Merge pull request #2920 from NixOS/lazy-flakes

Lazy flake input fetching

13 files changed, 460 insertions, 258 deletions

diff --git a/flake.lock b/flake.lock
index b85571b61..727d16d73 100644
--- a/flake.lock
+++ b/flake.lock
@@ -1,10 +1,13 @@
 {
     "inputs": {
         "nixpkgs": {
-            "narHash": "sha256-rMiWaLXkhizEEMEeMDutUl0Y/c+VEjfjvMkvBwvuQJU=",
-            "uri": "github:edolstra/nixpkgs/eeeffd24cd7e407cfaa99e98cfbb8f93bf4cc033"
+            "id": "nixpkgs",
+            "inputs": {},
+            "narHash": "sha256-eYtxncIMFVmOHaHBtTdPGcs/AnJqKqA6tHCm0UmPYQU=",
+            "nonFlakeInputs": {},
+            "uri": "github:edolstra/nixpkgs/e9d5882bb861dc48f8d46960e7c820efdbe8f9c1"
         }
     },
     "nonFlakeInputs": {},
-    "version": 1
+    "version": 2
 }
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 0f8a105b1..46c622ee8 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -7,7 +7,7 @@
 #include "eval-inline.hh"
 #include "download.hh"
 #include "json.hh"
-#include "primops/flake.hh"
+#include "flake/flake.hh"
 
 #include <algorithm>
 #include <cstring>
diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/flake/flake.cc
index e5035c53a..bb0543541 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -1,7 +1,8 @@
 #include "flake.hh"
+#include "lockfile.hh"
 #include "primops.hh"
 #include "eval-inline.hh"
-#include "fetchGit.hh"
+#include "primops/fetchGit.hh"
 #include "download.hh"
 #include "args.hh"
 
@@ -43,106 +44,13 @@ std::shared_ptr<FlakeRegistry> readRegistry(const Path & path)
 void writeRegistry(const FlakeRegistry & registry, const Path & path)
 {
     nlohmann::json json;
-    json["version"] = 1;
+    json["version"] = 2;
     for (auto elem : registry.entries)
         json["flakes"][elem.first.to_string()] = { {"uri", elem.second.to_string()} };
     createDirs(dirOf(path));
     writeFile(path, json.dump(4)); // The '4' is the number of spaces used in the indentation in the json file.
 }
 
-LockFile::FlakeEntry readFlakeEntry(nlohmann::json json)
-{
-    FlakeRef flakeRef(json["uri"]);
-    if (!flakeRef.isImmutable())
-        throw Error("cannot use mutable flake '%s' in pure mode", flakeRef);
-
-    LockFile::FlakeEntry entry(flakeRef, Hash((std::string) json["narHash"]));
-
-    auto nonFlakeInputs = json["nonFlakeInputs"];
-
-    for (auto i = nonFlakeInputs.begin(); i != nonFlakeInputs.end(); ++i) {
-        FlakeRef flakeRef(i->value("uri", ""));
-        if (!flakeRef.isImmutable())
-            throw Error("requested to fetch FlakeRef '%s' purely, which is mutable", flakeRef);
-        LockFile::NonFlakeEntry nonEntry(flakeRef, Hash(i->value("narHash", "")));
-        entry.nonFlakeEntries.insert_or_assign(i.key(), nonEntry);
-    }
-
-    auto inputs = json["inputs"];
-
-    for (auto i = inputs.begin(); i != inputs.end(); ++i)
-        entry.flakeEntries.insert_or_assign(i.key(), readFlakeEntry(*i));
-
-    return entry;
-}
-
-LockFile readLockFile(const Path & path)
-{
-    LockFile lockFile;
-
-    if (!pathExists(path))
-        return lockFile;
-
-    auto json = nlohmann::json::parse(readFile(path));
-
-    auto version = json.value("version", 0);
-    if (version != 1)
-        throw Error("lock file '%s' has unsupported version %d", path, version);
-
-    auto nonFlakeInputs = json["nonFlakeInputs"];
-
-    for (auto i = nonFlakeInputs.begin(); i != nonFlakeInputs.end(); ++i) {
-        FlakeRef flakeRef(i->value("uri", ""));
-        LockFile::NonFlakeEntry nonEntry(flakeRef, Hash(i->value("narHash", "")));
-        if (!flakeRef.isImmutable())
-            throw Error("found mutable FlakeRef '%s' in lockfile at path %s", flakeRef, path);
-        lockFile.nonFlakeEntries.insert_or_assign(i.key(), nonEntry);
-    }
-
-    auto inputs = json["inputs"];
-
-    for (auto i = inputs.begin(); i != inputs.end(); ++i)
-        lockFile.flakeEntries.insert_or_assign(i.key(), readFlakeEntry(*i));
-
-    return lockFile;
-}
-
-nlohmann::json flakeEntryToJson(const LockFile::FlakeEntry & entry)
-{
-    nlohmann::json json;
-    json["uri"] = entry.ref.to_string();
-    json["narHash"] = entry.narHash.to_string(SRI);
-    for (auto & x : entry.nonFlakeEntries) {
-        json["nonFlakeInputs"][x.first]["uri"] = x.second.ref.to_string();
-        json["nonFlakeInputs"][x.first]["narHash"] = x.second.narHash.to_string(SRI);
-    }
-    for (auto & x : entry.flakeEntries)
-        json["inputs"][x.first.to_string()] = flakeEntryToJson(x.second);
-    return json;
-}
-
-std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
-{
-    nlohmann::json json;
-    json["version"] = 1;
-    json["nonFlakeInputs"] = nlohmann::json::object();
-    for (auto & x : lockFile.nonFlakeEntries) {
-        json["nonFlakeInputs"][x.first]["uri"] = x.second.ref.to_string();
-        json["nonFlakeInputs"][x.first]["narHash"] = x.second.narHash.to_string(SRI);
-    }
-    json["inputs"] = nlohmann::json::object();
-    for (auto & x : lockFile.flakeEntries)
-        json["inputs"][x.first.to_string()] = flakeEntryToJson(x.second);
-    stream << json.dump(4); // '4' = indentation in json file
-    return stream;
-}
-
-void writeLockFile(const LockFile & lockFile, const Path & path)
-{
-    createDirs(dirOf(path));
-    writeFile(path, fmt("%s\n", lockFile));
-}
-
 Path getUserRegistryPath()
 {
     return getHome() + "/.config/nix/registry.json";
@@ -368,7 +276,7 @@ Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool impureIsAllowe
 }
 
