Redefine backend ID to be an index into the proc array

Previously, backend ID was an index into the ProcState array, in the
shared cache invalidation manager (sinvaladt.c). The entry in the
ProcState array was reserved at backend startup by scanning the array
for a free entry, and that was also when the backend got its backend
ID. Things become slightly simpler if we redefine backend ID to be the
index into the PGPROC array, and directly use it also as an index to
the ProcState array. This uses a little more memory, as we reserve a
few extra slots in the ProcState array for aux processes that don't
need them, but the simplicity is worth it.

Aux processes now also have a backend ID. This simplifies the
reservation of BackendStatusArray and ProcSignal slots.

You can now convert a backend ID into an index into the PGPROC array
simply by subtracting 1. We still use 0-based "pgprocnos" in various
places, for indexes into the PGPROC array, but the only difference now
is that backend IDs start at 1 while pgprocnos start at 0. (The next
commmit will get rid of the term "backend ID" altogether and make
everything 0-based.)

There is still a 'backendId' field in PGPROC, now part of 'vxid' which
encapsulates the backend ID and local transaction ID together. It's
needed for prepared xacts. For regular backends, the backendId is
always equal to pgprocno + 1, but for prepared xact PGPROC entries,
it's the ID of the original backend that processed the transaction.

Reviewed-by: Andres Freund, Reid Thompson
Discussion: https://www.postgresql.org/message-id/8171f1aa-496f-46a6-afc3-c46fe7a9b407@iki.fi

1 files changed, 11 insertions, 16 deletions

diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index 0f9f90d2c7b..199dd182253 100644
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -87,7 +87,7 @@ typedef struct
  * possible auxiliary process type.  (This scheme assumes there is not
  * more than one of any auxiliary process type at a time.)
  */
-#define NumProcSignalSlots	(MaxBackends + NUM_AUXPROCTYPES)
+#define NumProcSignalSlots	(MaxBackends + NUM_AUXILIARY_PROCS)
 
 /* Check whether the relevant type bit is set in the flags. */
 #define BARRIER_SHOULD_CHECK(flags, type) \
@@ -154,24 +154,23 @@ ProcSignalShmemInit(void)
 /*
  * ProcSignalInit
  *		Register the current process in the ProcSignal array
- *
- * The passed index should be my BackendId if the process has one,
- * or MaxBackends + aux process type if not.
  */
 void
-ProcSignalInit(int pss_idx)
+ProcSignalInit(void)
 {
 	ProcSignalSlot *slot;
 	uint64		barrier_generation;
 
-	Assert(pss_idx >= 1 && pss_idx <= NumProcSignalSlots);
-
-	slot = &ProcSignal->psh_slot[pss_idx - 1];
+	if (MyBackendId <= 0)
+		elog(ERROR, "MyBackendId not set");
+	if (MyBackendId > NumProcSignalSlots)
+		elog(ERROR, "unexpected MyBackendId %d in ProcSignalInit (max %d)", MyBackendId, NumProcSignalSlots);
+	slot = &ProcSignal->psh_slot[MyBackendId - 1];
 
 	/* sanity check */
 	if (slot->pss_pid != 0)
 		elog(LOG, "process %d taking over ProcSignal slot %d, but it's not empty",
-			 MyProcPid, pss_idx);
+			 MyProcPid, MyBackendId - 1);
 
 	/* Clear out any leftover signal reasons */
 	MemSet(slot->pss_signalFlags, 0, NUM_PROCSIGNALS * sizeof(sig_atomic_t));
@@ -200,7 +199,7 @@ ProcSignalInit(int pss_idx)
 	MyProcSignalSlot = slot;
 
 	/* Set up to release the slot on process exit */
-	on_shmem_exit(CleanupProcSignalState, Int32GetDatum(pss_idx));
+	on_shmem_exit(CleanupProcSignalState, (Datum) 0);
 }
 
 /*
@@ -212,11 +211,7 @@ ProcSignalInit(int pss_idx)
 static void
 CleanupProcSignalState(int status, Datum arg)
 {
-	int			pss_idx = DatumGetInt32(arg);
-	ProcSignalSlot *slot;
-
-	slot = &ProcSignal->psh_slot[pss_idx - 1];
-	Assert(slot == MyProcSignalSlot);
+	ProcSignalSlot *slot = MyProcSignalSlot;
 
 	/*
 	 * Clear MyProcSignalSlot, so that a SIGUSR1 received after this point
@@ -233,7 +228,7 @@ CleanupProcSignalState(int status, Datum arg)
 		 * infinite loop trying to exit
 		 */
 		elog(LOG, "process %d releasing ProcSignal slot %d, but it contains %d",
-			 MyProcPid, pss_idx, (int) slot->pss_pid);
+			 MyProcPid, (int) (slot - ProcSignal->psh_slot), (int) slot->pss_pid);
 		return;					/* XXX better to zero the slot anyway? */
 	}