Use tmpdir/replace to help prevent missing track race condition in li…

…ve update

The previous version had a rare race condition where snes9x trying to load an MSU track between it being deleted and it being replaced would result in no music playing until the next transition.  This change uses temporary copy/link and os.replace (POSIX guaranteed to be atomic) to hopefully eliminate this race condition.

Main.py

@@ -10,8 +10,9 @@
 import pprint
 import sched, time
 import datetime
+from tempfile import TemporaryDirectory
 
-__version__ = '0.7.1'
+__version__ = '0.7.2'
 
 # Creates a shuffled MSU-1 pack for ALttP Randomizer from one or more source
 # MSU-1 packs.
@@ -314,7 +315,7 @@ def delete_old_msu(args, rompath):
                 except PermissionError:
                     logger.info(f"WARNING: Failed to remove {path}")
 
-def copy_track(logger, srcpath, dst, rompath, dry_run, higan, forcerealcopy, live):
+def copy_track(logger, srcpath, dst, rompath, dry_run, higan, forcerealcopy, live, tmpdir):
     if higan:
         dstpath = higandir + "/track-" + str(dst) + ".pcm"
     else:
@@ -335,13 +336,16 @@ def copy_track(logger, srcpath, dst, rompath, dry_run, higan, forcerealcopy, liv
 
     if not dry_run:
         try:
-            if (os.path.exists(dstpath)):
-                os.remove(dstpath)
-
+            # Use a temporary file and os.replace to get around the fact that
+            # python doesn't have an atomic copy/hardlink with overwrite.
+            tmpname = os.path.join(tmpdir, f"tmp{os.path.basename(dstpath)}")
+
             if (forcerealcopy):
-                shutil.copy(srcpath, dstpath)
+                shutil.copy(srcpath, tmpname)
             else:
-                os.link(srcpath, dstpath)
+                os.link(srcpath, tmpname)
+
+            os.replace(tmpname, dstpath)
         except PermissionError:
             if not live:
                 logger.info(f"Failed to copy {srcpath} to {dstpath} during non-live update")
@@ -414,25 +418,26 @@ def shuffle_all_tracks(rompath, fullshuffle, singleshuffle, dry_run, higan, forc
     if not live:
         logger.info("Non-looping tracks:")
 
-    for i in nonloopingfoundtracks:
-        winner = random.choice(trackindex[i])
-        copy_track(logger, winner, i, rompath, dry_run, higan, forcerealcopy, live)
+    with TemporaryDirectory(dir='.') as tmpdir:
+        for i in nonloopingfoundtracks:
+            winner = random.choice(trackindex[i])
+            copy_track(logger, winner, i, rompath, dry_run, higan, forcerealcopy, live, tmpdir)
 
-    #For all found looping tracks, pick a random track from a random pack
-    #in the target directory, with a matching track number by default, or
-    #a shuffled different looping track number if fullshuffle or
-    #singleshuffle are enabled.
-    if not live:
-        logger.info("Looping tracks:")
-    for i in loopingfoundtracks:
-        if (args.fullshuffle or args.singleshuffle):
-            dst = i
-            src = shuffledloopingfoundtracks[loopingfoundtracks.index(i)]
-        else:
-            dst = i
-            src = i
-        winner = random.choice(trackindex[src])
-        copy_track(logger, winner, dst, rompath, dry_run, higan, forcerealcopy, live)
+        #For all found looping tracks, pick a random track from a random pack
+        #in the target directory, with a matching track number by default, or
+        #a shuffled different looping track number if fullshuffle or
+        #singleshuffle are enabled.
+        if not live:
+            logger.info("Looping tracks:")
+        for i in loopingfoundtracks:
+            if (args.fullshuffle or args.singleshuffle):
+                dst = i
+                src = shuffledloopingfoundtracks[loopingfoundtracks.index(i)]
+            else:
+                dst = i
+                src = i
+            winner = random.choice(trackindex[src])
+            copy_track(logger, winner, dst, rompath, dry_run, higan, forcerealcopy, live, tmpdir)
     if live:
         shuffletime = datetime.datetime.now() - shufflestarttime
         print("Reshuffling MSU pack every%s second%s, press ctrl+c or close the window to stop reshuffling. (shuffled in %d.%ds)" %(" " + str(int(live)) if int(live) != 1 else "", "s" if int(live) != 1 else "", shuffletime.seconds, shuffletime.microseconds))