Change the lock file to a graph

This enables support for cycles between flakes.

6 files changed, 294 insertions, 244 deletions

diff --git a/src/libexpr/flake/call-flake.nix b/src/libexpr/flake/call-flake.nix
index c59a0c75f..b9088e550 100644
--- a/src/libexpr/flake/call-flake.nix
+++ b/src/libexpr/flake/call-flake.nix
@@ -1,22 +1,27 @@
-locks: rootSrc: rootSubdir:
+lockFileStr: rootSrc: rootSubdir:
 
 let
 
-  callFlake = sourceInfo: subdir: locks:
-    let
-      flake = import (sourceInfo + "/" + subdir + "/flake.nix");
+  lockFile = builtins.fromJSON lockFileStr;
 
-      inputs = builtins.mapAttrs (n: v:
-        if v.flake or true
-        then callFlake (fetchTree (removeAttrs v.locked ["dir"])) (v.locked.dir or "") v.inputs
-        else fetchTree v.locked) locks;
+  allNodes =
+    builtins.mapAttrs
+      (key: node:
+        let
+          sourceInfo = if key == lockFile.root then rootSrc else fetchTree (removeAttrs node.locked ["dir"]);
+          subdir = if key == lockFile.root then rootSubdir else node.locked.dir or "";
+          flake = import (sourceInfo + (if subdir != "" then "/" else "") + subdir + "/flake.nix");
+          inputs = builtins.mapAttrs (inputName: key: allNodes.${key}) (node.inputs or {});
+          outputs = flake.outputs (inputs // { self = result; });
+          result = outputs // sourceInfo // { inherit inputs; inherit outputs; inherit sourceInfo; };
+        in
+          if node.flake or true then
+            assert flake.edition or flake.epoch or 0 == 201909;
+            assert builtins.isFunction flake.outputs;
+            result
+          else
+            sourceInfo
+      )
+      lockFile.nodes;
 
-      outputs = flake.outputs (inputs // { self = result; });
-
-      result = outputs // sourceInfo // { inherit inputs; inherit outputs; inherit sourceInfo; };
-    in
-      assert flake.edition == 201909;
-
-      result;
-
-in callFlake rootSrc rootSubdir (builtins.fromJSON locks).inputs
+in allNodes.${lockFile.root}
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 249ceba6c..c58b0eea9 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -293,52 +293,6 @@ Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup
     return getFlake(state, originalRef, {}, allowLookup, flakeCache);
 }
 
-static void flattenLockFile(
-    const LockedInputs & inputs,
-    const InputPath & prefix,
-    std::map<InputPath, const LockedInput *> & res)
-{
-    for (auto &[id, input] : inputs.inputs) {
-        auto inputPath(prefix);
-        inputPath.push_back(id);
-        res.emplace(inputPath, &input);
-        flattenLockFile(input, inputPath, res);
-    }
-}
-
-static std::string diffLockFiles(const LockedInputs & oldLocks, const LockedInputs & newLocks)
-{
-    std::map<InputPath, const LockedInput *> oldFlat, newFlat;
-    flattenLockFile(oldLocks, {}, oldFlat);
-    flattenLockFile(newLocks, {}, newFlat);
-
-    auto i = oldFlat.begin();
-    auto j = newFlat.begin();
-    std::string res;
-
-    while (i != oldFlat.end() || j != newFlat.end()) {
-        if (j != newFlat.end() && (i == oldFlat.end() || i->first > j->first)) {
-            res += fmt("* Added '%s': '%s'\n", concatStringsSep("/", j->first), j->second->lockedRef);
-            ++j;
-        } else if (i != oldFlat.end() && (j == newFlat.end() || i->first < j->first)) {
-            res += fmt("* Removed '%s'\n", concatStringsSep("/", i->first));
-            ++i;
-        } else {
-            if (!(i->second->lockedRef == j->second->lockedRef)) {
-                assert(i->second->lockedRef.to_string() != j->second->lockedRef.to_string());
-                res += fmt("* Updated '%s': '%s' -> '%s'\n",
-                    concatStringsSep("/", i->first),
-                    i->second->lockedRef,
-                    j->second->lockedRef);
-            }
-            ++i;
-            ++j;
-        }
-    }
-
-    return res;
-}
-
 /* Compute an in-memory lock file for the specified top-level flake,
    and optionally write it to file, it the flake is writable. */
 LockedFlake lockFlake(
@@ -380,8 +334,8 @@ LockedFlake lockFlake(
         /* Recurse into the flake inputs. */
         std::function<void(
             const FlakeInputs & flakeInputs,
-            const LockedInputs & oldLocks,
-            LockedInputs & newLocks,
+            std::shared_ptr<const Node> oldLocks,
+            std::shared_ptr<Node> newLocks,
             const InputPath & inputPathPrefix)>
             updateLocks;
 
