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, 4 insertions, 5 deletions

diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ed6071f3286..13b81228e5b 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -74,9 +74,9 @@ typedef struct
 #define SetInvalidVirtualTransactionId(vxid) \
 	((vxid).backendId = InvalidBackendId, \
 	 (vxid).localTransactionId = InvalidLocalTransactionId)
-#define GET_VXID_FROM_PGPROC(vxid, proc) \
-	((vxid).backendId = (proc).backendId, \
-	 (vxid).localTransactionId = (proc).lxid)
+#define GET_VXID_FROM_PGPROC(vxid_dst, proc) \
+	((vxid_dst).backendId = (proc).vxid.backendId, \
+		 (vxid_dst).localTransactionId = (proc).vxid.lxid)
 
 /* MAX_LOCKMODES cannot be larger than the # of bits in LOCKMASK */
 #define MAX_LOCKMODES		10
@@ -454,8 +454,7 @@ typedef struct LockInstanceData
 	LOCKTAG		locktag;		/* tag for locked object */
 	LOCKMASK	holdMask;		/* locks held by this PGPROC */
 	LOCKMODE	waitLockMode;	/* lock awaited by this PGPROC, if any */
-	BackendId	backend;		/* backend ID of this PGPROC */
-	LocalTransactionId lxid;	/* local transaction ID of this PGPROC */
+	VirtualTransactionId vxid;	/* virtual transaction ID of this PGPROC */
 	TimestampTz waitStart;		/* time at which this PGPROC started waiting
 								 * for lock */
 	int			pid;			/* pid of this PGPROC */