Prevent debug_query_string from producing garbage in apply worker logs

Initialize debug_query_string to NULL at apply worker startup, since it
is not a regular backend and has no client query string.

Also clear debug_query_string in execute_sql_command_error_cb after the
errcontext call, which already includes the SQL statement, to prevent it
from appearing a second time in the LOG output.

Author: danolivo

src/spock_apply.c

@@ -3282,6 +3282,12 @@ static void
 execute_sql_command_error_cb(void *arg)
 {
 	errcontext("during execution of queued SQL statement: %s", (char *) arg);
+	/*
+	 * The errcontext above already includes the SQL statement, so clear
+	 * debug_query_string to prevent it from appearing a second time in
+	 * the LOG output.
+	 */
+	debug_query_string = NULL;
 }
 
 /*
@@ -4145,6 +4151,13 @@ spock_apply_main(Datum main_arg)
 		apply_api.multi_insert_finish = spock_apply_spi_mi_finish;
 	}
 
+	/*
+	 * The apply worker is not a regular backend and has no client query
+	 * string. Initialize debug_query_string to NULL so that LOG reports
+	 * do not print arbitrary memory contents.
+	 */
+	debug_query_string = NULL;
+
 	/* Setup synchronous commit according to the user's wishes */
 	SetConfigOption("synchronous_commit",
 					spock_synchronous_commit ? "local" : "off",