@@ -389,8 +343,8 @@ LockedFlake lockFlake(
 
         updateLocks = [&](
             const FlakeInputs & flakeInputs,
-            const LockedInputs & oldLocks,
-            LockedInputs & newLocks,
+            std::shared_ptr<const Node> oldLocks,
+            std::shared_ptr<Node> newLocks,
             const InputPath & inputPathPrefix)
         {
             /* Get the overrides (i.e. attributes of the form
@@ -428,14 +382,16 @@ LockedFlake lockFlake(
                        input of the dwarffs input of the root flake),
                        but if it's from an override, it's relative to
                        the *root* of the lock file. */
-                    auto follows = (hasOverride ? newLockFile : newLocks).findInput(*input.follows);
+                    auto follows = (hasOverride ? newLockFile.root : newLocks)->findInput(*input.follows);
                     if (follows)
-                        newLocks.inputs.insert_or_assign(id, **follows);
+                        newLocks->inputs.insert_or_assign(id, follows);
                     else
                         /* We haven't processed the source of the
                            "follows" yet (e.g. "dwarffs/nixpkgs"). So
                            we'll need another round of the fixpoint
                            iteration. */
+                        // FIXME: now that LockFile is a graph, we
+                        // could pre-create the missing node.
                         unresolved.push_back(inputPath);
                     continue;
                 }
@@ -443,16 +399,25 @@ LockedFlake lockFlake(
                 /* Do we have an entry in the existing lock file? And
                    we don't have a --update-input flag for this
                    input? */
-                auto oldLock =
+                auto oldLockIt =
                     lockFlags.inputUpdates.count(inputPath)
-                    ? oldLocks.inputs.end()
-                    : oldLocks.inputs.find(id);
+                    ? oldLocks->inputs.end()
+                    : oldLocks->inputs.find(id);
+
+                std::shared_ptr<const LockedNode> oldLock;
+                if (oldLockIt != oldLocks->inputs.end()) {
+                    oldLock = std::dynamic_pointer_cast<const LockedNode>(oldLockIt->second);
+                    assert(oldLock);
+                }
 
-                if (oldLock != oldLocks.inputs.end() && oldLock->second.originalRef == input.ref && !hasOverride) {
+                if (oldLock
+                    && oldLock->originalRef == input.ref
+                    && !hasOverride)
+                {
                     /* Copy the input from the old lock file if its
                        flakeref didn't change and there is no override
                        from a higher level flake. */
-                    newLocks.inputs.insert_or_assign(id, oldLock->second);
+                    newLocks->inputs.insert_or_assign(id, std::make_shared<LockedNode>(*oldLock));
 
                     /* If we have an --update-input flag for an input
                        of this input, then we must fetch the flake to
@@ -466,11 +431,11 @@ LockedFlake lockFlake(
 
                     if (hasChildUpdate) {
                         auto inputFlake = getFlake(
-                            state, oldLock->second.lockedRef, oldLock->second.info, false, flakeCache);
+                            state, oldLock->lockedRef, oldLock->info, false, flakeCache);
 
