Add test for readFile keeping context

tests/local.mk

@@ -59,7 +59,8 @@ nix_tests = \
   ca/recursive.sh \
   ca/concurrent-builds.sh \
   ca/nix-copy.sh \
-  eval-store.sh
+  eval-store.sh \
+  readfile-context.sh
   # parallel.sh
 
 ifeq ($(HAVE_LIBCPUID), 1)

tests/readfile-context.builder.sh

@@ -0,0 +1 @@
+echo "$input" > $out

tests/readfile-context.nix

@@ -0,0 +1,19 @@
+with import ./config.nix;
+
+let
+
+  input = import ./simple.nix;
+
+  dependent = mkDerivation {
+    name = "dependent";
+    builder = ./readfile-context.builder.sh;
+    input = "${input}/hello";
+  };
+
+  readDependent = mkDerivation {
+    name = "read-dependent";
+    builder = ./readfile-context.builder.sh;
+    input = builtins.readFile dependent;
+  };
+
+in readDependent

tests/readfile-context.sh

@@ -0,0 +1,16 @@
+source common.sh
+
+clearStore
+
+outPath=$(nix-build --no-out-link readfile-context.nix)
+
+# Set a GC root.
+ln -s $outPath "$NIX_STATE_DIR"/gcroots/foo
+
+# Check that file exists.
+[ "$(cat $(cat $outPath))" = "Hello World!" ]
+
+nix-collect-garbage
+
+# Check that file still exists.
+[ "$(cat $(cat $outPath))" = "Hello World!" ]