From d5634cb189ab8cdbae01640dbeb47616441bc1c6 Mon Sep 17 00:00:00 2001
From: Tim Chang <changti@amazon.com>
Date: Mon, 6 Nov 2023 17:07:25 +0000
Subject: [PATCH] Added comments, added OID macros

---
 src/backend/access/transam/varsup.c | 46 ++++++++++++++++-------------
 1 file changed, 26 insertions(+), 20 deletions(-)

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index be48823cbff..640d4c183ae 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -13,6 +13,8 @@
 
 #include "postgres.h"
 
+#include <unistd.h>
+
 #include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/subtrans.h"
@@ -31,6 +33,9 @@
 /* Number of OIDs to prefetch (preallocate) per XLOG write */
 #define VAR_OID_PREFETCH		8192
 
+#define OID_TO_BUFFER_START(oid) 		((oid) + INT_MIN)
+#define BUFFER_START_TO_OID 			((Oid) (temp_oid_buffer_start) - INT_MIN)
+
 /* pointer to "variable cache" in shared memory (set up by shmem.c) */
 VariableCache ShmemVariableCache = NULL;
 
@@ -520,7 +525,6 @@ GetNewTempObjectId(void)
 {
 	Oid			result;
 	Oid			tempOidStart;
-	Oid			real_temp_oid_buffer_start;
 	static Oid 	nextTempOid = InvalidOid;
 
 	/* safety check, we should never get this far in a HS standby */
@@ -531,47 +535,57 @@ GetNewTempObjectId(void)
 	{
 		/* First check to see if another connection has already picked a start, then update. */
 		LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
-		if (OidIsValid(ShmemVariableCache->tempOidStart) && (ShmemVariableCache->tempOidBufferSize == temp_oid_buffer_size))
+		if (OidIsValid(ShmemVariableCache->tempOidStart))
 		{
-			/* If ShmemVariableCache->tempOidStart is valid, it could also mean the buffer size GUC has been altered, and we need to pick a new start. */
-			temp_oid_buffer_start = ShmemVariableCache->tempOidStart + INT_MIN;
+			temp_oid_buffer_start = OID_TO_BUFFER_START(ShmemVariableCache->tempOidStart);
 			nextTempOid = ShmemVariableCache->tempOidStart;
 		}
 		else
 		{
 			/* We need to pick a new start for the buffer range. */
 			tempOidStart = ShmemVariableCache->nextOid;
+
+			/*
+			 * Decrement ShmemVariableCache->oidCount to take into account the new buffer we're allocating
+			 */
 			if (ShmemVariableCache->oidCount < temp_oid_buffer_size)
 				ShmemVariableCache->oidCount = 0;
 			else
 				ShmemVariableCache->oidCount -= temp_oid_buffer_size;
 
+			/*
+			 * If ShmemVariableCache->nextOid is below FirstNormalObjectId then we can start at FirstNormalObjectId here and
+			 * GetNewObjectId will return the right value on the next call.  
+			 */
+			if (tempOidStart < FirstNormalObjectId)
+				tempOidStart = FirstNormalObjectId;
+
 			/* If the OID range would wraparound, start from beginning instead. */
 			if (tempOidStart + temp_oid_buffer_size < tempOidStart) 
-			{
 				tempOidStart = FirstNormalObjectId;
-			}
-			temp_oid_buffer_start = tempOidStart + INT_MIN;
+
+			temp_oid_buffer_start = OID_TO_BUFFER_START(tempOidStart);
 			ShmemVariableCache->tempOidStart = tempOidStart;
 			ShmemVariableCache->tempOidBufferSize = temp_oid_buffer_size;
 
 			nextTempOid = (Oid) tempOidStart;
+
+			/* Skip nextOid ahead to end of range here as well.  */
+			ShmemVariableCache->nextOid = (Oid) (tempOidStart + temp_oid_buffer_size);
 		}
 		LWLockRelease(OidGenLock);
 	}
 
-	real_temp_oid_buffer_start = temp_oid_buffer_start - INT_MIN;
-
 	/*
 	 * Check for wraparound of the temp OID buffer.
 	 */
-	if (nextTempOid >= (Oid) (real_temp_oid_buffer_start + temp_oid_buffer_size) || nextTempOid < (Oid) real_temp_oid_buffer_start)
+	if (nextTempOid >= (Oid) (BUFFER_START_TO_OID + temp_oid_buffer_size) 
+			|| nextTempOid < BUFFER_START_TO_OID)
 	{
-		nextTempOid = (Oid) real_temp_oid_buffer_start;
+		nextTempOid = BUFFER_START_TO_OID;
 	}
 
 	result = nextTempOid;
-
 	nextTempOid++;
 
 	return result;
@@ -598,14 +612,6 @@ GetNewObjectId(void)
 
 	LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
 
-	/* If we're in tempOid buffer range, skip ahead. */
-	if (OidIsValid(temp_oid_buffer_start)
-	&& ShmemVariableCache->nextOid >= (Oid) (temp_oid_buffer_start - INT_MIN)
-	&& ShmemVariableCache->nextOid < (Oid) (temp_oid_buffer_start + temp_oid_buffer_size - INT_MIN))
-	{
-		ShmemVariableCache->nextOid = (Oid) (temp_oid_buffer_start + temp_oid_buffer_size - INT_MIN);
-	}
-
 	/*
 	 * Check for wraparound of the OID counter.  We *must* not return 0
 	 * (InvalidOid), and in normal operation we mustn't return anything below