libexpr: add temp roots for external paths during evaluation

Protect store paths from previous sessions against concurrent GC
by adding addTempRoot at consumption sites in import and the
PackageInfo constructor. Current-session paths already have temp
roots from writeDerivation.

Author: martinjlowm

src/libexpr/get-drvs.cc

@@ -24,6 +24,11 @@ PackageInfo::PackageInfo(EvalState & state, ref<Store> store, const std::string
 
     this->drvPath = drvPath;
 
+    /* Prevent GC from deleting the .drv before we read it. This
+       constructor is only called from CLI entry points (nix-build)
+       where the path originates from a previous session, so it
+       has no temp root from the current process. */
+    store->addTempRoot(drvPath);
     auto drv = store->derivationFromPath(drvPath);
 
     name = drvPath.name();

src/libexpr/primops.cc

@@ -306,8 +306,14 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
     auto isValidDerivationInStore = [&]() -> std::optional<StorePath> {
         if (!state.store->isStorePath(path2))
             return std::nullopt;
+        if (!isDerivation(path2))
+            return std::nullopt;
         auto storePath = state.store->parseStorePath(path2);
-        if (!(state.store->isValidPath(storePath) && isDerivation(path2)))
+        /* Add a temp root before checking validity to prevent GC
+           from deleting the path between our check and the
+           subsequent readDerivation(). */
+        state.store->addTempRoot(storePath);
+        if (!state.store->isValidPath(storePath))
             return std::nullopt;
         return storePath;
     };