                         updateLocks(inputFlake.inputs,
-                            (const LockedInputs &) oldLock->second,
-                            newLocks.inputs.find(id)->second,
+                            oldLock,
+                            newLocks->inputs.find(id)->second,
                             inputPath);
 
                     } else {
@@ -480,12 +445,14 @@ LockedFlake lockFlake(
                            check those. */
                         FlakeInputs fakeInputs;
 
-                        for (auto & i : oldLock->second.inputs)
-                            fakeInputs.emplace(i.first, FlakeInput { .ref = i.second.originalRef });
+                        for (auto & i : oldLock->inputs)
+                            fakeInputs.emplace(i.first, FlakeInput {
+                                .ref = std::dynamic_pointer_cast<LockedNode>(i.second)->originalRef
+                            });
 
                         updateLocks(fakeInputs,
-                            oldLock->second,
-                            newLocks.inputs.find(id)->second,
+                            oldLock,
+                            newLocks->inputs.find(id)->second,
                             inputPath);
                     }
 
@@ -499,8 +466,8 @@ LockedFlake lockFlake(
                     if (input.isFlake) {
                         auto inputFlake = getFlake(state, input.ref, {}, lockFlags.useRegistries, flakeCache);
 
-                        newLocks.inputs.insert_or_assign(id,
-                            LockedInput(inputFlake.lockedRef, inputFlake.originalRef, inputFlake.sourceInfo->info));
+                        newLocks->inputs.insert_or_assign(id,
+                            std::make_shared<LockedNode>(inputFlake.lockedRef, inputFlake.originalRef, inputFlake.sourceInfo->info));
 
                         /* Recursively process the inputs of this
                            flake. Also, unless we already have this
@@ -515,25 +482,25 @@ LockedFlake lockFlake(
                         Finally cleanup([&]() { parents.pop_back(); });
 
                         updateLocks(inputFlake.inputs,
-                            oldLock != oldLocks.inputs.end()
-                            ? (const LockedInputs &) oldLock->second
+                            oldLock
+                            ? std::dynamic_pointer_cast<const Node>(oldLock)
                             : LockFile::read(
-                                inputFlake.sourceInfo->actualPath + "/" + inputFlake.lockedRef.subdir + "/flake.lock"),
-                            newLocks.inputs.find(id)->second,
+                                inputFlake.sourceInfo->actualPath + "/" + inputFlake.lockedRef.subdir + "/flake.lock").root,
+                            newLocks->inputs.find(id)->second,
                             inputPath);
                     }
 
                     else {
                         auto [sourceInfo, lockedRef] = fetchOrSubstituteTree(
                             state, input.ref, {}, lockFlags.useRegistries, flakeCache);
-                        newLocks.inputs.insert_or_assign(id,
-                            LockedInput(lockedRef, input.ref, sourceInfo.info, false));
+                        newLocks->inputs.insert_or_assign(id,
+                            std::make_shared<LockedNode>(lockedRef, input.ref, sourceInfo.info, false));
                     }
                 }
             }
         };
 
-        updateLocks(flake.inputs, oldLockFile, newLockFile, {});
+        updateLocks(flake.inputs, oldLockFile.root, newLockFile.root, {});
 
         /* Check if there is a cycle in the "follows" inputs. */
         if (!unresolved.empty() && unresolved == prevUnresolved) {
@@ -619,8 +586,7 @@ LockedFlake lockFlake(
 }
 
 void callFlake(EvalState & state,
-    const Flake & flake,
-    const LockedInputs & lockedInputs,
+    const LockedFlake & lockedFlake,
     Value & vRes)
 {
     auto vLocks = state.allocValue();
@@ -629,11 +595,11 @@ void callFlake(EvalState & state,
     auto vTmp1 = state.allocValue();
     auto vTmp2 = state.allocValue();
 
-    mkString(*vLocks, lockedInputs.to_string());
+    mkString(*vLocks, lockedFlake.lockFile.to_string());
 
