* Atomic file replacement is good.

scripts/generate-patches.pl

@@ -198,9 +198,17 @@ foreach my $p (keys %dstOutPaths) {
     
         my $finalName =
             "$narBz2Hash-$name-$closestVersion-to-$version.nar-diff";
-    
-        system("cp '$tmpdir/DIFF' '$patchesDir/$finalName'") == 0
+
+        if (-e "$patchesDir/$finalName") {
+            print "    not copying, already exists\n";
+            next;
+        }
+        
+        system("cp '$tmpdir/DIFF' '$patchesDir/$finalName.tmp'") == 0
             or die "cannot copy diff";
+
+        rename("$patchesDir/$finalName.tmp", "$patchesDir/$finalName")
+            or die "cannot rename $patchesDir/$finalName.tmp";
         
         # Add the patch to the manifest.
         addPatch \%dstPatches, $p,

scripts/readmanifest.pm.in

@@ -146,7 +146,7 @@ sub writeManifest
     my $patches = shift;
     my $successors = shift;
 
-    open MANIFEST, ">$manifest";
+    open MANIFEST, ">$manifest.tmp"; # !!! check exclusive
 
     foreach my $storePath (keys %{$narFiles}) {
         my $narFileList = $$narFiles{$storePath};
@@ -184,6 +184,9 @@ sub writeManifest
     
     
     close MANIFEST;
+
+    rename("$manifest.tmp", $manifest)
+        or die "cannot rename $manifest.tmp: $!";
 }