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/[email protected]

1 files changed, 1 insertions, 11 deletions

diff --git a/src/backend/postmaster/auxprocess.c b/src/backend/postmaster/auxprocess.c
index ab86e802f21..39171fea06b 100644
--- a/src/backend/postmaster/auxprocess.c
+++ b/src/backend/postmaster/auxprocess.c
@@ -107,17 +107,7 @@ AuxiliaryProcessMain(AuxProcType auxtype)
 
 	BaseInit();
 
-	/*
-	 * Assign the ProcSignalSlot for an auxiliary process.  Since it doesn't
-	 * have a BackendId, the slot is statically allocated based on the
-	 * auxiliary process type (MyAuxProcType).  Backends use slots indexed in
-	 * the range from 1 to MaxBackends (inclusive), so we use MaxBackends +
-	 * AuxProcType + 1 as the index of the slot for an auxiliary process.
-	 *
-	 * This will need rethinking if we ever want more than one of a particular
-	 * auxiliary process type.
-	 */
-	ProcSignalInit(MaxBackends + MyAuxProcType + 1);
+	ProcSignalInit();
 
 	/*
 	 * Auxiliary processes don't run transactions, but they may need a