-    emitTreeAttrs(state, *flake.sourceInfo, flake.lockedRef.input, *vRootSrc);
+    emitTreeAttrs(state, *lockedFlake.flake.sourceInfo, lockedFlake.flake.lockedRef.input, *vRootSrc);
 
-    mkString(*vRootSubdir, flake.lockedRef.subdir);
+    mkString(*vRootSubdir, lockedFlake.flake.lockedRef.subdir);
 
     static Value * vCallFlake = nullptr;
 
@@ -649,13 +615,6 @@ void callFlake(EvalState & state,
     state.callFunction(*vTmp2, *vRootSubdir, vRes, noPos);
 }
 
-void callFlake(EvalState & state,
-    const LockedFlake & lockedFlake,
-    Value & v)
-{
-    callFlake(state, lockedFlake.flake, lockedFlake.lockFile, v);
-}
-
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     callFlake(state,
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index e5c6c1bb0..88e386be0 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -96,13 +96,7 @@ LockedFlake lockFlake(
 
 void callFlake(
     EvalState & state,
-    const Flake & flake,
-    const LockedInputs & inputs,
-    Value & v);
-
-void callFlake(
-    EvalState & state,
-    const LockedFlake & resFlake,
+    const LockedFlake & lockedFlake,
     Value & v);
 
 }
diff --git a/src/libexpr/flake/lockfile.cc b/src/libexpr/flake/lockfile.cc
index fdbba44de..b7a9e20c1 100644
--- a/src/libexpr/flake/lockfile.cc
+++ b/src/libexpr/flake/lockfile.cc
@@ -67,9 +67,8 @@ static TreeInfo parseTreeInfo(const nlohmann::json & json)
     throw Error("attribute 'info' missing in lock file");
 }
 
-LockedInput::LockedInput(const nlohmann::json & json)
-    : LockedInputs(json)
-    , lockedRef(getFlakeRef(json, "url", "uri", "locked"))
+LockedNode::LockedNode(const nlohmann::json & json)
+    : lockedRef(getFlakeRef(json, "url", "uri", "locked"))
     , originalRef(getFlakeRef(json, "originalUrl", "originalUri", "original"))
     , info(parseTreeInfo(json))
     , isFlake(json.find("flake") != json.end() ? (bool) json["flake"] : true)
@@ -90,103 +89,139 @@ static nlohmann::json treeInfoToJson(const TreeInfo & info)
     return json;
 }
 
-nlohmann::json LockedInput::toJson() const
-{
-    auto json = LockedInputs::toJson();
-    json["original"] = fetchers::attrsToJson(originalRef.toAttrs());
-    json["locked"] = fetchers::attrsToJson(lockedRef.toAttrs());
-    json["info"] = treeInfoToJson(info);
-    if (!isFlake) json["flake"] = false;
-    return json;
-}
-
-StorePath LockedInput::computeStorePath(Store & store) const
+StorePath LockedNode::computeStorePath(Store & store) const
 {
     return info.computeStorePath(store);
 }
 
-LockedInputs::LockedInputs(const nlohmann::json & json)
-{
-    for (auto & i : json["inputs"].items())
-        inputs.insert_or_assign(i.key(), LockedInput(i.value()));
-}
-
-nlohmann::json LockedInputs::toJson() const
-{
-    nlohmann::json json;
-    {
-        auto j = nlohmann::json::object();
-        for (auto & i : inputs)
-            j[i.first] = i.second.toJson();
-        json["inputs"] = std::move(j);
-    }
-    return json;
-}
-
-std::string LockedInputs::to_string() const
-{
-    return toJson().dump(2);
-}
-
-bool LockedInputs::isImmutable() const
-{
-    for (auto & i : inputs)
-        if (!i.second.lockedRef.input->isImmutable() || !i.second.isImmutable()) return false;
-
-    return true;
-}
-
-std::optional<LockedInput *> LockedInputs::findInput(const InputPath & path)
+std::shared_ptr<Node> Node::findInput(const InputPath & path)
 {
     assert(!path.empty());
 
