Remove numFreeProcs from ProcGlobal.

Variable numFreeProcs is added by GPDB to accelerate HaveNFreeProcs(),
but considering the argument size of calls to HaveNFreeProcs(), this
optimization may be not worth it.
In addition, the current codes calculate numFreeProcs wrong.

So remove numFreeProcs to keep the same as PostgreSQL.

Relevant thread from gpdb-dev: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/NG8t5XXYQB0/7BDVoQQLBAAJ

src/backend/storage/lmgr/proc.c

@@ -295,13 +295,6 @@ InitProcGlobal(void)
 	/* Create ProcStructLock spinlock, too */
 	ProcStructLock = (slock_t *) ShmemAlloc(sizeof(slock_t));
 	SpinLockInit(ProcStructLock);
-
-	/*
-	 * GPDB: numFreeProcs is used to keep track of the
-	 * number of free PGPROC entries in freeProcs list,
-	 * and will accelerate HaveNFreeProcs().
-	 */
-	ProcGlobal->numFreeProcs = MaxConnections;
 }
 
 /*
@@ -351,17 +344,9 @@ InitProcess(void)
 	if (MyProc != NULL)
 	{
 		if (IsAnyAutoVacuumProcess())
-		{
 			procglobal->autovacFreeProcs = (PGPROC *) MyProc->links.next;
-		}
 		else
-		{
 			procglobal->freeProcs = (PGPROC *) MyProc->links.next;
-
-			procglobal->numFreeProcs--;     /* we removed an entry from the list. */
-			Assert(procglobal->numFreeProcs >= 0);
-		}
-
 		SpinLockRelease(ProcStructLock);
 	}
 	else
@@ -712,13 +697,30 @@ GetStartupBufferPinWaitBufId(void)
 
 /*
  * Check whether there are at least N free PGPROC objects.
+ *
+ * Note: this is designed on the assumption that N will generally be small.
  */
 bool
 HaveNFreeProcs(int n)
 {
-	Assert(n >= 0);
+	PGPROC	   *proc;
 
-	return (ProcGlobal->numFreeProcs >= n);
+	/* use volatile pointer to prevent code rearrangement */
+	volatile PROC_HDR *procglobal = ProcGlobal;
+
+	SpinLockAcquire(ProcStructLock);
+
+	proc = procglobal->freeProcs;
+
+	while (n > 0 && proc != NULL)
+	{
+		proc = (PGPROC *) proc->links.next;
+		n--;
+	}
+
+	SpinLockRelease(ProcStructLock);
+
+	return (n <= 0);
 }
 
 /*
@@ -950,8 +952,6 @@ ProcKill(int code, Datum arg)
 	{
 		proc->links.next = (SHM_QUEUE *) procglobal->freeProcs;
 		procglobal->freeProcs = proc;
-
-		procglobal->numFreeProcs++;	/* we added an entry */
 	}
 
 

src/include/storage/proc.h

@@ -254,9 +254,6 @@ typedef struct PROC_HDR
 
     /* Counter for assigning serial numbers to processes */
     int         mppLocalProcessCounter;
-
-	/* Number of free PGPROC entries in freeProcs list. */
-	int			numFreeProcs;
 } PROC_HDR;
 
 extern PROC_HDR *ProcGlobal;