Improve comments for postmaster.c's BackendList.
This had gotten a little disjointed over time, and some of the grammar
was sloppy.  Rewrite for more clarity.
In passing, re-pgindent some recently added comments.
No code changes.
* authorization phase). This is used mainly to keep track of how many
* children we have and send them appropriate signals when necessary.
*
- * "Special" children such as the startup, bgwriter and autovacuum launcher
- * tasks are not in this list. Autovacuum worker and walsender are in it.
+ * As shown in the above set of backend types, this list includes not only
+ * "normal" client sessions, but also autovacuum workers, walsenders, and
+ * background workers. (Note that at the time of launch, walsenders are
+ * labeled BACKEND_TYPE_NORMAL; we relabel them to BACKEND_TYPE_WALSND
+ * upon noticing they've changed their PMChildFlags entry. Hence that check
+ * must be done before any operation that needs to distinguish walsenders
+ * from normal backends.)
+ *
* Also, "dead_end" children are in it: these are children launched just for
* the purpose of sending a friendly rejection message to a would-be client.
* We must track them because they are attached to shared memory, but we know
* they will never become live backends. dead_end children are not assigned a
- * PMChildSlot.
+ * PMChildSlot. dead_end children have bkend_type NORMAL.
*
- * Background workers are in this list, too.
+ * "Special" children such as the startup, bgwriter and autovacuum launcher
+ * tasks are not in this list. They are tracked via StartupPID and other
+ * pid_t variables below. (Thus, there can't be more than one of any given
+ * "special" child process type. We use BackendList entries for any child
+ * process there can be more than one of.)
*/
typedef struct bkend
{
pid_t pid; /* process id of backend */
int32 cancel_key; /* cancel key for cancels for this backend */
int child_slot; /* PMChildSlot for this backend, if any */
-
- /*
- * Flavor of backend or auxiliary process. Note that BACKEND_TYPE_WALSND
- * backends initially announce themselves as BACKEND_TYPE_NORMAL, so if
- * bkend_type is normal, you should check for a recent transition.
- */
- int bkend_type;
+ int bkend_type; /* child process flavor, see above */
bool dead_end; /* is it going to send an error and quit? */
bool bgworker_notify; /* gets bgworker start/stop notifications */
dlist_node elem; /* list link in BackendList */
@@ -1059,10 +1063,9 @@ PostmasterMain(int argc, char *argv[])
* only during a few moments during a standby promotion. However there is
* a race condition: if pg_ctl promote is executed and creates the files
* during a promotion, the files can stay around even after the server is
- * brought up to be the primary. Then, if a new standby starts by using the
- * backup taken from the new primary, the files can exist at the server
- * startup and should be removed in order to avoid an unexpected
- * promotion.
+ * brought up to be the primary. Then, if a new standby starts by using
+ * the backup taken from the new primary, the files can exist at server
+ * startup and must be removed in order to avoid an unexpected promotion.
*
* Note that promotion signal files need to be removed before the startup
* process is invoked. Because, after that, they can be used by
@@ -5336,8 +5339,8 @@ sigusr1_handler(SIGNAL_ARGS)
/*
* Tell startup process to finish recovery.
*
- * Leave the promote signal file in place and let the Startup
- * process do the unlink.
+ * Leave the promote signal file in place and let the Startup process
+ * do the unlink.
*/
signal_child(StartupPID, SIGUSR2);
}
