libexpr: add temp roots during evaluation to prevent GC races

src/libexpr/eval.cc

@@ -1041,6 +1041,7 @@ std::string EvalState::mkSingleDerivedPathStringRaw(const SingleDerivedPath & p)
                 auto optStaticOutputPath = std::visit(
                     overloaded{
                         [&](const SingleDerivedPath::Opaque & o) {
+                            store->addTempRoot(o.path);
                             waitForPath(o.path);
                             auto drv = store->readDerivation(o.path);
                             auto i = drv.outputs.find(b.output);

src/libexpr/get-drvs.cc

@@ -25,6 +25,9 @@ PackageInfo::PackageInfo(EvalState & state, ref<Store> store, const std::string
 
     this->drvPath = drvPath;
 
+    /* Prevent GC from deleting the .drv between now and when the
+       build phase adds its own temp root. */
+    store->addTempRoot(drvPath);
     auto drv = store->derivationFromPath(drvPath);
 
     name = drvPath.name();

src/libexpr/primops.cc

@@ -309,6 +309,10 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         if (!state.store->isStorePath(path2))
             return std::nullopt;
         auto storePath = state.store->parseStorePath(path2);
+        /* Add a temp root before checking validity or reading the
+           derivation, to prevent GC from deleting the path between
+           our validity check and the subsequent readDerivation(). */
+        state.store->addTempRoot(storePath);
         state.waitForPath(storePath);
         if (!(state.store->isValidPath(storePath) && isDerivation(path2)))
             return std::nullopt;
@@ -1760,9 +1764,14 @@ static void derivationStrictInternal(
                     /* !!! This doesn't work if readOnlyMode is set. */
                     StorePathSet refs;
                     // FIXME: don't need to wait, we only need the references.
+                    state.store->addTempRoot(d.drvPath);
                     state.waitForPath(d.drvPath);
                     state.store->computeFSClosure(d.drvPath, refs);
                     for (auto & j : refs) {
+                        /* Prevent GC from deleting derivations in the
+                           closure before we can read them. */
+                        if (j.isDerivation())
+                            state.store->addTempRoot(j);
                         drv.inputSrcs.insert(j);
                         if (j.isDerivation()) {
                             drv.inputDrvs.map[j].value = state.store->readDerivation(j).outputNames();