libstore/build: pathContentsGood is physical path aware now

`pathContentsGood` is used to assess the validity of a path as part of
derivation goals *in repair mode*.

When repair is used with a diverted store, i.e. a store where
fsPath(toRealPath(s)) != fsPath(s) for s a store path, this result in
utterly broken behavior because it will attempt to assess the goodness
of the *logical* store locations, most of the time: /nix/store/...

So, if you are repairing your system using a live NixOS ISO. Your ISO
contains a `/nix/store` (assumed to be good) and you repair your system
which is rooted at /mnt and contains its own /nix/store, that is, a Nix
store at /mnt/nix/store.

Performing the following operation `nix-store --verify --repair --store
/mnt` will assess the contents goodness of the ISO's Nix store.

To avoid this, we assess the path existence of the *physical path*, aka
the result of `store.toRealPath` applied to a *logical* store path
string representation and we verify the hash of the *physical path*.

The error messages are not taken care of in this CL as those are purely
cosmetic and helps the user understand what is going on.

Fixes #892.

Change-Id: Ib9e0153cb5683edcf37f1963ebf065ceba5e5dfb
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libstore/build/worker.cc

@@ -389,10 +389,11 @@ try {
     printInfo("checking path '%s'...", store.toRealPath(store.printStorePath(path)));
     auto info = TRY_AWAIT(store.queryPathInfo(path));
     bool res;
-    if (!pathExists(store.printStorePath(path)))
+    if (!pathExists(store.toRealPath(store.printStorePath(path)))) {
         res = false;
-    else {
-        HashResult current = hashPath(info->narHash.type, store.printStorePath(path));
+    } else {
+        HashResult current =
+            hashPath(info->narHash.type, store.toRealPath(store.printStorePath(path)));
         Hash nullHash(HashType::SHA256);
         res = info->narHash == nullHash || info->narHash == current.first;
     }

tests/functional/meson.build

@@ -67,6 +67,7 @@ functional_tests_scripts = [
   'nix-build.sh',
   'gc-concurrent.sh',
   'repair.sh',
+  'repair-chroot.sh',
   'fixed.sh',
   'export-graph.sh',
   'timeout.sh',

tests/functional/repair-chroot.sh

@@ -0,0 +1,36 @@
+source common.sh
+
+if [[ $(uname) == Darwin ]]; then skipTest "Darwin does not have diverted stores"; fi
+needLocalStore "--repair needs a local store"
+
+clearStore
+
+path=$(nix-build dependencies.nix -o $TEST_ROOT/result)
+path2=$(nix-store -qR $path | grep input-2)
+path2_basename=$(basename $path2)
+
+# Corrupt a path in a *chroot* store and check whether nix-build --repair can fix it.
+extra_chroot_store_path="$TEST_ROOT/extra-chroot-store"
+# Required because otherwise storeDir=/build/... in the sandbox.
+special_sandbox_build_dir="/build-tmp"
+chroot_path=$(nix-build dependencies.nix -o $TEST_ROOT/chroot-result --sandbox-build-dir "$special_sandbox_build_dir" --store "$extra_chroot_store_path" --extra-sandbox-paths /nix/store)
+chroot_path2="$extra_chroot_store_path/nix/store/$path2_basename"
+
+[ -d $chroot_path2 ] || fail "chroot path $chroot_path2 does not exist"
+
+nix-store --verify --check-contents --sandbox-build-dir "$special_sandbox_build_dir" -v --store "$extra_chroot_store_path" |& grepQuiet "$extra_chroot_store_path" || fail "$extra_chroot_store_path does not occur in the store verification, the chroot store parameter is ignored."
+chroot_hash=$(nix-hash $path2)
+
+chmod u+w $chroot_path2
+touch $chroot_path2/bad
+
+! nix-store --verify --check-contents -v --store "$extra_chroot_store_path" --sandbox-build-dir "$special_sandbox_build_dir"
+
+# The path can be repaired by rebuilding the derivation.
+nix-store --verify --check-contents --repair --store "$extra_chroot_store_path" --extra-sandbox-paths /nix/store --sandbox-build-dir "$special_sandbox_build_dir"
+
+! [ -e $chroot_path2/bad ]
+! [ -w $chroot_path2 ]
+
+# NOTE: verify path must be given the *LOGICAL* path here. And not the physical path. Confusing? I know.
+nix-store --verify-path $path2 --store "$extra_chroot_store_path"