Test nested sandboxing, and make nicer error

We were bedeviled by sandboxing issues when working on the layered
store. The problem ended up being that when we have nested nix builds,
and the inner store is inside the build dir (e.g. store is
`/build/nix-test/$name/store`, build dir is `/build`) bind mounts
clobber each other and store paths cannot be found.

After thoroughly cleaning up `local-derivation-goal.cc`, we might be
able to make that work. But that is a lot of work. For now, we just fail
earlier with a proper error message.

Finally, test this: nested sandboxing without the problematic store dir
should work, and with should fail with the expected error message.

src/libstore/build/local-derivation-goal.cc

@@ -594,6 +594,10 @@ void LocalDerivationGoal::startBuilder()
             else
                 dirsInChroot[i.substr(0, p)] = {i.substr(p + 1), optional};
         }
+        if (hasPrefix(worker.store.storeDir, tmpDirInSandbox))
+        {
+            throw Error("Inside the sandbox, `sandbox-build-dir` must not contain `store?store`");
+        }
         dirsInChroot[tmpDirInSandbox] = tmpDir;
 
         /* Add the closure of store paths to the chroot. */

tests/local.mk

@@ -138,7 +138,8 @@ nix_tests = \
   path-from-hash-part.sh \
   test-libstoreconsumer.sh \
   toString-path.sh \
-  read-only-store.sh
+  read-only-store.sh \
+  nested-sandboxing.sh
 
 ifeq ($(HAVE_LIBCPUID), 1)
 	nix_tests += compute-levels.sh

tests/nested-sandboxing.sh

@@ -0,0 +1,10 @@
+source common.sh
+[[ -d /nix/store ]] || skipTest "running this test without Nix's deps being drawn from /nix/store is not yet supported"
+
+requireSandboxSupport
+
+source ./nested-sandboxing/command.sh
+
+expectStderr 100 runNixBuild badStoreUrl 2 | grepQuiet '`sandbox-build-dir` must not contain'
+
+runNixBuild goodStoreUrl 5

tests/nested-sandboxing/command.sh

@@ -0,0 +1,29 @@
+export NIX_BIN_DIR=$(dirname $(type -p nix))
+# TODO Get Nix and its closure more flexibly
+export EXTRA_SANDBOX="/nix/store $(dirname $NIX_BIN_DIR)"
+
+badStoreUrl () {
+    local count=$1
+    echo $TEST_ROOT/store-$count
+}
+
+goodStoreUrl () {
+    local count=$1
+    echo $("badStoreUrl" "$count")?store=/foo-$count
+}
+
+# The non-standard sandbox-build-dir helps ensure that we get the same behavior
+# whether this test is being run in a derivation as part of the nix build or
+# being manually run by a developer outside a derivation
+runNixBuild () {
+    local storeFun=$1
+    local count=$2
+    nix-build \
+        --no-substitute --no-out-link \
+        --store "$("$storeFun" "$count")" \
+        --extra-sandbox-paths "$EXTRA_SANDBOX" \
+        ./nested-sandboxing/runner.nix \
+        --arg count "$((count - 1))" \
+        --argstr storeFun "$storeFun" \
+        --sandbox-build-dir /build-non-standard
+}

tests/nested-sandboxing/runner.nix

@@ -0,0 +1,24 @@
+{ count, storeFun }:
+
+with import ../config.nix;
+
+mkDerivation {
+  name = "nested-sandboxing";
+  busybox = builtins.getEnv "busybox";
+  EXTRA_SANDBOX = builtins.getEnv "EXTRA_SANDBOX";
+  buildCommand = if count == 0 then ''
+    echo Deep enough! > $out
+  '' else ''
+    cp -r ${../common} ./common
+    cp ${../common.sh} ./common.sh
+    cp ${../config.nix} ./config.nix
+    cp -r ${./.} ./nested-sandboxing
+
+    export PATH=${builtins.getEnv "NIX_BIN_DIR"}:$PATH
+
+    source common.sh
+    source ./nested-sandboxing/command.sh
+
+    runNixBuild ${storeFun} ${toString count} >> $out
+  '';
+}