libstore/profiles: do not dereference current generation if it doesn't exist

If the profile inode is invalid, e.g. invalid symlink, the current
generation cannot be discovered.

Nonetheless, this should not be a reason for an assert failure, instead
of crashing, just raise an error.

Fixes fj#801.

Change-Id: I63937672173bc3bf37196de98307800adc5757e1
Signed-off-by: Raito Bezarius <raito@lix.systems>
Co-authored-by: Qyriad <qyriad@qyriad.me>

lix/libstore/profiles.cc

@@ -153,8 +153,11 @@ void deleteGenerations(
 
     auto [gens, curGen] = findGenerations(profile);
 
+    if (!curGen)
+        throw Error("cannot find current generation of profile '%1%', is it a valid symlink?", profile);
+
     if (gensToDelete.count(*curGen))
-        throw Error("cannot delete current version of profile %1%'", profile);
+        throw Error("cannot delete current version of profile '%1%'", profile);
 
     for (auto & i : gens) {
         if (!gensToDelete.count(i.number)) continue;

tests/functional/common/vars-and-functions.sh.in

@@ -68,6 +68,11 @@ clearProfiles() {
     rm -rf "$profiles"
 }
 
+linkProfilesTo() {
+    target="$1"
+    ln -s "$HOME"/.local/state/nix/profiles "$target"
+}
+
 clearStore() {
     echo "clearing store..."
     chmod -R +w "$NIX_STORE_DIR"

tests/functional/user-envs.sh

@@ -4,6 +4,13 @@ if [ -z "${storeCleared-}" ]; then
     clearStore
 fi
 
+# Test if invalid profiles symlinks are supported.
+# https://git.lix.systems/lix-project/lix/issues/801
+VOID=$(mktemp -d)
+rm -rf "$VOID"
+linkProfilesTo $VOID
+expectStderr 1 nix-env --delete-generations 0 | grepQuiet -E "error: cannot find current generation of profile '.*local/state/nix/profiles/profile', is it a valid symlink?"
+
 clearProfiles
 
 # Query installed: should be empty.