-    LockedInputs * pos = this;
+    auto pos = shared_from_this();
 
     for (auto & elem : path) {
         auto i = pos->inputs.find(elem);
         if (i == pos->inputs.end())
             return {};
-        pos = &i->second;
+        pos = i->second;
     }
 
-    return (LockedInput *) pos;
+    return pos;
 }
 
-void LockedInputs::removeInput(const InputPath & path)
+LockFile::LockFile(const nlohmann::json & json, const Path & path)
 {
-    assert(!path.empty());
-
-    LockedInputs * pos = this;
+    auto version = json.value("version", 0);
+    if (version < 3 || version > 5)
+        throw Error("lock file '%s' has unsupported version %d", path, version);
+
+    if (version < 5) {
+        std::function<void(Node & node, const nlohmann::json & json)> getInputs;
+
+        getInputs = [&](Node & node, const nlohmann::json & json)
+        {
+            for (auto & i : json["inputs"].items()) {
+                auto input = std::make_shared<LockedNode>(i.value());
+                getInputs(*input, i.value());
+                node.inputs.insert_or_assign(i.key(), input);
+            }
+        };
+
+        getInputs(*root, json);
+    }
 
-    for (size_t n = 0; n < path.size(); n++) {
-        auto i = pos->inputs.find(path[n]);
-        if (i == pos->inputs.end()) return;
-        if (n + 1 == path.size())
-            pos->inputs.erase(i);
-        else
-            pos = &i->second;
+    else {
+        std::unordered_map<std::string, std::shared_ptr<Node>> nodeMap;
+
+        std::function<void(Node & node, const nlohmann::json & jsonNode)> getInputs;
+
+        getInputs = [&](Node & node, const nlohmann::json & jsonNode)
+        {
+            if (jsonNode.find("inputs") == jsonNode.end()) return;
+            for (auto & i : jsonNode["inputs"].items()) {
+                std::string inputKey = i.value();
+                auto k = nodeMap.find(inputKey);
+                if (k == nodeMap.end()) {
+                    auto jsonNode2 = json["nodes"][inputKey];
+                    auto input = std::make_shared<LockedNode>(jsonNode2);
+                    k = nodeMap.insert_or_assign(inputKey, input).first;
+                    getInputs(*input, jsonNode2);
+                }
+                node.inputs.insert_or_assign(i.key(), k->second);
+            }
+        };
+
+        std::string rootKey = json["root"];
+        nodeMap.insert_or_assign(rootKey, root);
+        getInputs(*root, json["nodes"][rootKey]);
     }
 }
 
 nlohmann::json LockFile::toJson() const
 {
-    auto json = LockedInputs::toJson();
-    json["version"] = 4;
+    nlohmann::json nodes;
+    std::unordered_map<std::shared_ptr<const Node>, std::string> nodeKeys;
+    std::unordered_set<std::string> keys;
+
+    std::function<std::string(const std::string & key, std::shared_ptr<const Node> node)> dumpNode;
+
+    dumpNode = [&](std::string key, std::shared_ptr<const Node> node) -> std::string
+    {
+        auto k = nodeKeys.find(node);
+        if (k != nodeKeys.end())
+            return k->second;
+
+        if (!keys.insert(key).second) {
+            for (int n = 2; ; ++n) {
+                auto k = fmt("%s_%d", key, n);
+                if (keys.insert(k).second) {
+                    key = k;
+                    break;
+                }
+            }
+        }
+
+        nodeKeys.insert_or_assign(node, key);
+
+        auto n = nlohmann::json::object();
+
+        if (!node->inputs.empty()) {
+            auto inputs = nlohmann::json::object();
+            for (auto & i : node->inputs)
+                inputs[i.first] = dumpNode(i.first, i.second);
+            n["inputs"] = std::move(inputs);
+        }
+
+        if (auto lockedNode = std::dynamic_pointer_cast<const LockedNode>(node)) {
+            n["original"] = fetchers::attrsToJson(lockedNode->originalRef.toAttrs());
+            n["locked"] = fetchers::attrsToJson(lockedNode->lockedRef.toAttrs());
+            n["info"] = treeInfoToJson(lockedNode->info);
+            if (!lockedNode->isFlake) n["flake"] = false;
+        }
+
+        nodes[key] = std::move(n);
+
+        return key;
+    };
+
+    nlohmann::json json;
+    json["version"] = 5;
+    json["root"] = dumpNode("root", root);
+    json["nodes"] = std::move(nodes);
+
     return json;
 }
 