 // Get the `NonFlake` corresponding to a `FlakeRef`.
-NonFlake getNonFlake(EvalState & state, const FlakeRef & flakeRef, FlakeAlias alias, bool impureIsAllowed = false)
+NonFlake getNonFlake(EvalState & state, const FlakeRef & flakeRef, bool impureIsAllowed = false)
 {
     auto sourceInfo = fetchFlake(state, flakeRef, impureIsAllowed);
     debug("got non-flake source '%s' with flakeref %s", sourceInfo.storePath, sourceInfo.resolvedRef.to_string());
@@ -382,38 +290,9 @@ NonFlake getNonFlake(EvalState & state, const FlakeRef & flakeRef, FlakeAlias al
     if (state.allowedPaths)
         state.allowedPaths->insert(nonFlake.sourceInfo.storePath);
 
-    nonFlake.alias = alias;
-
     return nonFlake;
 }
 
-LockFile entryToLockFile(const LockFile::FlakeEntry & entry)
-{
-    LockFile lockFile;
-    lockFile.flakeEntries = entry.flakeEntries;
-    lockFile.nonFlakeEntries = entry.nonFlakeEntries;
-    return lockFile;
-}
-
-LockFile::FlakeEntry dependenciesToFlakeEntry(const ResolvedFlake & resolvedFlake)
-{
-    LockFile::FlakeEntry entry(
-        resolvedFlake.flake.sourceInfo.resolvedRef,
-        resolvedFlake.flake.sourceInfo.narHash);
-
-    for (auto & info : resolvedFlake.flakeDeps)
-        entry.flakeEntries.insert_or_assign(info.first.to_string(), dependenciesToFlakeEntry(info.second));
-
-    for (auto & nonFlake : resolvedFlake.nonFlakeDeps) {
-        LockFile::NonFlakeEntry nonEntry(
-            nonFlake.sourceInfo.resolvedRef,
-            nonFlake.sourceInfo.narHash);
-        entry.nonFlakeEntries.insert_or_assign(nonFlake.alias, nonEntry);
-    }
-
-    return entry;
-}
-
 bool allowedToWrite(HandleLockFile handle)
 {
     return handle == UpdateLockFile || handle == RecreateLockFile;
@@ -435,70 +314,84 @@ bool allowedToUseRegistries(HandleLockFile handle, bool isTopRef)
     else assert(false);
 }
 
-ResolvedFlake resolveFlakeFromLockFile(EvalState & state, const FlakeRef & flakeRef,
-    HandleLockFile handleLockFile, LockFile lockFile = {}, bool topRef = false)
+/* Given a flakeref and its subtree of the lockfile, return an updated
+   subtree of the lockfile. That is, if the 'flake.nix' of the
+   referenced flake has inputs that don't have a corresponding entry
+   in the lockfile, they're added to the lockfile; conversely, any
+   lockfile entries that don't have a corresponding entry in flake.nix
+   are removed.
+
+   Note that this is lazy: we only recursively fetch inputs that are
+   not in the lockfile yet. */
+static std::pair<Flake, FlakeInput> updateLocks(
+    EvalState & state,
+    const Flake & flake,
+    HandleLockFile handleLockFile,
+    const FlakeInputs & oldEntry,
+    bool topRef)
 {
-    Flake flake = getFlake(state, flakeRef, allowedToUseRegistries(handleLockFile, topRef));
-
-    ResolvedFlake deps(flake);
-
-    for (auto & nonFlakeInfo : flake.nonFlakeInputs) {
-        FlakeRef ref = nonFlakeInfo.second;
-        auto i = lockFile.nonFlakeEntries.find(nonFlakeInfo.first);
-        if (i != lockFile.nonFlakeEntries.end()) {
-            NonFlake nonFlake = getNonFlake(state, i->second.ref, nonFlakeInfo.first);
-            if (nonFlake.sourceInfo.narHash != i->second.narHash)
-                throw Error("the content hash of flakeref '%s' doesn't match", i->second.ref.to_string());
-            deps.nonFlakeDeps.push_back(nonFlake);
+    FlakeInput newEntry(
+        flake.id,
+        flake.sourceInfo.resolvedRef,
+        flake.sourceInfo.narHash);
+
+    for (auto & input : flake.nonFlakeInputs) {
+        auto & id = input.first;
+        auto & ref = input.second;
+        auto i = oldEntry.nonFlakeInputs.find(id);
+        if (i != oldEntry.nonFlakeInputs.end()) {
+            newEntry.nonFlakeInputs.insert_or_assign(i->first, i->second);
         } else {
             if (handleLockFile == AllPure || handleLockFile == TopRefUsesRegistries)
-                throw Error("cannot update non-flake dependency '%s' in pure mode", nonFlakeInfo.first);
-            deps.nonFlakeDeps.push_back(getNonFlake(state, nonFlakeInfo.second, nonFlakeInfo.first, allowedToUseRegistries(handleLockFile, false)));
+                throw Error("cannot update non-flake dependency '%s' in pure mode", id);
+            auto nonFlake = getNonFlake(state, ref, allowedToUseRegistries(handleLockFile, false));
+            newEntry.nonFlakeInputs.insert_or_assign(id,
+                NonFlakeInput(
+                    nonFlake.sourceInfo.resolvedRef,
+                    nonFlake.sourceInfo.narHash));
         }
     }
 
