Eval cache: fix cache regressions

- Fix eval cache not being persisted in `nix develop` (since #10570)
- Fallback to flake path so the eval cache isn't disabled for flakes evaluated by a relative `path:` (since #10149, #10176)
- Don't attempt to commit cache transaction if there is no active transaction, which will spew errors in edge cases
- Drive-by: trivial typo fix

src/libcmd/installable-value.hh

@@ -66,7 +66,7 @@ struct ExtraPathInfoValue : ExtraPathInfo
 };
 
 /**
- * An Installable which corresponds a Nix langauge value, in addition to
+ * An Installable which corresponds a Nix language value, in addition to
  * a collection of \ref DerivedPath "derived paths".
  */
 struct InstallableValue : Installable

src/libexpr/eval-cache.cc

@@ -95,7 +95,7 @@ struct AttrDb
     {
         try {
             auto state(_state->lock());
-            if (!failed)
+            if (!failed && state->txn->active)
                 state->txn->commit();
             state->txn.reset();
         } catch (...) {

src/libflake/flake/flake.cc

@@ -947,8 +947,10 @@ std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
 {
     if (lockFile.isUnlocked()) return std::nullopt;
 
-    auto fingerprint = flake.lockedRef.input.getFingerprint(store);
-    if (!fingerprint) return std::nullopt;
+    auto fingerprint = flake.lockedRef.input.getFingerprint(store)
+        // Fingerprints are null for relative path references so we use
+        // the flake path as a fallback to avoid losing caching.
+        .value_or(flake.path.to_string());
 
     *fingerprint += fmt(";%s;%s", flake.lockedRef.subdir, lockFile);
 

src/nix/develop.cc

@@ -697,6 +697,10 @@ struct CmdDevelop : Common, MixEnvironment
             }
         }
 
+        // Release our references to eval caches to ensure they are persisted to disk, because
+        // we are about to exec out of this process without running C++ destructors.
+        getEvalState()->evalCaches.clear();
+
         runProgramInStore(store, UseLookupPath::Use, shell, args, buildEnvironment.getSystem());
 #endif
     }