Add a test for the non-blocking GC

src/libstore/gc.cc

@@ -735,6 +735,10 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         }
     };
 
+    /* Synchronisation point for testing, see tests/gc-concurrent.sh. */
+    if (auto p = getEnv("_NIX_TEST_GC_SYNC"))
+        readFile(*p);
+
     /* Either delete all garbage paths, or just the specified
        paths (for gcDeleteSpecific). */
     if (options.action == GCOptions::gcDeleteSpecific) {

tests/gc-non-blocking.sh

@@ -0,0 +1,33 @@
+# Test whether the collector is non-blocking, i.e. a build can run in
+# parallel with it.
+source common.sh
+
+needLocalStore "the GC test needs a synchronisation point"
+
+clearStore
+
+fifo=$TEST_ROOT/test.fifo
+mkfifo "$fifo"
+
+dummy=$(nix store add-path ./simple.nix)
+
+running=$TEST_ROOT/running
+touch $running
+
+(_NIX_TEST_GC_SYNC=$fifo nix-store --gc -vvvvv; rm $running) &
+pid=$!
+
+sleep 2
+
+outPath=$(nix-build -o "$TEST_ROOT/result" -E "
+  with import ./config.nix;
+  mkDerivation {
+    name = \"non-blocking\";
+    buildCommand = \"set -x; test -e $running; mkdir \$out; echo > $fifo\";
+  }")
+
+wait $pid
+
+(! test -e $running)
+(! test -e $dummy)
+test -e $outPath

tests/local.mk

@@ -4,6 +4,7 @@ nix_tests = \
   gc.sh \
   ca/gc.sh \
   gc-concurrent.sh \
+  gc-non-blocking.sh \
   gc-auto.sh \
   referrers.sh user-envs.sh logging.sh nix-build.sh misc.sh fixed.sh \
   gc-runtime.sh check-refs.sh filter-source.sh \