Add missing temproots for cached sources and existing derivations

src/libfetchers/fetch-to-store.cc

@@ -47,6 +47,12 @@ std::pair<StorePath, Hash> fetchToStore2(
             auto hash = Hash::parseSRI(fetchers::getStrAttr(*res, "hash"));
             auto storePath =
                 store.makeFixedOutputPathFromCA(name, ContentAddressWithReferences::fromParts(method, hash, {}));
+
+            /* Add a temproot before the call to isValidPath to prevent accidental GC in case the
+               input is cached. Note that this must be done before to avoid races. */
+            if (mode != FetchMode::DryRun)
+                store.addTempRoot(storePath);
+
             if (mode == FetchMode::DryRun || store.isValidPath(storePath)) {
                 debug(
                     "source path '%s' cache hit in '%s' (hash '%s')",

src/libstore/derivations.cc

@@ -139,6 +139,12 @@ StorePath Store::writeDerivation(const Derivation & drv, RepairFlag repair)
 {
     auto [suffix, contents, references, path] = infoForDerivation(*this, drv);
 
+    /* In case the derivation is already valid, we bail out early since that's
+       faster. But we need to make sure that the derivation has a corresponding
+       temproot. It is added by the remote in addToStoreFromDump, but we'd like
+       to avoid sending a lot of drv contents to the daemon. */
+    addTempRoot(path);
+
     if (isValidPath(path) && !repair)
         return path;