If we can't write a lock file, pretend the top-level flake is dirty

Alternative to #4639. You can still read flake.lock, but at least in
reproducible workflows like NixOS configurations where you require a
non-dirty tree, evaluation will fail because there is no rev.

src/libexpr/flake/flake.cc

@@ -635,8 +635,10 @@ LockedFlake lockFlake(
                     }
                 } else
                     throw Error("cannot write modified lock file of flake '%s' (use '--no-write-lock-file' to ignore)", topRef);
-            } else
+            } else {
                 warn("not writing modified lock file of flake '%s':\n%s", topRef, chomp(diff));
+                flake.forceDirty = true;
+            }
         }
 
         return LockedFlake { .flake = std::move(flake), .lockFile = std::move(newLockFile) };
@@ -659,7 +661,13 @@ void callFlake(EvalState & state,
 
     mkString(*vLocks, lockedFlake.lockFile.to_string());
 
-    emitTreeAttrs(state, *lockedFlake.flake.sourceInfo, lockedFlake.flake.lockedRef.input, *vRootSrc);
+    emitTreeAttrs(
+        state,
+        *lockedFlake.flake.sourceInfo,
+        lockedFlake.flake.lockedRef.input,
+        *vRootSrc,
+        false,
+        lockedFlake.flake.forceDirty);
 
     mkString(*vRootSubdir, lockedFlake.flake.lockedRef.subdir);
 

src/libexpr/flake/flake.hh

@@ -61,6 +61,7 @@ struct Flake
     FlakeRef originalRef; // the original flake specification (by the user)
     FlakeRef resolvedRef; // registry references and caching resolved to the specific underlying flake
     FlakeRef lockedRef; // the specific local store result of invoking the fetcher
+    bool forceDirty = false; // pretend that 'lockedRef' is dirty
     std::optional<std::string> description;
     std::shared_ptr<const fetchers::Tree> sourceInfo;
     FlakeInputs inputs;
@@ -140,6 +141,8 @@ void emitTreeAttrs(
     EvalState & state,
     const fetchers::Tree & tree,
     const fetchers::Input & input,
-    Value & v, bool emptyRevFallback = false);
+    Value & v,
+    bool emptyRevFallback = false,
+    bool forceDirty = false);
 
 }

src/libexpr/primops/fetchTree.cc

@@ -16,7 +16,8 @@ void emitTreeAttrs(
     const fetchers::Tree & tree,
     const fetchers::Input & input,
     Value & v,
-    bool emptyRevFallback)
+    bool emptyRevFallback,
+    bool forceDirty)
 {
     assert(input.isImmutable());
 
@@ -33,6 +34,12 @@ void emitTreeAttrs(
     mkString(*state.allocAttr(v, state.symbols.create("narHash")),
         narHash->to_string(SRI, true));
 
+    if (input.getType() == "git")
+        mkBool(*state.allocAttr(v, state.symbols.create("submodules")),
+            fetchers::maybeGetBoolAttr(input.attrs, "submodules").value_or(true));
+
+    if (!forceDirty) {
+
         if (auto rev = input.getRev()) {
             mkString(*state.allocAttr(v, state.symbols.create("rev")), rev->gitRev());
             mkString(*state.allocAttr(v, state.symbols.create("shortRev")), rev->gitShortRev());
@@ -43,15 +50,13 @@ void emitTreeAttrs(
             mkString(*state.allocAttr(v, state.symbols.create("shortRev")), emptyHash.gitShortRev());
         }
 
-    if (input.getType() == "git")
-        mkBool(*state.allocAttr(v, state.symbols.create("submodules")),
-            fetchers::maybeGetBoolAttr(input.attrs, "submodules").value_or(true));
-
         if (auto revCount = input.getRevCount())
             mkInt(*state.allocAttr(v, state.symbols.create("revCount")), *revCount);
         else if (emptyRevFallback)
             mkInt(*state.allocAttr(v, state.symbols.create("revCount")), 0);
 
+    }
+
     if (auto lastModified = input.getLastModified()) {
         mkInt(*state.allocAttr(v, state.symbols.create("lastModified")), *lastModified);
         mkString(*state.allocAttr(v, state.symbols.create("lastModifiedDate")),
@@ -167,7 +172,7 @@ static void fetchTree(
     if (state.allowedPaths)
         state.allowedPaths->insert(tree.actualPath);
 
-    emitTreeAttrs(state, tree, input2, v, params.emptyRevFallback);
+    emitTreeAttrs(state, tree, input2, v, params.emptyRevFallback, false);
 }
 
 static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, Value & v)