2 files changed, 11 insertions, 3 deletions
diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/primops/flake.cc
index c08c30c9c..9131080bf 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/primops/flake.cc
@@ -309,10 +309,17 @@ Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool impureIsAllowe
 
     state.forceAttrs(vInfo);
 
+    if (auto epoch = vInfo.attrs->get(state.symbols.create("epoch"))) {
+        flake.epoch = state.forceInt(*(**epoch).value, *(**epoch).pos);
+        if (flake.epoch > 2019)
+            throw Error("flake '%s' requires unsupported epoch %d; please upgrade Nix", flakeRef, flake.epoch);
+    } else
+        throw Error("flake '%s' lacks attribute 'epoch'", flakeRef);
+
     if (auto name = vInfo.attrs->get(state.sName))
         flake.id = state.forceStringNoCtx(*(**name).value, *(**name).pos);
     else
-        throw Error("flake lacks attribute 'name'");
+        throw Error("flake '%s' lacks attribute 'name'", flakeRef);
 
     if (auto description = vInfo.attrs->get(state.sDescription))
         flake.description = state.forceStringNoCtx(*(**description).value, *(**description).pos);
@@ -337,7 +344,7 @@ Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool impureIsAllowe
         state.forceFunction(*(**provides).value, *(**provides).pos);
         flake.vProvides = (**provides).value;
     } else
-        throw Error("flake lacks attribute 'provides'");
+        throw Error("flake '%s' lacks attribute 'provides'", flakeRef);
 
     return flake;
 }
diff --git a/src/libexpr/primops/flake.hh b/src/libexpr/primops/flake.hh
index 8eaac9d96..983c0eab6 100644
--- a/src/libexpr/primops/flake.hh
+++ b/src/libexpr/primops/flake.hh
@@ -101,7 +101,8 @@ struct Flake
     std::map<FlakeAlias, FlakeRef> nonFlakeRequires;
     Value * vProvides; // FIXME: gc
     // date
-    // content hash
+    unsigned int epoch;
+
     Flake(const FlakeRef & origRef, const SourceInfo & sourceInfo) : originalRef(origRef),
         resolvedRef(sourceInfo.resolvedRef), revCount(sourceInfo.revCount), storePath(sourceInfo.storePath) {};
 };