eval-cache: Add derivation result as temproot

Closes the window where the derivation could be garbage collected after
we checked isValidPath.

Author: dramforever

src/libexpr/eval-cache.cc

@@ -707,13 +707,16 @@ StorePath AttrCursor::forceDerivation()
     auto aDrvPath = getAttr(root->state.s.drvPath);
     auto drvPath = root->state.store->parseStorePath(aDrvPath->getString());
     drvPath.requireDerivation();
-    if (!root->state.store->isValidPath(drvPath) && !settings.readOnlyMode) {
-        /* The eval cache contains 'drvPath', but the actual path has
-           been garbage-collected. So force it to be regenerated. */
-        aDrvPath->forceValue();
-        if (!root->state.store->isValidPath(drvPath))
-            throw Error(
-                "don't know how to recreate store derivation '%s'!", root->state.store->printStorePath(drvPath));
+    if (!settings.readOnlyMode) {
+        root->state.store->addTempRoot(drvPath);
+        if (!root->state.store->isValidPath(drvPath)) {
+            /* The eval cache contains 'drvPath', but the actual path has
+               been garbage-collected. So force it to be regenerated. */
+            aDrvPath->forceValue();
+            if (!root->state.store->isValidPath(drvPath))
+                throw Error(
+                    "don't know how to recreate store derivation '%s'!", root->state.store->printStorePath(drvPath));
+        }
     }
     return drvPath;
 }