-    for (auto newFlakeRef : flake.inputs) {
-        auto i = lockFile.flakeEntries.find(newFlakeRef);
-        if (i != lockFile.flakeEntries.end()) { // Propagate lockFile downwards if possible
-            ResolvedFlake newResFlake = resolveFlakeFromLockFile(state, i->second.ref, handleLockFile, entryToLockFile(i->second));
-            if (newResFlake.flake.sourceInfo.narHash != i->second.narHash)
-                throw Error("the content hash of flakeref '%s' doesn't match", i->second.ref.to_string());
-            deps.flakeDeps.insert_or_assign(newFlakeRef, newResFlake);
+    for (auto & inputRef : flake.inputs) {
+        auto i = oldEntry.flakeInputs.find(inputRef);
+        if (i != oldEntry.flakeInputs.end()) {
+            newEntry.flakeInputs.insert_or_assign(inputRef, i->second);
         } else {
             if (handleLockFile == AllPure || handleLockFile == TopRefUsesRegistries)
-                throw Error("cannot update flake dependency '%s' in pure mode", newFlakeRef.to_string());
-            deps.flakeDeps.insert_or_assign(newFlakeRef, resolveFlakeFromLockFile(state, newFlakeRef, handleLockFile));
+                throw Error("cannot update flake dependency '%s' in pure mode", inputRef);
+            newEntry.flakeInputs.insert_or_assign(inputRef,
+                updateLocks(state,
+                    getFlake(state, inputRef, allowedToUseRegistries(handleLockFile, false)),
+                    handleLockFile, {}, false).second);
         }
     }
 
-    return deps;
+    return {flake, newEntry};
 }
 
-/* Given a flake reference, recursively fetch it and its dependencies.
-   FIXME: this should return a graph of flakes.
-*/
+/* Compute an in-memory lockfile for the specified top-level flake,
+   and optionally write it to file, it the flake is writable. */
 ResolvedFlake resolveFlake(EvalState & state, const FlakeRef & topRef, HandleLockFile handleLockFile)
 {
-    Flake flake = getFlake(state, topRef, allowedToUseRegistries(handleLockFile, true));
+    auto flake = getFlake(state, topRef, allowedToUseRegistries(handleLockFile, true));
+
     LockFile oldLockFile;
 
     if (!recreateLockFile(handleLockFile)) {
         // If recreateLockFile, start with an empty lockfile
         // FIXME: symlink attack
-        oldLockFile = readLockFile(
+        oldLockFile = LockFile::read(
             state.store->toRealPath(flake.sourceInfo.storePath)
             + "/" + flake.sourceInfo.resolvedRef.subdir + "/flake.lock");
     }
 
-    LockFile lockFile(oldLockFile);
-
-    ResolvedFlake resFlake = resolveFlakeFromLockFile(state, topRef, handleLockFile, lockFile, true);
-    lockFile = entryToLockFile(dependenciesToFlakeEntry(resFlake));
+    LockFile lockFile(updateLocks(
+            state, flake, handleLockFile, oldLockFile, true).second);
 
     if (!(lockFile == oldLockFile)) {
         if (allowedToWrite(handleLockFile)) {
             if (auto refData = std::get_if<FlakeRef::IsPath>(&topRef.data)) {
-                writeLockFile(lockFile, refData->path + (topRef.subdir == "" ? "" : "/" + topRef.subdir) + "/flake.lock");
+                lockFile.write(refData->path + (topRef.subdir == "" ? "" : "/" + topRef.subdir) + "/flake.lock");
 
                 // Hack: Make sure that flake.lock is visible to Git, so it ends up in the Nix store.
                 runProgram("git", true, { "-C", refData->path, "add",
@@ -509,7 +402,7 @@ ResolvedFlake resolveFlake(EvalState & state, const FlakeRef & topRef, HandleLoc
             warn("using updated lockfile without writing it to file");
     }
 
