Move dynamically-allocated LWLock tranche names to shared memory.

There are two ways for shared libraries to allocate their own
LWLock tranches.  One way is to call RequestNamedLWLockTranche() in
a shmem_request_hook, which requires the library to be loaded via
shared_preload_libraries.  The other way is to call
LWLockNewTrancheId(), which is not subject to the same
restrictions.  However, LWLockNewTrancheId() does require each
backend to store the tranche's name in backend-local memory via
LWLockRegisterTranche().  This API is a little cumbersome and leads
to things like unhelpful pg_stat_activity.wait_event values in
backends that haven't loaded the library.

This commit moves these LWLock tranche names to shared memory, thus
eliminating the need for each backend to call
LWLockRegisterTranche().  Instead, the tranche name must be
provided to LWLockNewTrancheId(), which immediately makes the name
available to all backends.  Since the tranche name array is
append-only, lookups can ordinarily avoid locking as long as their
local copy of the LWLock counter is greater than the requested
tranche ID.

One downside of this approach is that we now have a hard limit on
both the length of tranche names (NAMEDATALEN-1 bytes) and the
number of dynamically-allocated tranches (256).  Besides a limit of
NAMEDATALEN-1 bytes for tranche names registered via
RequestNamedLWLockTranche(), no such limits previously existed.  We
could avoid these new limits by using dynamic shared memory, but
the complexity involved didn't seem worth it.  We briefly
considered making the tranche limit user-configurable but
ultimately decided against that, too.  Since there is still a lot
of time left in the v19 development cycle, it's possible we will
revisit this choice.

Author: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Rahila Syed <rahilasyed90@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAA5RZ0vvED3naph8My8Szv6DL4AxOVK3eTPS0qXsaKi%3DbVdW2A%40mail.gmail.com

1 files changed, 5 insertions, 20 deletions

diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index f9cf57f8d26..0e9cf81a4c7 100644
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -73,14 +73,7 @@ typedef union LWLockPadded
 
 extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
 
-/* struct for storing named tranche information */
-typedef struct NamedLWLockTranche
-{
-	int			trancheId;
-	char	   *trancheName;
-} NamedLWLockTranche;
-
-extern PGDLLIMPORT NamedLWLockTranche *NamedLWLockTrancheArray;
+extern PGDLLIMPORT char **LWLockTrancheNames;
 extern PGDLLIMPORT int NamedLWLockTrancheRequests;
 extern PGDLLIMPORT int *LWLockCounter;
 
@@ -158,19 +151,11 @@ extern LWLockPadded *GetNamedLWLockTranche(const char *tranche_name);
 
 /*
  * There is another, more flexible method of obtaining lwlocks. First, call
- * LWLockNewTrancheId just once to obtain a tranche ID; this allocates from
- * a shared counter.  Next, each individual process using the tranche should
- * call LWLockRegisterTranche() to associate that tranche ID with a name.
- * Finally, LWLockInitialize should be called just once per lwlock, passing
- * the tranche ID as an argument.
- *
- * It may seem strange that each process using the tranche must register it
- * separately, but dynamic shared memory segments aren't guaranteed to be
- * mapped at the same address in all coordinating backends, so storing the
- * registration in the main shared memory segment wouldn't work for that case.
+ * LWLockNewTrancheId to obtain a tranche ID; this allocates from a shared
+ * counter.  Second, LWLockInitialize should be called just once per lwlock,
+ * passing the tranche ID as an argument.
  */
-extern int	LWLockNewTrancheId(void);
-extern void LWLockRegisterTranche(int tranche_id, const char *tranche_name);
+extern int	LWLockNewTrancheId(const char *name);
 extern void LWLockInitialize(LWLock *lock, int tranche_id);
 
 /*