Support empty hash in fetchers

fetchTarball, fetchTree, and fetchGit all have *optional* hash attrs.
This means that we need to be careful with what we allow to avoid
accidentally making these defaults. When ‘hash = ""’ we assume the
empty hash is wanted.

1 files changed, 8 insertions, 3 deletions

diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index c5a0d9886..745f65adf 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -102,9 +102,14 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             string n(attr.name);
             if (n == "url")
                 url = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (n == "sha256")
-                expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
-            else if (n == "name")
+            else if (n == "sha256") {
+                auto hashStr = state.forceStringNoCtx(*attr.value, *attr.pos);
+                if (hashStr == "") {
+                    expectedHash = Hash(htSHA256);
+                    printError("warning: found empty hash, assuming you wanted '%s'", expectedHash->to_string());
+                } else
+                    expectedHash = Hash(hashStr, htSHA256);
+            } else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
                 throw EvalError("unsupported argument '%s' to '%s', at %s",