Allow empty hash in derivations

follow up of https://github.com/NixOS/nix/pull/3544

This allows hash="" so that it can be used for debugging purposes. For
instance, this gives you an error message like:

  warning: found empty hash, assuming you wanted 'sha256:0000000000000000000000000000000000000000000000000000'
  hash mismatch in fixed-output derivation '/nix/store/asx6qw1r1xk6iak6y6jph4n58h4hdmbm-nix':
    wanted: sha256:0000000000000000000000000000000000000000000000000000
    got:    sha256:0fpfhipl9v1mfzw2ffmxiyyzqwlkvww22bh9wcy4qrfslb4jm429

1 files changed, 7 insertions, 1 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index d458ab272..de04fd2be 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -719,7 +719,13 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             throw Error(format("multiple outputs are not supported in fixed-output derivations, at %1%") % posDrvName);
 
         HashType ht = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
-        Hash h(*outputHash, ht);
+
+        Hash h;
+        if (outputHash->empty()) {
+            h = Hash(ht);
+            printError("warning: found empty hash, assuming you wanted '%s'", h.to_string());
+        } else
+            h = Hash(*outputHash, ht);
 
         auto outPath = state.store->makeFixedOutputPath(ingestionMethod, h, drvName);
         if (!jsonObject) drv.env["out"] = state.store->printStorePath(outPath);