From 10f9a8e77d504354deafbcf7351a6d46d4528542 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 15 Oct 2021 16:52:37 +0200
Subject: [PATCH] Add a test for the non-blocking GC

---
 src/libstore/gc.cc       |  4 ++++
 tests/gc-non-blocking.sh | 33 +++++++++++++++++++++++++++++++++
 tests/local.mk           |  1 +
 3 files changed, 38 insertions(+)
 create mode 100644 tests/gc-non-blocking.sh

diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 648295684..5e8cbf8fc 100644
--- a/src/libstore/gc.cc
+++ b/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) {
diff --git a/tests/gc-non-blocking.sh b/tests/gc-non-blocking.sh
new file mode 100644
index 000000000..8b21c6f1c
--- /dev/null
+++ b/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
diff --git a/tests/local.mk b/tests/local.mk
index d88c3a875..8d454ee19 100644
--- a/tests/local.mk
+++ b/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 \