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, 14 insertions, 14 deletions

diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c70a1adb9ad..e62968b4a86 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -3625,8 +3625,8 @@ GetLockStatusData(void)
 								 proc->fpRelId[f]);
 			instance->holdMask = lockbits << FAST_PATH_LOCKNUMBER_OFFSET;
 			instance->waitLockMode = NoLock;
-			instance->backend = proc->backendId;
-			instance->lxid = proc->lxid;
+			instance->vxid.backendId = proc->vxid.backendId;
+			instance->vxid.localTransactionId = proc->vxid.lxid;
 			instance->pid = proc->pid;
 			instance->leaderPid = proc->pid;
 			instance->fastpath = true;
@@ -3652,15 +3652,15 @@ GetLockStatusData(void)
 					repalloc(data->locks, sizeof(LockInstanceData) * els);
 			}
 
-			vxid.backendId = proc->backendId;
+			vxid.backendId = proc->vxid.backendId;
 			vxid.localTransactionId = proc->fpLocalTransactionId;
 
 			instance = &data->locks[el];
 			SET_LOCKTAG_VIRTUALTRANSACTION(instance->locktag, vxid);
 			instance->holdMask = LOCKBIT_ON(ExclusiveLock);
 			instance->waitLockMode = NoLock;
-			instance->backend = proc->backendId;
-			instance->lxid = proc->lxid;
+			instance->vxid.backendId = proc->vxid.backendId;
+			instance->vxid.localTransactionId = proc->vxid.lxid;
 			instance->pid = proc->pid;
 			instance->leaderPid = proc->pid;
 			instance->fastpath = true;
@@ -3712,8 +3712,8 @@ GetLockStatusData(void)
 			instance->waitLockMode = proc->waitLockMode;
 		else
 			instance->waitLockMode = NoLock;
-		instance->backend = proc->backendId;
-		instance->lxid = proc->lxid;
+		instance->vxid.backendId = proc->vxid.backendId;
+		instance->vxid.localTransactionId = proc->vxid.lxid;
 		instance->pid = proc->pid;
 		instance->leaderPid = proclock->groupLeader->pid;
 		instance->fastpath = false;
@@ -3888,8 +3888,8 @@ GetSingleProcBlockerStatusData(PGPROC *blocked_proc, BlockedProcsData *data)
 			instance->waitLockMode = proc->waitLockMode;
 		else
 			instance->waitLockMode = NoLock;
-		instance->backend = proc->backendId;
-		instance->lxid = proc->lxid;
+		instance->vxid.backendId = proc->vxid.backendId;
+		instance->vxid.localTransactionId = proc->vxid.lxid;
 		instance->pid = proc->pid;
 		instance->leaderPid = proclock->groupLeader->pid;
 		instance->fastpath = false;
@@ -4374,8 +4374,8 @@ lock_twophase_postabort(TransactionId xid, uint16 info,
  *		lockers, as we haven't advertised this vxid via the ProcArray yet.
  *
  *		Since MyProc->fpLocalTransactionId will normally contain the same data
- *		as MyProc->lxid, you might wonder if we really need both.  The
- *		difference is that MyProc->lxid is set and cleared unlocked, and
+ *		as MyProc->vxid.lxid, you might wonder if we really need both.  The
+ *		difference is that MyProc->vxid.lxid is set and cleared unlocked, and
  *		examined by procarray.c, while fpLocalTransactionId is protected by
  *		fpInfoLock and is used only by the locking subsystem.  Doing it this
  *		way makes it easier to verify that there are no funny race conditions.
@@ -4391,7 +4391,7 @@ VirtualXactLockTableInsert(VirtualTransactionId vxid)
 
 	LWLockAcquire(&MyProc->fpInfoLock, LW_EXCLUSIVE);
 
-	Assert(MyProc->backendId == vxid.backendId);
+	Assert(MyProc->vxid.backendId == vxid.backendId);
 	Assert(MyProc->fpLocalTransactionId == InvalidLocalTransactionId);
 	Assert(MyProc->fpVXIDLock == false);
 
@@ -4413,7 +4413,7 @@ VirtualXactLockTableCleanup(void)
 	bool		fastpath;
 	LocalTransactionId lxid;
 
-	Assert(MyProc->backendId != InvalidBackendId);
+	Assert(MyProc->vxid.backendId != InvalidBackendId);
 
 	/*
 	 * Clean up shared memory state.
@@ -4541,7 +4541,7 @@ VirtualXactLock(VirtualTransactionId vxid, bool wait)
 	 */
 	LWLockAcquire(&proc->fpInfoLock, LW_EXCLUSIVE);
 
-	if (proc->backendId != vxid.backendId
+	if (proc->vxid.backendId != vxid.backendId
 		|| proc->fpLocalTransactionId != vxid.localTransactionId)
 	{
 		/* VXID ended */