was done as
a superuser or not.  The plperl regression tests contain an example where
this is useful.

src/backend/utils/misc/guc-file.l

@@ -296,11 +296,7 @@ ProcessConfigFile(GucContext context)
 								  GUC_ACTION_SET, true);
 		if (scres > 0)
 		{
-			/* variable was updated, so remember the source location */
-			set_config_sourcefile(item->name, item->filename,
-								  item->sourceline);
-
-			/* and log the change if appropriate */
+			/* variable was updated, so log the change if appropriate */
 			if (pre_value)
 			{
 				const char *post_value = GetConfigOption(item->name, true, false);
@@ -315,7 +311,17 @@ ProcessConfigFile(GucContext context)
 		}
 		else if (scres == 0)
 			error = true;
-		/* else no error but variable was not changed, do nothing */
+		/* else no error but variable's active value was not changed */
+
+		/*
+		 * We should update source location unless there was an error, since
+		 * even if the active value didn't change, the reset value might have.
+		 * (In the postmaster, there won't be a difference, but it does matter
+		 * in backends.)
+		 */
+		if (scres != 0)
+			set_config_sourcefile(item->name, item->filename,
+								  item->sourceline);
 
 		if (pre_value)
 			pfree(pre_value);

src/include/utils/guc_tables.h

@@ -118,9 +118,11 @@ typedef struct guc_stack
 	int			nest_level;		/* nesting depth at which we made entry */
 	GucStackState state;		/* see enum above */
 	GucSource	source;			/* source of the prior value */
+	/* masked value's source must be PGC_S_SESSION, so no need to store it */
+	GucContext	scontext;		/* context that set the prior value */
+	GucContext	masked_scontext; /* context that set the masked value */
 	config_var_value prior;		/* previous value of variable */
 	config_var_value masked;	/* SET value in a GUC_SET_LOCAL entry */
-	/* masked value's source must be PGC_S_SESSION, so no need to store it */
 } GucStack;
 
 /*
@@ -143,21 +145,21 @@ struct config_generic
 	enum config_group group;	/* to help organize variables by function */
 	const char *short_desc;		/* short desc. of this variable's purpose */
 	const char *long_desc;		/* long desc. of this variable's purpose */
-	int			flags;			/* flag bits, see below */
+	int			flags;			/* flag bits, see guc.h */
 	/* variable fields, initialized at runtime: */
 	enum config_type vartype;	/* type of variable (set only at startup) */
 	int			status;			/* status bits, see below */
-	GucSource	reset_source;	/* source of the reset_value */
 	GucSource	source;			/* source of the current actual value */
+	GucSource	reset_source;	/* source of the reset_value */
+	GucContext	scontext;		/* context that set the current value */
+	GucContext	reset_scontext;	/* context that set the reset value */
 	GucStack   *stack;			/* stacked prior values */
 	void	   *extra;			/* "extra" pointer for current actual value */
 	char	   *sourcefile;		/* file current setting is from (NULL if not
-								 * file) */
+								 * set in config file) */
 	int			sourceline;		/* line in source file */
 };
 
-/* bit values in flags field are defined in guc.h */
-
 /* bit values in status field */
 #define GUC_IS_IN_FILE		0x0001		/* found it in config file */
 /*

src/pl/plperl/expected/plperl_init.out

 -- test plperl.on_plperl_init errors are fatal
--- Must load plperl before we can set on_plperl_init
+-- This test tests setting on_plperl_init after loading plperl
 LOAD 'plperl';
 SET SESSION plperl.on_plperl_init = ' system("/nonesuch") ';
 SHOW plperl.on_plperl_init;

src/pl/plperl/expected/plperl_shared.out

 -- test plperl.on_plperl_init via the shared hash
 -- (must be done before plperl is first used)
--- Must load plperl before we can set on_plperl_init
-LOAD 'plperl';
+-- This test tests setting on_plperl_init before loading plperl
 -- testing on_plperl_init gets run, and that it can alter %_SHARED
 SET plperl.on_plperl_init = '$_SHARED{on_init} = 42';
 -- test the shared hash

src/pl/plperl/expected/plperlu.out

 -- Use ONLY plperlu tests here. For plperl/plerlu combined tests
 -- see plperl_plperlu.sql
--- Must load plperl before we can set on_plperlu_init
+-- This test tests setting on_plperlu_init after loading plperl
 LOAD 'plperl';
 -- Test plperl.on_plperlu_init gets run
 SET plperl.on_plperlu_init = '$_SHARED{init} = 42';

src/pl/plperl/sql/plperl_init.sql

 -- test plperl.on_plperl_init errors are fatal
 
--- Must load plperl before we can set on_plperl_init
+-- This test tests setting on_plperl_init after loading plperl
 LOAD 'plperl';
 
 SET SESSION plperl.on_plperl_init = ' system("/nonesuch") ';

src/pl/plperl/sql/plperl_shared.sql

 -- test plperl.on_plperl_init via the shared hash
 -- (must be done before plperl is first used)
 
--- Must load plperl before we can set on_plperl_init
-LOAD 'plperl';
+-- This test tests setting on_plperl_init before loading plperl
 
 -- testing on_plperl_init gets run, and that it can alter %_SHARED
 SET plperl.on_plperl_init = '$_SHARED{on_init} = 42';

src/pl/plperl/sql/plperlu.sql

 -- Use ONLY plperlu tests here. For plperl/plerlu combined tests
 -- see plperl_plperlu.sql
 
--- Must load plperl before we can set on_plperlu_init
+-- This test tests setting on_plperlu_init after loading plperl
 LOAD 'plperl';
 
 -- Test plperl.on_plperlu_init gets run