-    return resFlake;
+    return ResolvedFlake(std::move(flake), std::move(lockFile));
 }
 
 void updateLockFile(EvalState & state, const FlakeRef & flakeRef, bool recreateLockFile)
@@ -520,7 +413,7 @@ void updateLockFile(EvalState & state, const FlakeRef & flakeRef, bool recreateL
 static void emitSourceInfoAttrs(EvalState & state, const SourceInfo & sourceInfo, Value & vAttrs)
 {
     auto & path = sourceInfo.storePath;
-    state.store->isValidPath(path);
+    assert(state.store->isValidPath(path));
     mkString(*state.allocAttr(vAttrs, state.sOutPath), path, {path});
 
     if (sourceInfo.resolvedRef.rev) {
@@ -539,40 +432,104 @@ static void emitSourceInfoAttrs(EvalState & state, const SourceInfo & sourceInfo
                 std::put_time(std::gmtime(&*sourceInfo.lastModified), "%Y%m%d%H%M%S")));
 }
 
-void callFlake(EvalState & state, const ResolvedFlake & resFlake, Value & v)
+/* Helper primop to make callFlake (below) fetch/call its inputs
+   lazily. Note that this primop cannot be called by user code since
+   it doesn't appear in 'builtins'. */
+static void prim_callFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    // Construct the resulting attrset '{description, outputs,
-    // ...}'. This attrset is passed lazily as an argument to 'outputs'.
+    auto lazyFlake = (FlakeInput *) args[0]->attrs;
+    auto flake = getFlake(state, lazyFlake->ref, false);
 
-    state.mkAttrs(v, resFlake.flakeDeps.size() + resFlake.nonFlakeDeps.size() + 8);
+    if (flake.sourceInfo.narHash != lazyFlake->narHash)
+        throw Error("the content hash of flake '%s' doesn't match the hash recorded in the referring lockfile", flake.sourceInfo.resolvedRef);
 
-    for (auto info : resFlake.flakeDeps) {
-        const ResolvedFlake newResFlake = info.second;
-        auto vFlake = state.allocAttr(v, newResFlake.flake.id);
-        callFlake(state, newResFlake, *vFlake);
-    }
+    callFlake(state, flake, *lazyFlake, v);
+}
+
+static void prim_callNonFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    auto lazyNonFlake = (NonFlakeInput *) args[0]->attrs;
+
+    auto nonFlake = getNonFlake(state, lazyNonFlake->ref);
 
-    for (const NonFlake nonFlake : resFlake.nonFlakeDeps) {
-        auto vNonFlake = state.allocAttr(v, nonFlake.alias);
-        state.mkAttrs(*vNonFlake, 8);
+    if (nonFlake.sourceInfo.narHash != lazyNonFlake->narHash)
+        throw Error("the content hash of repository '%s' doesn't match the hash recorded in the referring lockfile", nonFlake.sourceInfo.resolvedRef);
 
-        state.store->isValidPath(nonFlake.sourceInfo.storePath);
-        mkString(*state.allocAttr(*vNonFlake, state.sOutPath),
-            nonFlake.sourceInfo.storePath, {nonFlake.sourceInfo.storePath});
+    state.mkAttrs(v, 8);
+
+    assert(state.store->isValidPath(nonFlake.sourceInfo.storePath));
+
+    mkString(*state.allocAttr(v, state.sOutPath),
+        nonFlake.sourceInfo.storePath, {nonFlake.sourceInfo.storePath});
+
+    emitSourceInfoAttrs(state, nonFlake.sourceInfo, v);
+}
 
-        emitSourceInfoAttrs(state, nonFlake.sourceInfo, *vNonFlake);
+void callFlake(EvalState & state,
+    const Flake & flake,
+    const FlakeInputs & inputs,
+    Value & vRes)
+{
+    // Construct the resulting attrset '{outputs, ...}'. This attrset
+    // is passed lazily as an argument to the 'outputs' function.
+
+    auto & v = *state.allocValue();
+
+    state.mkAttrs(v,
+        inputs.flakeInputs.size() +
+        inputs.nonFlakeInputs.size() + 8);
+
+    for (auto & dep : inputs.flakeInputs) {
+        auto vFlake = state.allocAttr(v, dep.second.id);
+        auto vPrimOp = state.allocValue();
+        static auto primOp = new PrimOp(prim_callFlake, 1, state.symbols.create("callFlake"));
+        vPrimOp->type = tPrimOp;
+        vPrimOp->primOp = primOp;
+        auto vArg = state.allocValue();
+        vArg->type = tNull;
+        // FIXME: leak
+        vArg->attrs = (Bindings *) new FlakeInput(dep.second); // evil! also inefficient
+        mkApp(*vFlake, *vPrimOp, *vArg);
+    }
+
+    for (auto & dep : inputs.nonFlakeInputs) {
+        auto vNonFlake = state.allocAttr(v, dep.first);
+        auto vPrimOp = state.allocValue();
+        static auto primOp = new PrimOp(prim_callNonFlake, 1, state.symbols.create("callNonFlake"));
+        vPrimOp->type = tPrimOp;
+        vPrimOp->primOp = primOp;
+        auto vArg = state.allocValue();
+        vArg->type = tNull;
+        // FIXME: leak
+        vArg->attrs = (Bindings *) new NonFlakeInput(dep.second); // evil! also inefficient
+        mkApp(*vNonFlake, *vPrimOp, *vArg);
     }
 
-    mkString(*state.allocAttr(v, state.sDescription), resFlake.flake.description);
+    mkString(*state.allocAttr(v, state.sDescription), flake.description);
 
