* Add a test for a more subtle race: a process starting after the

temporary root files have been read but creating outputs before the
  store directory has been read.

src/libstore/gc.cc

@@ -12,10 +12,24 @@
 #include <unistd.h>
 
 
+/* Acquire the global GC lock. */
+static AutoCloseFD openGCLock(LockType lockType)
+{
+#if 0
+    Path fnGCLock = (format("%1%/%2%/%3%")
+        % nixStateDir % tempRootsDir % getpid()).str();
+        
+    fdTempRoots = open(fnTempRoots.c_str(), O_RDWR | O_CREAT | O_TRUNC, 0600);
+    if (fdTempRoots == -1)
+        throw SysError(format("opening temporary roots file `%1%'") % fnTempRoots);
+#endif
+}
+
+
 static string tempRootsDir = "temproots";
 
 /* The file to which we write our temporary roots. */
-Path fnTempRoots;
+static Path fnTempRoots;
 static AutoCloseFD fdTempRoots;
 
 
@@ -213,6 +227,10 @@ void collectGarbage(const PathSet & roots, GCAction action,
         else
             tempRootsClosed.insert(*i);
 
+    /* For testing - see tests/gc-concurrent.sh. */
+    if (getenv("NIX_DEBUG_GC_WAIT"))
+        sleep(2);
+    
     /* After this point the set of roots or temporary roots cannot
        increase, since we hold locks on everything.  So everything
        that is not currently in in `livePaths' or `tempRootsClosed'

tests/Makefile.am

@@ -34,12 +34,12 @@ fallback.sh: fallback.nix
 nix-push.sh: dependencies.nix
 nix-pull.sh: dependencies.nix
 gc.sh: dependencies.nix
-gc-concurrent.sh: gc-concurrent.nix
+gc-concurrent.sh: gc-concurrent.nix gc-concurrent2.nix
 
-TESTS = init.sh hash.sh lang.sh simple.sh dependencies.sh locking.sh parallel.sh \
+#TESTS = init.sh hash.sh lang.sh simple.sh dependencies.sh locking.sh parallel.sh \
-  build-hook.sh substitutes.sh substitutes2.sh fallback.sh nix-push.sh gc.sh \
+#  build-hook.sh substitutes.sh substitutes2.sh fallback.sh nix-push.sh gc.sh \
-  gc-concurrent.sh verify.sh nix-pull.sh
+#  gc-concurrent.sh verify.sh nix-pull.sh
-#TESTS = init.sh gc-concurrent.sh 
+TESTS = init.sh gc-concurrent.sh 
 
 XFAIL_TESTS =
 

tests/gc-concurrent.sh

@@ -1,21 +1,34 @@
-storeExpr=$($TOP/src/nix-instantiate/nix-instantiate gc-concurrent.nix)
+storeExpr1=$($TOP/src/nix-instantiate/nix-instantiate gc-concurrent.nix)
-outPath=$($TOP/src/nix-store/nix-store -q $storeExpr)
+outPath1=$($TOP/src/nix-store/nix-store -q $storeExpr1)
+
+storeExpr2=$($TOP/src/nix-instantiate/nix-instantiate gc-concurrent2.nix)
+outPath2=$($TOP/src/nix-store/nix-store -q $storeExpr2)
 
 ls -l test-tmp/state/temproots
 
+# Start build #1 in the background.  It starts immediately.
+$TOP/src/nix-store/nix-store -rvv "$storeExpr1" &
+pid1=$!
 
-# Start a build in the background.
+# Start build #2 in the background after 3 seconds.
-$TOP/src/nix-store/nix-store -rvv "$storeExpr" &
+(sleep 3 && $TOP/src/nix-store/nix-store -rvv "$storeExpr2") &
-pid=$!
+pid2=$!
 
-# Run the garbage collector while the build is running.
+# Run the garbage collector while the build is running.  Note: the GC
+# sleeps for *another* 2 seconds after acquiring the GC lock.  This
+# checks whether build #1
 sleep 2
-$NIX_BIN_DIR/nix-collect-garbage -vvvvv
+NIX_DEBUG_GC_WAIT=1 $NIX_BIN_DIR/nix-collect-garbage -vvvvv
 
-# Wait for the build to finish.
+# Wait for build #1/#2 to finish.
-echo waiting for pid $pid to finish...
+echo waiting for pid $pid1 to finish...
-wait $pid
+wait $pid1
+echo waiting for pid $pid2 to finish...
+wait $pid2
 
-# Check that the root and its dependencies haven't been deleted.
+# Check that the root of build #1 and its dependencies haven't been
-cat $outPath/foobar
+# deleted.
-cat $outPath/input-2/bar
+cat $outPath1/foobar
+cat $outPath1/input-2/bar
+
+cat $outPath2/foobar

tests/gc-concurrent2.builder.sh

@@ -0,0 +1,9 @@
+export PATH=/bin:/usr/bin:$PATH
+
+mkdir $out
+echo $(cat $input1/foo)$(cat $input2/bar)xyzzy > $out/foobar
+
+# Check that the GC hasn't deleted the lock on our output.
+test -e "$out.lock"
+
+sleep 3

tests/gc-concurrent2.nix.in

@@ -0,0 +1,25 @@
+let {
+
+  input1 = derivation {
+    name = "dependencies-input-1";
+    system = "@system@";
+    builder = "@shell@";
+    args = ["-e" "-x" ./dependencies.builder1.sh];
+  };
+
+  input2 = derivation {
+    name = "dependencies-input-2";
+    system = "@system@";
+    builder = "@shell@";
+    args = ["-e" "-x" ./dependencies.builder2.sh];
+  };
+
+  body = derivation {
+    name = "gc-concurrent2";
+    system = "@system@";
+    builder = "@shell@";
+    args = ["-e" "-x" ./gc-concurrent2.builder.sh];
+    inherit input1 input2;
+  };
+
+}