-LockFile LockFile::read(const Path & path)
+std::string LockFile::to_string() const
 {
-    if (pathExists(path)) {
-        auto json = nlohmann::json::parse(readFile(path));
-
-        auto version = json.value("version", 0);
-        if (version != 3 && version != 4)
-            throw Error("lock file '%s' has unsupported version %d", path, version);
+    return toJson().dump(2);
+}
 
-        return LockFile(json);
-    } else
-        return LockFile();
+LockFile LockFile::read(const Path & path)
+{
+    if (!pathExists(path)) return LockFile();
+    return LockFile(nlohmann::json::parse(readFile(path)), path);
 }
 
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
@@ -201,6 +236,35 @@ void LockFile::write(const Path & path) const
     writeFile(path, fmt("%s\n", *this));
 }
 
+bool LockFile::isImmutable() const
+{
+    std::unordered_set<std::shared_ptr<const Node>> nodes;
+
+    std::function<void(std::shared_ptr<const Node> node)> visit;
+
+    visit = [&](std::shared_ptr<const Node> node)
+    {
+        if (!nodes.insert(node).second) return;
+        for (auto & i : node->inputs) visit(i.second);
+    };
+
+    visit(root);
+
+    for (auto & i : nodes) {
+        if (i == root) continue;
+        auto lockedNode = std::dynamic_pointer_cast<const LockedNode>(i);
+        if (lockedNode && !lockedNode->lockedRef.input->isImmutable()) return false;
+    }
+
+    return true;
+}
+
+bool LockFile::operator ==(const LockFile & other) const
+{
+    // FIXME: slow
+    return toJson() == other.toJson();
+}
+
 InputPath parseInputPath(std::string_view s)
 {
     InputPath path;
@@ -217,4 +281,52 @@ InputPath parseInputPath(std::string_view s)
     return path;
 }
 
+static void flattenLockFile(
+    std::shared_ptr<const Node> node,
+    const InputPath & prefix,
+    std::map<InputPath, std::shared_ptr<const LockedNode>> & res)
+{
+    // FIXME: handle cycles
+    for (auto &[id, input] : node->inputs) {
+        auto inputPath(prefix);
+        inputPath.push_back(id);
+        if (auto lockedInput = std::dynamic_pointer_cast<const LockedNode>(input))
+            res.emplace(inputPath, lockedInput);
+        flattenLockFile(input, inputPath, res);
+    }
+}
+
+std::string diffLockFiles(const LockFile & oldLocks, const LockFile & newLocks)
+{
+    std::map<InputPath, std::shared_ptr<const LockedNode>> oldFlat, newFlat;
+    flattenLockFile(oldLocks.root, {}, oldFlat);
+    flattenLockFile(newLocks.root, {}, newFlat);
+
+    auto i = oldFlat.begin();
+    auto j = newFlat.begin();
+    std::string res;
+
+    while (i != oldFlat.end() || j != newFlat.end()) {
+        if (j != newFlat.end() && (i == oldFlat.end() || i->first > j->first)) {
+            res += fmt("* Added '%s': '%s'\n", concatStringsSep("/", j->first), j->second->lockedRef);
+            ++j;
+        } else if (i != oldFlat.end() && (j == newFlat.end() || i->first < j->first)) {
+            res += fmt("* Removed '%s'\n", concatStringsSep("/", i->first));
+            ++i;
+        } else {
+            if (!(i->second->lockedRef == j->second->lockedRef)) {
+                assert(i->second->lockedRef.to_string() != j->second->lockedRef.to_string());
+                res += fmt("* Updated '%s': '%s' -> '%s'\n",
+                    concatStringsSep("/", i->first),
+                    i->second->lockedRef,
+                    j->second->lockedRef);
+            }
+            ++i;
+            ++j;
+        }
+    }
+
+    return res;
+}
+
 }
diff --git a/src/libexpr/flake/lockfile.hh b/src/libexpr/flake/lockfile.hh
index 82cbffd19..c34939ebc 100644
--- a/src/libexpr/flake/lockfile.hh
+++ b/src/libexpr/flake/lockfile.hh
@@ -15,35 +15,26 @@ using namespace fetchers;
 