-    emitSourceInfoAttrs(state, resFlake.flake.sourceInfo, v);
+    emitSourceInfoAttrs(state, flake.sourceInfo, v);
 
     auto vOutputs = state.allocAttr(v, state.symbols.create("outputs"));
-    mkApp(*vOutputs, *resFlake.flake.vOutputs, v);
+    mkApp(*vOutputs, *flake.vOutputs, v);
 
     v.attrs->push_back(Attr(state.symbols.create("self"), &v));
 
     v.attrs->sort();
+
+    /* For convenience, put the outputs directly in the result, so you
+       can refer to an output of an input as 'inputs.foo.bar' rather
+       than 'inputs.foo.outputs.bar'. */
+    auto v2 = *state.allocValue();
+    state.eval(state.parseExprFromString("res: res.outputs // res", "/"), v2);
+
+    state.callFunction(v2, v, vRes, noPos);
+}
+
+void callFlake(EvalState & state,
+    const ResolvedFlake & resFlake,
+    Value & v)
+{
+    callFlake(state, resFlake.flake, resFlake.lockFile, v);
 }
 
 // This function is exposed to be used in nix files.
diff --git a/src/libexpr/primops/flake.hh b/src/libexpr/flake/flake.hh
index 82b0973f6..b8d0da252 100644
--- a/src/libexpr/primops/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -1,7 +1,8 @@
+#pragma once
+
 #include "types.hh"
 #include "flakeref.hh"
-
-#include <variant>
+#include "lockfile.hh"
 
 namespace nix {
 
@@ -19,50 +20,11 @@ struct FlakeRegistry
     std::map<FlakeRef, FlakeRef> entries;
 };
 
-struct LockFile
-{
-    struct NonFlakeEntry
-    {
-        FlakeRef ref;
-        Hash narHash;
-        NonFlakeEntry(const FlakeRef & flakeRef, const Hash & hash) : ref(flakeRef), narHash(hash) {};
-
-        bool operator ==(const NonFlakeEntry & other) const
-        {
-            return ref == other.ref && narHash == other.narHash;
-        }
-    };
-
-    struct FlakeEntry
-    {
-        FlakeRef ref;
-        Hash narHash;
-        std::map<FlakeRef, FlakeEntry> flakeEntries;
-        std::map<FlakeAlias, NonFlakeEntry> nonFlakeEntries;
-        FlakeEntry(const FlakeRef & flakeRef, const Hash & hash) : ref(flakeRef), narHash(hash) {};
-
-        bool operator ==(const FlakeEntry & other) const
-        {
-            return
-                ref == other.ref
-                && narHash == other.narHash
-                && flakeEntries == other.flakeEntries
-                && nonFlakeEntries == other.nonFlakeEntries;
-        }
-    };
-
-    std::map<FlakeRef, FlakeEntry> flakeEntries;
-    std::map<FlakeAlias, NonFlakeEntry> nonFlakeEntries;
+typedef std::vector<std::shared_ptr<FlakeRegistry>> Registries;
 
-    bool operator ==(const LockFile & other) const
-    {
-        return
-            flakeEntries == other.flakeEntries
-            && nonFlakeEntries == other.nonFlakeEntries;
-    }
-};
+std::shared_ptr<FlakeRegistry> readRegistry(const Path &);
 
-typedef std::vector<std::shared_ptr<FlakeRegistry>> Registries;
+void writeRegistry(const FlakeRegistry &, const Path &);
 
 Path getUserRegistryPath();
 
@@ -75,10 +37,6 @@ enum HandleLockFile : unsigned int
     , UseNewLockFile // `RecreateLockFile` without writing to file
     };
 
-std::shared_ptr<FlakeRegistry> readRegistry(const Path &);
-
-void writeRegistry(const FlakeRegistry &, const Path &);
-
 struct SourceInfo
 {
     // Immutable flakeref that this source tree was obtained from.
@@ -117,7 +75,6 @@ struct Flake
 
 struct NonFlake
 {
-    FlakeAlias alias;
     FlakeRef originalRef;
     SourceInfo sourceInfo;
     NonFlake(const FlakeRef & origRef, const SourceInfo & sourceInfo)
@@ -129,14 +86,21 @@ Flake getFlake(EvalState &, const FlakeRef &, bool impureIsAllowed);
 struct ResolvedFlake
 {
     Flake flake;
-    std::map<FlakeRef, ResolvedFlake> flakeDeps; // The key in this map, is the originalRef as written in flake.nix
-    std::vector<NonFlake> nonFlakeDeps;
-    ResolvedFlake(const Flake & flake) : flake(flake) {}
+    LockFile lockFile;
+    ResolvedFlake(Flake && flake, LockFile && lockFile)
+        : flake(flake), lockFile(lockFile) {}
 };
 
 ResolvedFlake resolveFlake(EvalState &, const FlakeRef &, HandleLockFile);
 
