When restoring GUCs in parallel workers, show an error context.

Otherwise it can be hard to see where an error is coming from, when
the parallel worker sets all the GUCs that it received from the
leader.  Bug #15726.  Back-patch to 9.5, where RestoreGUCState()
appeared.

Reported-by: Tiago Anastacio
Reviewed-by: Daniel Gustafsson, Tom Lane
Discussion: https://postgr.es/m/15726-6d67e4fa14f027b3%40postgresql.org

src/backend/utils/misc/guc.c

@@ -10209,6 +10209,21 @@ read_gucstate_binary(char **srcptr, char *srcend, void *dest, Size size)
 	*srcptr += size;
 }
 
+/*
+ * Callback used to add a context message when reporting errors that occur
+ * while trying to restore GUCs in parallel workers.
+ */
+static void
+guc_restore_error_context_callback(void *arg)
+{
+	char	  **error_context_name_and_value = (char **) arg;
+
+	if (error_context_name_and_value)
+		errcontext("while setting parameter \"%s\" to \"%s\"",
+				   error_context_name_and_value[0],
+				   error_context_name_and_value[1]);
+}
+
 /*
  * RestoreGUCState:
  * Reads the GUC state at the specified address and updates the GUCs with the
@@ -10227,6 +10242,7 @@ RestoreGUCState(void *gucstate)
 	char	   *srcend;
 	Size		len;
 	int			i;
+	ErrorContextCallback error_context_callback;
 
 	/* See comment at can_skip_gucvar(). */
 	for (i = 0; i < num_guc_variables; i++)
@@ -10239,9 +10255,16 @@ RestoreGUCState(void *gucstate)
 	srcptr += sizeof(len);
 	srcend = srcptr + len;
 
+	/* If the GUC value check fails, we want errors to show useful context. */
+	error_context_callback.callback = guc_restore_error_context_callback;
+	error_context_callback.previous = error_context_stack;
+	error_context_callback.arg = NULL;
+	error_context_stack = &error_context_callback;
+
 	while (srcptr < srcend)
 	{
 		int			result;
+		char	   *error_context_name_and_value[2];
 
 		varname = read_gucstate(&srcptr, srcend);
 		varvalue = read_gucstate(&srcptr, srcend);
@@ -10256,6 +10279,9 @@ RestoreGUCState(void *gucstate)
 		read_gucstate_binary(&srcptr, srcend,
 							 &varscontext, sizeof(varscontext));
 
+		error_context_name_and_value[0] = varname;
+		error_context_name_and_value[1] = varvalue;
+		error_context_callback.arg = &error_context_name_and_value[0];
 		result = set_config_option(varname, varvalue, varscontext, varsource,
 								   GUC_ACTION_SET, true, ERROR, true);
 		if (result <= 0)
@@ -10264,7 +10290,10 @@ RestoreGUCState(void *gucstate)
 					 errmsg("parameter \"%s\" could not be set", varname)));
 		if (varsourcefile[0])
 			set_config_sourcefile(varname, varsourcefile, varsourceline);
+		error_context_callback.arg = NULL;
 	}
+
+	error_context_stack = error_context_callback.previous;
 }
 
 /*