 typedef std::vector<FlakeId> InputPath;
 
-struct LockedInput;
-
-/* Lock file information about the dependencies of a flake. */
-struct LockedInputs
+/* A node in the lock file. It has outgoing edges to other nodes (its
+   inputs). Only the root node has this type; all other nodes have
+   type LockedNode. */
+struct Node : std::enable_shared_from_this<Node>
 {
-    std::map<FlakeId, LockedInput> inputs;
-
-    LockedInputs() {};
-    LockedInputs(const nlohmann::json & json);
+    std::map<FlakeId, std::shared_ptr<Node>> inputs;
 
-    nlohmann::json toJson() const;
-
-    std::string to_string() const;
+    virtual ~Node() { }
 
-    bool isImmutable() const;
-
-    std::optional<LockedInput *> findInput(const InputPath & path);
-
-    void removeInput(const InputPath & path);
+    std::shared_ptr<Node> findInput(const InputPath & path);
 };
 
-/* Lock file information about a flake input. */
-struct LockedInput : LockedInputs
+/* A non-root node in the lock file. */
+struct LockedNode : Node
 {
     FlakeRef lockedRef, originalRef;
     TreeInfo info;
     bool isFlake = true;
 
-    LockedInput(
+    LockedNode(
         const FlakeRef & lockedRef,
         const FlakeRef & originalRef,
         const TreeInfo & info,
@@ -51,51 +42,36 @@ struct LockedInput : LockedInputs
         : lockedRef(lockedRef), originalRef(originalRef), info(info), isFlake(isFlake)
     { }
 
-    LockedInput(const nlohmann::json & json);
-
-    bool operator ==(const LockedInput & other) const
-    {
-        return
-            lockedRef == other.lockedRef
-            && originalRef == other.originalRef
-            && info == other.info
-            && inputs == other.inputs
-            && isFlake == other.isFlake;
-    }
-
-    nlohmann::json toJson() const;
+    LockedNode(const nlohmann::json & json);
 
     StorePath computeStorePath(Store & store) const;
 };
 
-/* An entire lock file. Note that this cannot be a FlakeInput for the
-   top-level flake, because then the lock file would need to contain
-   the hash of the top-level flake, but committing the lock file
-   would invalidate that hash. */
-struct LockFile : LockedInputs
+struct LockFile
 {
-    bool operator ==(const LockFile & other) const
-    {
-        return inputs == other.inputs;
-    }
-
-    LockFile() {}
-    LockFile(const nlohmann::json & json) : LockedInputs(json) {}
-    LockFile(LockedInput && dep)
-    {
-        inputs = std::move(dep.inputs);
-    }
+    std::shared_ptr<Node> root = std::make_shared<Node>();
+
+    LockFile() {};
+    LockFile(const nlohmann::json & json, const Path & path);
 
     nlohmann::json toJson() const;
 
