BUG #16376: ALTER SYSTEM incorrectly quotes empty list

The following bug has been logged on the website:

Bug reference: 16376
Logged by: Teja Mupparti
Email address: tejeswarm(at)hotmail(dot)com
PostgreSQL version: 11.0
Operating system: Linux
Description:

Some applications set Postgres parameters using GUI(s) Once a value is set
and deselected later, it issues a SQL
ALTER SYSTEM set shared_preload_libraries = '';
is translating into shared_preload_libraries = '""' in the
postgresql.auto.conf, which will prevent Postgres server from starting
(illegal value of "")
FATAL: could not access file "": No such file or directory.

The quick fix is in quote_identifier()
change
safe = ((ident[0] >= 'a' && ident[0] <= 'z') || ident[0] == '_');
to
safe = ((ident[0] >= 'a' && ident[0] <= 'z') || ident[0] == '_' || ident[0]
== '\0');

Which will avoid the unwnated double-quotes, but is opening a can of
worms.

Easy fix is in Alter system code path

+#define EMPTY_QUOTES "\"\""
+
/*
* Precision with which REAL type guc values are to be printed for GUC
* serialization.
@@ -7886,6 +7888,22 @@ AlterSystemSetConfigFile(AlterSystemStmt
*altersysstmt)
FreeFile(infile);
}

+ /*
+ * There is a special case where an empty list '' is
getting
+ * translated into '""' by the quoted_identifier() logic.
+ * For example, set shared_preload_libraries = '' is
written
+ * as shared_preload_libraries = '""' in the autoconfig
file
+ * and the subsequent restart fails with the below error.
+ *
+ * FATAL: could not access file "": No such file or
directory
+ *
+ * Fixing quoted_identifier() breaks other parts of the
code,
+ * where it depends on translating '' to "". If the list
is
+ * empty, set the value to NULL (this will remove the
entry
+ * from the auto-config file)
+ */
+ if (!strcmp(value, EMPTY_QUOTES))
+ value = '\0';
--