-void callFlake(EvalState & state, const ResolvedFlake & resFlake, Value & v);
+void callFlake(EvalState & state,
+    const Flake & flake,
+    const FlakeInputs & inputs,
+    Value & v);
+
+void callFlake(EvalState & state,
+    const ResolvedFlake & resFlake,
+    Value & v);
 
 void updateLockFile(EvalState &, const FlakeRef & flakeRef, bool recreateLockFile);
 
diff --git a/src/libexpr/primops/flakeref.cc b/src/libexpr/flake/flakeref.cc
index 24af09124..24af09124 100644
--- a/src/libexpr/primops/flakeref.cc
+++ b/src/libexpr/flake/flakeref.cc
diff --git a/src/libexpr/primops/flakeref.hh b/src/libexpr/flake/flakeref.hh
index 52bb82ddb..52bb82ddb 100644
--- a/src/libexpr/primops/flakeref.hh
+++ b/src/libexpr/flake/flakeref.hh
diff --git a/src/libexpr/flake/lockfile.cc b/src/libexpr/flake/lockfile.cc
new file mode 100644
index 000000000..97c748c66
--- /dev/null
+++ b/src/libexpr/flake/lockfile.cc
@@ -0,0 +1,102 @@
+#include "lockfile.hh"
+#include "store-api.hh"
+
+namespace nix::flake {
+
+AbstractInput::AbstractInput(const nlohmann::json & json)
+    : ref(json["uri"])
+    , narHash(Hash((std::string) json["narHash"]))
+{
+    if (!ref.isImmutable())
+        throw Error("lockfile contains mutable flakeref '%s'", ref);
+}
+
+nlohmann::json AbstractInput::toJson() const
+{
+    nlohmann::json json;
+    json["uri"] = ref.to_string();
+    json["narHash"] = narHash.to_string(SRI);
+    return json;
+}
+
+Path AbstractInput::computeStorePath(Store & store) const
+{
+    return store.makeFixedOutputPath(true, narHash, "source");
+}
+
+FlakeInput::FlakeInput(const nlohmann::json & json)
+    : FlakeInputs(json)
+    , AbstractInput(json)
+    , id(json["id"])
+{
+}
+
+nlohmann::json FlakeInput::toJson() const
+{
+    auto json = FlakeInputs::toJson();
+    json.update(AbstractInput::toJson());
+    json["id"] = id;
+    return json;
+}
+
+FlakeInputs::FlakeInputs(const nlohmann::json & json)
+{
+    for (auto & i : json["nonFlakeInputs"].items())
+        nonFlakeInputs.insert_or_assign(i.key(), NonFlakeInput(i.value()));
+
+    for (auto & i : json["inputs"].items())
+        flakeInputs.insert_or_assign(i.key(), FlakeInput(i.value()));
+}
+
+nlohmann::json FlakeInputs::toJson() const
+{
+    nlohmann::json json;
+    {
+        auto j = nlohmann::json::object();
+        for (auto & i : nonFlakeInputs)
+            j[i.first] = i.second.toJson();
+        json["nonFlakeInputs"] = std::move(j);
+    }
+    {
+        auto j = nlohmann::json::object();
+        for (auto & i : flakeInputs)
+            j[i.first.to_string()] = i.second.toJson();
+        json["inputs"] = std::move(j);
+    }
+    return json;
+}
+
+nlohmann::json LockFile::toJson() const
+{
+    auto json = FlakeInputs::toJson();
+    json["version"] = 2;
+    return json;
+}
+
+LockFile LockFile::read(const Path & path)
+{
+    if (pathExists(path)) {
+        auto json = nlohmann::json::parse(readFile(path));
+
+        auto version = json.value("version", 0);
+        if (version != 2)
+            throw Error("lock file '%s' has unsupported version %d", path, version);
+
+        return LockFile(json);
+    } else
+        return LockFile();
+}
+
+std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile)
+{
+    stream << lockFile.toJson().dump(4); // '4' = indentation in json file
+    return stream;
+}
+
+void LockFile::write(const Path & path) const
+{
+    createDirs(dirOf(path));
+    writeFile(path, fmt("%s\n", *this));
+}
+
+}
diff --git a/src/libexpr/flake/lockfile.hh b/src/libexpr/flake/lockfile.hh
new file mode 100644
index 000000000..b76124190
--- /dev/null
+++ b/src/libexpr/flake/lockfile.hh
@@ -0,0 +1,112 @@
+#pragma once
+
+#include "flakeref.hh"
+
+#include <nlohmann/json.hpp>
+
+namespace nix {
+class Store;
+}
+
+namespace nix::flake {
+
+/* Common lock file information about a flake input, namely the
+   immutable ref and the NAR hash. */
+struct AbstractInput
+{
+    FlakeRef ref;
+    Hash narHash;
+
+    AbstractInput(const FlakeRef & flakeRef, const Hash & narHash)
+        : ref(flakeRef), narHash(narHash)
+    {
+        assert(ref.isImmutable());
+    };
+
+    AbstractInput(const nlohmann::json & json);
+
+    nlohmann::json toJson() const;
+
+    Path computeStorePath(Store & store) const;
+};
+
+/* Lock file information about a non-flake input. */
+struct NonFlakeInput : AbstractInput
+{
+    using AbstractInput::AbstractInput;
+
+    bool operator ==(const NonFlakeInput & other) const
+    {
+        return ref == other.ref && narHash == other.narHash;
+    }
+};
+
+struct FlakeInput;
+
+/* Lock file information about the dependencies of a flake. */
+struct FlakeInputs
+{
+    std::map<FlakeRef, FlakeInput> flakeInputs;
+    std::map<FlakeAlias, NonFlakeInput> nonFlakeInputs;
+
+    FlakeInputs() {};
+    FlakeInputs(const nlohmann::json & json);
+
+    nlohmann::json toJson() const;
+};
+
+/* Lock file information about a flake input. */
+struct FlakeInput : FlakeInputs, AbstractInput
+{
+    FlakeId id;
+
+    FlakeInput(const FlakeId & id, const FlakeRef & flakeRef, const Hash & narHash)
+        : AbstractInput(flakeRef, narHash), id(id) {};
+
+    FlakeInput(const nlohmann::json & json);
+
+    bool operator ==(const FlakeInput & other) const
+    {
+        return
+            id == other.id
+            && ref == other.ref
+            && narHash == other.narHash
+            && flakeInputs == other.flakeInputs
+            && nonFlakeInputs == other.nonFlakeInputs;
+    }
+
+    nlohmann::json toJson() 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 : FlakeInputs
+{
+    bool operator ==(const LockFile & other) const
+    {
+        return
+            flakeInputs == other.flakeInputs
+            && nonFlakeInputs == other.nonFlakeInputs;
+    }
+
+    LockFile() {}
+    LockFile(const nlohmann::json & json) : FlakeInputs(json) {}
+    LockFile(FlakeInput && dep)
+    {
+        flakeInputs = std::move(dep.flakeInputs);
+        nonFlakeInputs = std::move(dep.nonFlakeInputs);
+    }
+
+    nlohmann::json toJson() const;
+
+    static LockFile read(const Path & path);
+
+    void write(const Path & path) const;
+};
+
+std::ostream & operator <<(std::ostream & stream, const LockFile & lockFile);
+
+}
+
diff --git a/src/libexpr/local.mk b/src/libexpr/local.mk
index ccd5293e4..a9cb6b7b6 100644
--- a/src/libexpr/local.mk
+++ b/src/libexpr/local.mk
@@ -4,7 +4,12 @@ libexpr_NAME = libnixexpr
 
