v3-0001-Don-t-use-strcmp-with-nullable-pointers.patch

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 39d3775e80..ccffaaad02 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1473,7 +1473,9 @@ check_GUC_init(struct config_generic *gconf)
 			{
 				struct config_string *conf = (struct config_string *) gconf;
 
-				if (*conf->variable != NULL && strcmp(*conf->variable, conf->boot_val) != 0)
+				if (*conf->variable != NULL &&
+					(conf->boot_val == NULL ||
+					 strcmp(*conf->variable, conf->boot_val) != 0))
 				{
 					elog(LOG, "GUC (PGC_STRING) %s, boot_val=%s, C-var=%s",
 						 conf->gen.name, conf->boot_val ? conf->boot_val : "<null>", *conf->variable);
@@ -4213,8 +4215,7 @@ SetConfigOption(const char *name, const char *value,
 /*
  * Fetch the current value of the option `name', as a string.
  *
- * If the option doesn't exist, return NULL if missing_ok is true (NOTE that
- * this cannot be distinguished from a string variable with a NULL value!),
+ * If the option doesn't exist, return NULL if missing_ok is true,
  * otherwise throw an ereport and don't return.
  *
  * If restrict_privileged is true, we also enforce that only superusers and
@@ -4257,7 +4258,8 @@ GetConfigOption(const char *name, bool missing_ok, bool restrict_privileged)
 			return buffer;
 
 		case PGC_STRING:
-			return *((struct config_string *) record)->variable;
+			return *((struct config_string *) record)->variable ?
+				*((struct config_string *) record)->variable : "";
 
 		case PGC_ENUM:
 			return config_enum_lookup_by_value((struct config_enum *) record,
@@ -4304,7 +4306,8 @@ GetConfigOptionResetString(const char *name)
 			return buffer;
 
 		case PGC_STRING:
-			return ((struct config_string *) record)->reset_val;
+			return ((struct config_string *) record)->reset_val ?
+				((struct config_string *) record)->reset_val : "";
 
 		case PGC_ENUM:
 			return config_enum_lookup_by_value((struct config_enum *) record,
@@ -5255,7 +5258,14 @@ get_explain_guc_options(int *num)
 				{
 					struct config_string *lconf = (struct config_string *) conf;
 
-					modified = (strcmp(lconf->boot_val, *(lconf->variable)) != 0);
+					if (lconf->boot_val == NULL &&
+						*lconf->variable == NULL)
+						modified = false;
+					else if (lconf->boot_val == NULL ||
+							 *lconf->variable == NULL)
+						modified = true;
+					else
+						modified = (strcmp(lconf->boot_val, *(lconf->variable)) != 0);
 				}
 				break;
 
@@ -5482,7 +5492,8 @@ write_one_nondefault_variable(FILE *fp, struct config_generic *gconf)
 			{
 				struct config_string *conf = (struct config_string *) gconf;
 
-				fprintf(fp, "%s", *conf->variable);
+				if (*conf->variable)
+					fprintf(fp, "%s", *conf->variable);
 			}
 			break;
 
@@ -6142,7 +6153,8 @@ RestoreGUCState(void *gucstate)
 				{
 					struct config_string *conf = (struct config_string *) gconf;
 
-					guc_free(*conf->variable);
+					if (*conf->variable)
+						guc_free(*conf->variable);
 					if (conf->reset_val && conf->reset_val != *conf->variable)
 						guc_free(conf->reset_val);
 					if (conf->reset_extra && conf->reset_extra != gconf->extra)
diff --git a/src/include/utils/guc_tables.h b/src/include/utils/guc_tables.h
index 1ec9575570..0c38255961 100644
--- a/src/include/utils/guc_tables.h
+++ b/src/include/utils/guc_tables.h
@@ -240,6 +240,16 @@ struct config_real
 	void	   *reset_extra;
 };
 
+/*
+ * A note about string GUCs: the boot_val is allowed to be NULL, which leads
+ * to the reset_val and the actual variable value (*variable) also being NULL.
+ * However, there is no way to set a NULL value subsequently using
+ * set_config_option or any other GUC API.  Also, GUC APIs such as SHOW will
+ * display a NULL value as an empty string.  Callers that choose to use a NULL
+ * boot_val should overwrite the setting later in startup, or else be careful
+ * that NULL doesn't have semantics that are visibly different from an empty
+ * string.
+ */
 struct config_string
 {
 	struct config_generic gen;