bufmgr: Add some more error checking [infrastructure] around pinning

This adds a few more assertions against a buffer being local in places we
don't expect, and extracts the check for a buffer being pinned exactly once
from LockBufferForCleanup() into its own function. Later commits will use this
function.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: http://postgr.es/m/419312fd-9255-078c-c3e3-f0525f911d7f@iki.fi

src/backend/storage/buffer/bufmgr.c

@@ -1735,6 +1735,8 @@ PinBuffer(BufferDesc *buf, BufferAccessStrategy strategy)
 	bool		result;
 	PrivateRefCountEntry *ref;
 
+	Assert(!BufferIsLocal(b));
+
 	ref = GetPrivateRefCountEntry(b, true);
 
 	if (ref == NULL)
@@ -1880,6 +1882,8 @@ UnpinBuffer(BufferDesc *buf)
 	PrivateRefCountEntry *ref;
 	Buffer		b = BufferDescriptorGetBuffer(buf);
 
+	Assert(!BufferIsLocal(b));
+
 	/* not moving as we're likely deleting it soon anyway */
 	ref = GetPrivateRefCountEntry(b, false);
 	Assert(ref != NULL);
@@ -4253,6 +4257,29 @@ ConditionalLockBuffer(Buffer buffer)
 									LW_EXCLUSIVE);
 }
 
+/*
+ * Verify that this backend is pinning the buffer exactly once.
+ *
+ * NOTE: Like in BufferIsPinned(), what we check here is that *this* backend
+ * holds a pin on the buffer.  We do not care whether some other backend does.
+ */
+void
+CheckBufferIsPinnedOnce(Buffer buffer)
+{
+	if (BufferIsLocal(buffer))
+	{
+		if (LocalRefCount[-buffer - 1] != 1)
+			elog(ERROR, "incorrect local pin count: %d",
+				 LocalRefCount[-buffer - 1]);
+	}
+	else
+	{
+		if (GetPrivateRefCount(buffer) != 1)
+			elog(ERROR, "incorrect local pin count: %d",
+				 GetPrivateRefCount(buffer));
+	}
+}
+
 /*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
@@ -4280,20 +4307,11 @@ LockBufferForCleanup(Buffer buffer)
 	Assert(BufferIsPinned(buffer));
 	Assert(PinCountWaitBuf == NULL);
 
+	CheckBufferIsPinnedOnce(buffer);
+
+	/* Nobody else to wait for */
 	if (BufferIsLocal(buffer))
-	{
-		/* There should be exactly one pin */
-		if (LocalRefCount[-buffer - 1] != 1)
-			elog(ERROR, "incorrect local pin count: %d",
-				 LocalRefCount[-buffer - 1]);
-		/* Nobody else to wait for */
 		return;
-	}
-
-	/* There should be exactly one local pin */
-	if (GetPrivateRefCount(buffer) != 1)
-		elog(ERROR, "incorrect local pin count: %d",
-			 GetPrivateRefCount(buffer));
 
 	bufHdr = GetBufferDescriptor(buffer - 1);
 
@@ -4794,6 +4812,8 @@ LockBufHdr(BufferDesc *desc)
 	SpinDelayStatus delayStatus;
 	uint32		old_buf_state;
 
+	Assert(!BufferIsLocal(BufferDescriptorGetBuffer(desc)));
+
 	init_local_spin_delay(&delayStatus);
 
 	while (true)

src/include/storage/bufmgr.h

@@ -134,6 +134,7 @@ extern void ReleaseBuffer(Buffer buffer);
 extern void UnlockReleaseBuffer(Buffer buffer);
 extern void MarkBufferDirty(Buffer buffer);
 extern void IncrBufferRefCount(Buffer buffer);
+extern void CheckBufferIsPinnedOnce(Buffer buffer);
 extern Buffer ReleaseAndReadBuffer(Buffer buffer, Relation relation,
 								   BlockNumber blockNum);