+    std::string to_string() const;
+
     static LockFile read(const Path & path);
 
     void write(const Path & path) const;
+
+    bool isImmutable() const;
+
+    bool operator ==(const LockFile & other) const;
 };
 
 std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile);
 
 InputPath parseInputPath(std::string_view s);
 
+std::string diffLockFiles(const LockFile & oldLocks, const LockFile & newLocks);
+
 }
 
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 0887fb402..82357aef8 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -199,24 +199,25 @@ struct CmdFlakeListInputs : FlakeCommand, MixJSON
         stopProgressBar();
 
         if (json)
-            std::cout << ((LockedInputs &) flake.lockFile).toJson() << "\n";
+            std::cout << flake.lockFile.toJson() << "\n";
         else {
             std::cout << fmt("%s\n", flake.flake.lockedRef);
 
-            std::function<void(const LockedInputs & inputs, const std::string & prefix)> recurse;
+            std::function<void(const Node & node, const std::string & prefix)> recurse;
 
-            recurse = [&](const LockedInputs & inputs, const std::string & prefix)
+            recurse = [&](const Node & node, const std::string & prefix)
             {
-                for (const auto & [i, input] : enumerate(inputs.inputs)) {
+                for (const auto & [i, input] : enumerate(node.inputs)) {
                     //auto tree2 = tree.child(i + 1 == inputs.inputs.size());
-                    bool last = i + 1 == inputs.inputs.size();
+                    bool last = i + 1 == node.inputs.size();
                     std::cout << fmt("%s" ANSI_BOLD "%s" ANSI_NORMAL ": %s\n",
-                        prefix + (last ? treeLast : treeConn), input.first, input.second.lockedRef);
-                    recurse(input.second, prefix + (last ? treeNull : treeLine));
+                        prefix + (last ? treeLast : treeConn), input.first,
+                        std::dynamic_pointer_cast<const LockedNode>(input.second)->lockedRef);
+                    recurse(*input.second, prefix + (last ? treeNull : treeLine));
                 }
             };
 
-            recurse(flake.lockFile, "");
+            recurse(*flake.lockFile.root, "");
         }
     }
 };
@@ -664,23 +665,26 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
         if (jsonRoot)
             jsonRoot->attr("path", store->printStorePath(flake.flake.sourceInfo->storePath));
 
-        std::function<void(const LockedInputs & inputs, std::optional<JSONObject> & jsonObj)> traverse;
-        traverse = [&](const LockedInputs & inputs, std::optional<JSONObject> & jsonObj)
+        // FIXME: use graph output, handle cycles.
+        std::function<void(const Node & node, std::optional<JSONObject> & jsonObj)> traverse;
+        traverse = [&](const Node & node, std::optional<JSONObject> & jsonObj)
         {
             auto jsonObj2 = jsonObj ? jsonObj->object("inputs") : std::optional<JSONObject>();
-            for (auto & input : inputs.inputs) {
+            for (auto & input : node.inputs) {
+                auto lockedInput = std::dynamic_pointer_cast<const LockedNode>(input.second);
+                assert(lockedInput);
                 auto jsonObj3 = jsonObj2 ? jsonObj2->object(input.first) : std::optional<JSONObject>();
                 if (!dryRun)
-                    input.second.lockedRef.input->fetchTree(store);
-                auto storePath = input.second.computeStorePath(*store);
+                    lockedInput->lockedRef.input->fetchTree(store);
+                auto storePath = lockedInput->computeStorePath(*store);
                 if (jsonObj3)
                     jsonObj3->attr("path", store->printStorePath(storePath));
                 sources.insert(std::move(storePath));
-                traverse(input.second, jsonObj3);
+                traverse(*lockedInput, jsonObj3);
             }
         };
 
-        traverse(flake.lockFile, jsonRoot);
+        traverse(*flake.lockFile.root, jsonRoot);
 
         if (!dryRun && !dstUri.empty()) {
             ref<Store> dstStore = dstUri.empty() ? openStore() : openStore(dstUri);