(cloudberry) 05/13: Be more wary about NULL values for GUC string variables.

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avamingli pushed a commit to branch main
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commit 8b0f06b6f4301f587879134c5fc6cbfcea36fbbd
Author: Tom Lane <tg...@sss.pgh.pa.us>
AuthorDate: Fri Nov 3 05:04:49 2023 -0400

    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
    
    Upstream commit: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=7704a1a72e879daaefa1742916e01505edd2ce47
    Discussion: https://postgr.es/m/CACpMh+AyDx5YUpPaAgzVwC1d8zfOL4JoD-uyFDnNSa1z0EsDQQ@mail.gmail.com
---
 src/backend/utils/misc/guc.c   |  3 ++-
 src/include/utils/guc_tables.h | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 474b840e0c..d38b05a3a4 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -10801,7 +10801,8 @@ 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 db1a2240b3..8d1eb62fbc 100644
--- a/src/include/utils/guc_tables.h
+++ b/src/include/utils/guc_tables.h
@@ -272,6 +272,16 @@ 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;


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