 libexpr_DIR := $(d)
 
-libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(d)/primops/*.cc) $(d)/lexer-tab.cc $(d)/parser-tab.cc
+libexpr_SOURCES := \
+  $(wildcard $(d)/*.cc) \
+  $(wildcard $(d)/primops/*.cc) \
+  $(wildcard $(d)/flake/*.cc) \
+  $(d)/lexer-tab.cc \
+  $(d)/parser-tab.cc
 
 libexpr_LIBS = libutil libstore
 
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 8d6716391..af1a361b3 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -4,7 +4,7 @@
 #include "progress-bar.hh"
 #include "eval.hh"
 #include "eval-inline.hh"
-#include "primops/flake.hh"
+#include "flake/flake.hh"
 #include "get-drvs.hh"
 #include "store-api.hh"
 
@@ -122,6 +122,7 @@ static nlohmann::json flakeToJson(const Flake & flake)
     return j;
 }
 
+#if 0
 static void printNonFlakeInfo(const NonFlake & nonFlake)
 {
     std::cout << fmt("ID:            %s\n", nonFlake.alias);
@@ -173,6 +174,7 @@ struct CmdFlakeDeps : FlakeCommand
         }
     }
 };
+#endif
 
 struct CmdFlakeUpdate : FlakeCommand
 {
@@ -232,6 +234,7 @@ struct CmdFlakeInfo : FlakeCommand, MixJSON
         if (json) {
             auto json = flakeToJson(flake);
 
+#if 0
             auto state = getEvalState();
 
             auto vFlake = state->allocValue();
@@ -254,6 +257,7 @@ struct CmdFlakeInfo : FlakeCommand, MixJSON
                 });
 
             json["outputs"] = std::move(outputs);
+#endif
 
             std::cout << json.dump() << std::endl;
         } else
@@ -518,7 +522,7 @@ struct CmdFlake : virtual MultiCommand, virtual Command
             , make_ref<CmdFlakeUpdate>()
             , make_ref<CmdFlakeInfo>()
             , make_ref<CmdFlakeCheck>()
-            , make_ref<CmdFlakeDeps>()
+            //, make_ref<CmdFlakeDeps>()
             , make_ref<CmdFlakeAdd>()
             , make_ref<CmdFlakeRemove>()
             , make_ref<CmdFlakePin>()
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index b6f05b314..a85295a09 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -7,7 +7,7 @@
 #include "get-drvs.hh"
 #include "store-api.hh"
 #include "shared.hh"
-#include "primops/flake.hh"
+#include "flake/flake.hh"
 
 #include <regex>
 #include <queue>
@@ -195,16 +195,28 @@ void makeFlakeClosureGCRoot(Store & store,
     /* Get the store paths of all non-local flakes. */
     PathSet closure;
 
-    std::queue<std::reference_wrapper<const flake::ResolvedFlake>> queue;
-    queue.push(resFlake);
+    assert(store.isValidPath(resFlake.flake.sourceInfo.storePath));
+    closure.insert(resFlake.flake.sourceInfo.storePath);
+
+    std::queue<std::reference_wrapper<const flake::FlakeInputs>> queue;
+    queue.push(resFlake.lockFile);
 
     while (!queue.empty()) {
-        const flake::ResolvedFlake & flake = queue.front();
+        const flake::FlakeInputs & flake = queue.front();
         queue.pop();
-        if (!std::get_if<FlakeRef::IsPath>(&flake.flake.sourceInfo.resolvedRef.data))
-            closure.insert(flake.flake.sourceInfo.storePath);
-        for (const auto & dep : flake.flakeDeps)
+        /* Note: due to lazy fetching, these paths might not exist
+           yet. */
+        for (auto & dep : flake.flakeInputs) {
+            auto path = dep.second.computeStorePath(store);
+            if (store.isValidPath(path))
+                closure.insert(path);
             queue.push(dep.second);
+        }
+        for (auto & dep : flake.nonFlakeInputs) {
+            auto path = dep.second.computeStorePath(store);
+            if (store.isValidPath(path))
+                closure.insert(path);
+        }
     }
 
