From 6bd78a44338ad11ff03a2ac554e6fd5bd5ed8418 Mon Sep 17 00:00:00 2001
From: H.G. Muller <h.g.muller@hccnet.nl>
Date: Sun, 7 Feb 2010 16:43:36 +0100
Subject: [PATCH] Remove race condition in clock switching

The move number is now incremented inside SwitchClock, when no timer
interrupt is scheduled, so the interrupt cannot occur at the wrong side
of the increment, and decrement the wrong clock.
---
 backend.c |   19 ++++++++++---------
 1 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/backend.c b/backend.c
index 2f7ee57..ecf338c 100644
--- a/backend.c
+++ b/backend.c
@@ -189,7 +189,7 @@ void ParseGameHistory P((char *game));
 void ParseBoard12 P((char *string));
 void KeepAlive P((void));
 void StartClocks P((void));
-void SwitchClocks P((void));
+void SwitchClocks P((int nr));
 void StopClocks P((void));
 void ResetClocks P((void));
 char *PGNDate P((void));
@@ -3048,10 +3048,10 @@ read_from_ics(isr, closure, data, count, error)
 		    looking_at(buf, &i, "It is not your move")) {
 		    /* Illegal move */
 		    if (ics_user_moved && forwardMostMove > backwardMostMove) { // only backup if we already moved
-			currentMove = --forwardMostMove;
+			currentMove = forwardMostMove-1;
 			DisplayMove(currentMove - 1); /* before DMError */
 			DrawPosition(FALSE, boards[currentMove]);
-			SwitchClocks();
+			SwitchClocks(forwardMostMove-1); // [HGM] race
 			DisplayBothClocks();
 		    }
 		    DisplayMoveError(_("Illegal move (rejected by ICS)")); // [HGM] but always relay error msg
@@ -6657,9 +6657,9 @@ if(appData.debugMode) fprintf(debugFP, "nodes = %d, %lld\n", (int) programStats.
 	    gameMode = EditGame;
 	    ModeHighlight();
 	}
-	currentMove = --forwardMostMove;
+	currentMove = forwardMostMove-1;
 	DisplayMove(currentMove-1); /* before DisplayMoveError */
-	SwitchClocks();
+	SwitchClocks(forwardMostMove-1); // [HGM] race
 	DisplayBothClocks();
 	sprintf(buf1, _("Illegal move \"%s\" (rejected by %s chess program)"),
 		parseList[currentMove], cps->which);
@@ -7737,8 +7737,8 @@ MakeMove(fromX, fromY, toX, toY, promoChar)
     {int i; for(i=0; i<BOARD_SIZE; i++) castlingRights[forwardMostMove+1][i] = castlingRights[forwardMostMove][i];}
     ApplyMove(fromX, fromY, toX, toY, promoChar, boards[forwardMostMove+1], 
 				castlingRights[forwardMostMove+1], &epStatus[forwardMostMove+1]);
-    forwardMostMove++; // [HGM] bare: moved to after ApplyMove, to make sure clock interrupt finds complete board
-    SwitchClocks(); // uses forwardMostMove, so must be done after incrementing it !
+    // forwardMostMove++; // [HGM] bare: moved to after ApplyMove, to make sure clock interrupt finds complete board
+    SwitchClocks(forwardMostMove+1); // [HGM] race: incrementing move nr inside
     timeRemaining[0][forwardMostMove] = whiteTimeRemaining;
     timeRemaining[1][forwardMostMove] = blackTimeRemaining;
     gameInfo.result = GameUnfinished;
@@ -13363,7 +13363,7 @@ DecrementClocks()
    from the color that is *not* on move now.
 */
 void
-SwitchClocks()
+SwitchClocks(int newMoveNr)
 {
     long lastTickLength;
     TimeMark now;
@@ -13373,7 +13373,7 @@ SwitchClocks()
 
     if (StopClockTimer() && appData.clockMode) {
 	lastTickLength = SubtractTimeMarks(&now, &tickStartTM);
-	if (WhiteOnMove(forwardMostMove)) {
+	if (!WhiteOnMove(forwardMostMove)) {
 	    if(blackNPS >= 0) lastTickLength = 0;
 	    blackTimeRemaining -= lastTickLength;
            /* [HGM] PGNtime: save time for PGN file if engine did not give it */
@@ -13390,6 +13390,7 @@ SwitchClocks()
 	}
 	flagged = CheckFlags();
     }
+    forwardMostMove = newMoveNr; // [HGM] race: change stm when no timer interrupt scheduled
     CheckTimeControl();
 
     if (flagged || !appData.clockMode) return;
-- 
1.7.0.4