Fix assertion failure at end of PITR.

InitXLogInsert() cannot be called in a critical section, because it
allocates memory. But CreateCheckPoint() did that, when called for the
end-of-recovery checkpoint by the startup process.

In the passing, fix the scratch space allocation in InitXLogInsert to go to
the right memory context. Also update the comment at InitXLOGAccess, which
hasn't been totally accurate since hot standby was introduced (in a hot
standby backend, InitXLOGAccess isn't called at backend startup).

Reported by Michael Paquier

src/backend/access/transam/xlog.c

@@ -7254,10 +7254,9 @@ ReadCheckpointRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
 }
 
 /*
- * This must be called during startup of a backend process, except that
+ * This must be called in a backend process before creating WAL records
- * it need not be called in a standalone backend (which does StartupXLOG
+ * (except in a standalone backend, which does StartupXLOG instead).  We need
- * instead).  We need to initialize the local copies of ThisTimeLineID and
+ * to initialize the local copies of ThisTimeLineID and RedoRecPtr.
- * RedoRecPtr.
  *
  * Note: before Postgres 8.0, we went to some effort to keep the postmaster
  * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
@@ -7594,6 +7593,16 @@ CreateCheckPoint(int flags)
 	if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
 		elog(ERROR, "can't create a checkpoint during recovery");
 
+	/*
+	 * Initialize InitXLogInsert working areas before entering the critical
+	 * section.  Normally, this is done by the first call to
+	 * RecoveryInProgress() or LocalSetXLogInsertAllowed(), but when creating
+	 * an end-of-recovery checkpoint, the LocalSetXLogInsertAllowed call is
+	 * done below in a critical section, and InitXLogInsert cannot be called
+	 * in a critical section.
+	 */
+	InitXLogInsert();
+
 	/*
 	 * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
 	 * (This is just pro forma, since in the present system structure there is

src/backend/access/transam/xloginsert.c

@@ -891,5 +891,6 @@ InitXLogInsert(void)
 	 * Allocate a buffer to hold the header information for a WAL record.
 	 */
 	if (hdr_scratch == NULL)
-		hdr_scratch = palloc0(HEADER_SCRATCH_SIZE);
+		hdr_scratch = MemoryContextAllocZero(xloginsert_cxt,
+											 HEADER_SCRATCH_SIZE);
 }