     if (closure.empty()) return;
diff --git a/src/nlohmann/json_fwd.hpp b/src/nlohmann/json_fwd.hpp
new file mode 100644
index 000000000..ae6e4c64f
--- /dev/null
+++ b/src/nlohmann/json_fwd.hpp
@@ -0,0 +1,10 @@
+#pragma once
+
+namespace nlohmann {
+
+struct json : basic_json<>
+{
+    using basic_json<>::basic_json;
+};
+
+}
diff --git a/tests/flakes.sh b/tests/flakes.sh
index 998abfd09..c380b405b 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -55,7 +55,7 @@ cat > $flake2Dir/flake.nix <<EOF
   description = "Fnord";
 
   outputs = inputs: rec {
-    packages.bar = inputs.flake1.outputs.packages.foo;
+    packages.bar = inputs.flake1.packages.foo;
   };
 }
 EOF
@@ -74,7 +74,7 @@ cat > $flake3Dir/flake.nix <<EOF
   description = "Fnord";
 
   outputs = inputs: rec {
-    packages.xyzzy = inputs.flake2.outputs.packages.bar;
+    packages.xyzzy = inputs.flake2.packages.bar;
   };
 }
 EOF
@@ -83,7 +83,7 @@ git -C $flake3Dir add flake.nix
 git -C $flake3Dir commit -m 'Initial'
 
 cat > $nonFlakeDir/README.md <<EOF
-Not much
+FNORD
 EOF
 
 git -C $nonFlakeDir add README.md
@@ -176,8 +176,8 @@ cat > $flake3Dir/flake.nix <<EOF
   description = "Fnord";
 
   outputs = inputs: rec {
-    packages.xyzzy = inputs.flake2.outputs.packages.bar;
-    packages.sth = inputs.flake1.outputs.packages.foo;
+    packages.xyzzy = inputs.flake2.packages.bar;
+    packages.sth = inputs.flake1.packages.foo;
   };
 }
 EOF
@@ -191,6 +191,8 @@ nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth
 # Check whether it saved the lockfile
 [[ ! (-z $(git -C $flake3Dir diff master)) ]]
 
+git -C $flake3Dir commit -m 'Add lockfile'
+
 # Unsupported epochs should be an error.
 sed -i $flake3Dir/flake.nix -e s/201906/201909/
 nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth 2>&1 | grep 'unsupported epoch'
@@ -202,6 +204,7 @@ nix flake list --flake-registry file://$registry --tarball-ttl 0 | grep -q flake
 mv $registry.tmp $registry
 
 # Test whether flakes are registered as GC roots for offline use.
+# FIXME: use tarballs rather than git.
 rm -rf $TEST_HOME/.cache
 nix build -o $TEST_ROOT/result --flake-registry file://$registry file://$flake2Dir:bar
 mv $flake1Dir $flake1Dir.tmp
@@ -230,14 +233,44 @@ cat > $flake3Dir/flake.nix <<EOF
   description = "Fnord";
 
   outputs = inputs: rec {
-    packages.xyzzy = inputs.flake2.outputs.packages.bar;
-    packages.sth = inputs.flake1.outputs.packages.foo;
+    packages.xyzzy = inputs.flake2.packages.bar;
+    packages.sth = inputs.flake1.packages.foo;
+    packages.fnord =
+      with import ./config.nix;
+      mkDerivation {
+        inherit system;
+        name = "fnord";
+        buildCommand = ''
+          cat \${inputs.nonFlake}/README.md > \$out
+        '';
+      };
   };
 }
 EOF
 
-git -C $flake3Dir add flake.nix
+cp ./config.nix $flake3Dir
+
+git -C $flake3Dir add flake.nix config.nix
 git -C $flake3Dir commit -m 'Add nonFlakeInputs'
 
-# Check whether `nix build` works with a lockfile which is missing a nonFlakeInputs
+# Check whether `nix build` works with a lockfile which is missing a
+# nonFlakeInputs.
 nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth
+
+git -C $flake3Dir commit -m 'Update nonFlakeInputs'
+
+nix build -o $TEST_ROOT/result --flake-registry $registry flake3:fnord
+[[ $(cat $TEST_ROOT/result) = FNORD ]]
+
+# Check whether flake input fetching is lazy: flake3:sth does not
+# depend on flake2, so this shouldn't fail.
+rm -rf $TEST_HOME/.cache
+clearStore
+mv $flake2Dir $flake2Dir.tmp
+mv $nonFlakeDir $nonFlakeDir.tmp
+nix build -o $TEST_ROOT/result --flake-registry $registry flake3:sth
+(! nix build -o $TEST_ROOT/result --flake-registry $registry flake3:xyzzy)
+(! nix build -o $TEST_ROOT/result --flake-registry $registry flake3:fnord)
+mv $flake2Dir.tmp $flake2Dir
+mv $nonFlakeDir.tmp $nonFlakeDir
+nix build -o $TEST_ROOT/result --flake-registry $registry flake3:xyzzy flake3:fnord