Be more wary about NULL values for GUC string variables.

get_explain_guc_options() crashed if a string GUC marked GUC_EXPLAIN
has a NULL boot_val.  Nosing around found a couple of other places
that seemed insufficiently cautious about NULL string values, although
those are likely unreachable in practice.  Add some commentary
defining the expectations for NULL values of string variables,
in hopes of forestalling future additions of more such bugs.

Xing Guo, Aleksander Alekseev, Tom Lane

Discussion: https://postgr.es/m/CACpMh+AyDx5YUpPaAgzVwC1d8zfOL4JoD-uyFDnNSa1z0EsDQQ@mail.gmail.com

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index bd03965a7593e269d154251597cf4d015ac2f95a..22d43287e365003b1cf4913b2435161ffe817e0d 100644 (file)
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -8965,7 +8965,14 @@ 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;
 
@@ -9720,7 +9727,8 @@ 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;
 

diff --git a/src/include/utils/guc_tables.h b/src/include/utils/guc_tables.h
index d68976fafa94defa8d637ae59f3d24697c7f030f..8b842fca5fc76fd2e4810a7f0d0f05d81a4366a8 100644 (file)
--- a/src/include/utils/guc_tables.h
+++ b/src/include/utils/guc_tables.h
@@ -223,6 +223,16 @@ 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;