From 01cb7d0bf5a761d47cf89cc60358458e2e77fc7f Mon Sep 17 00:00:00 2001
From: Maxim Orlov <orlovmg@gmail.com>
Date: Mon, 20 Mar 2023 18:23:55 +0300
Subject: [PATCH v58 3/3] Make use FullTransactionId in 2PC filenames

Switch from using TransactionId to FullTransactionId in naming of 2PC files.
Transaction state file in the pg_twophase directory now have extra 8 bytes in
the name to address an epoch of a given xid.

Author: Maxim Orlov
Discussion: https://postgr.es/m/CAJ7c6TPDOYBYrnCAeyndkBktO0WG2xSdYduTF0nxq%2BvfkmTF5Q%40mail.gmail.com
---
 src/backend/access/transam/twophase.c | 50 ++++++++++++++++++++++++---
 1 file changed, 45 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 068e59bec0..e103ffb6d3 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -941,8 +941,46 @@ TwoPhaseGetDummyProc(TransactionId xid, bool lock_held)
 /* State file support													*/
 /************************************************************************/
 
-#define TwoPhaseFilePath(path, xid) \
-	snprintf(path, MAXPGPATH, TWOPHASE_DIR "/%08X", xid)
+/*
+ * Compute the FullTransactionId for the given TransactionId.
+ *
+ * The wrap logic is safe here because the span of active xids cannot exceed one
+ * epoch at any given time.
+ */
+static inline FullTransactionId
+AdjustToFullTransactionId(TransactionId xid)
+{
+	FullTransactionId	nextFullXid;
+	TransactionId		nextXid;
+	uint32				epoch;
+
+	Assert(TransactionIdIsValid(xid));
+
+	LWLockAcquire(XidGenLock, LW_SHARED);
+	nextFullXid = ShmemVariableCache->nextXid;
+	LWLockRelease(XidGenLock);
+
+	nextXid = XidFromFullTransactionId(nextFullXid);
+	epoch = EpochFromFullTransactionId(nextFullXid);
+	if (unlikely(xid > nextXid))
+	{
+		/* Wraparound occured, must be from a prev epoch. */
+		Assert(epoch > 0);
+		epoch--;
+	}
+
+	return FullTransactionIdFromEpochAndXid(epoch, xid);
+}
+
+static inline int
+TwoPhaseFilePath(char *path, TransactionId xid)
+{
+	FullTransactionId fxid = AdjustToFullTransactionId(xid);
+
+	return snprintf(path, MAXPGPATH, TWOPHASE_DIR "/%08X%08X",
+					EpochFromFullTransactionId(fxid),
+					XidFromFullTransactionId(fxid));
+}
 
 /*
  * 2PC state file format:
@@ -1881,13 +1919,15 @@ restoreTwoPhaseData(void)
 	cldir = AllocateDir(TWOPHASE_DIR);
 	while ((clde = ReadDir(cldir, TWOPHASE_DIR)) != NULL)
 	{
-		if (strlen(clde->d_name) == 8 &&
-			strspn(clde->d_name, "0123456789ABCDEF") == 8)
+		if (strlen(clde->d_name) == 16 &&
+			strspn(clde->d_name, "0123456789ABCDEF") == 16)
 		{
 			TransactionId xid;
+			FullTransactionId fxid;
 			char	   *buf;
 
-			xid = (TransactionId) strtoul(clde->d_name, NULL, 16);
+			fxid = FullTransactionIdFromU64(strtou64(clde->d_name, NULL, 16));
+			xid = XidFromFullTransactionId(fxid);
 
 			buf = ProcessTwoPhaseBuffer(xid, InvalidXLogRecPtr,
 										true, false, false);
-- 
2.41.0