From e414bde6f9f58e599d48307ff3cb0ab64cb47d9a Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 22 May 2019 14:31:40 +0200
Subject: [PATCH] Check the flake epoch

Closes #2883.
---
 src/libexpr/primops/flake.cc | 11 +++++++++--
 src/libexpr/primops/flake.hh |  3 ++-
 src/nix/flake.cc             |  2 ++
 tests/flakes.sh              |  6 +++++-
 4 files changed, 18 insertions(+), 4 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) {};
 };
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index ecbb3b81f..2dcdfc663 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -83,6 +83,7 @@ void printFlakeInfo(const Flake & flake, bool json) {
         if (flake.revCount)
             j["revCount"] = *flake.revCount;
         j["path"] = flake.storePath;
+        j["epoch"] = flake.epoch;
         std::cout << j.dump(4) << std::endl;
     } else {
         std::cout << "ID:          " << flake.id << "\n";
@@ -95,6 +96,7 @@ void printFlakeInfo(const Flake & flake, bool json) {
         if (flake.revCount)
             std::cout << "Revcount:    " << *flake.revCount << "\n";
         std::cout << "Path:        " << flake.storePath << "\n";
+        std::cout << "Epoch:       " << flake.epoch << "\n";
     }
 }
 
diff --git a/tests/flakes.sh b/tests/flakes.sh
index 5137bc39a..6c987ad14 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -146,7 +146,7 @@ nix build -o $TEST_ROOT/result file://$flake2Dir:bar
 # Test whether indirect dependencies work.
 nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:xyzzy
 
-# Add dependency to flake3
+# Add dependency to flake3.
 rm $flake3Dir/flake.nix
 
 cat > $flake3Dir/flake.nix <<EOF
@@ -174,3 +174,7 @@ nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth
 
 # Check whether it saved the lockfile
 [[ ! (-z $(git -C $flake3Dir diff master)) ]]
+
+# Unsupported epochs should be an error.
+sed -i $flake3Dir/flake.nix -e s/2019/2030/
+nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth 2>&1 | grep 'unsupported epoch'