lix/src/libexpr/flake/flake.cc

#include "flake.hh"
#include "eval.hh"
#include "lockfile.hh"
#include "primops.hh"
#include "eval-inline.hh"
#include "store-api.hh"
#include "fetchers.hh"
#include "finally.hh"
#include "fetch-settings.hh"

namespace nix {

using namespace flake;

namespace flake {

typedef std::pair<fetchers::Tree, FlakeRef> FetchedFlake;
typedef std::vector<std::pair<FlakeRef, FetchedFlake>> FlakeCache;

static std::optional<FetchedFlake> lookupInFlakeCache(
    const FlakeCache & flakeCache,
    const FlakeRef & flakeRef)
{
    // FIXME: inefficient.
    for (auto & i : flakeCache) {
        if (flakeRef == i.first) {
            debug("mapping '%s' to previously seen input '%s' -> '%s",
                flakeRef, i.first, i.second.second);
            return i.second;
        }
    }

    return std::nullopt;
}

static std::tuple<fetchers::Tree, FlakeRef, FlakeRef> fetchOrSubstituteTree(
    EvalState & state,
    const FlakeRef & originalRef,
    bool allowLookup,
    FlakeCache & flakeCache)
{
    auto fetched = lookupInFlakeCache(flakeCache, originalRef);
    FlakeRef resolvedRef = originalRef;

    if (!fetched) {
        if (originalRef.input.isDirect()) {
            fetched.emplace(originalRef.fetchTree(state.store));
        } else {
            if (allowLookup) {
                resolvedRef = originalRef.resolve(state.store);
                auto fetchedResolved = lookupInFlakeCache(flakeCache, originalRef);
                if (!fetchedResolved) fetchedResolved.emplace(resolvedRef.fetchTree(state.store));
                flakeCache.push_back({resolvedRef, *fetchedResolved});
                fetched.emplace(*fetchedResolved);
            }
            else {
                throw Error("'%s' is an indirect flake reference, but registry lookups are not allowed", originalRef);
            }
        }
        flakeCache.push_back({originalRef, *fetched});
    }

    auto [tree, lockedRef] = *fetched;

    debug("got tree '%s' from '%s'",
        state.store->printStorePath(tree.storePath), lockedRef);

    state.allowPath(tree.storePath);

    assert(!originalRef.input.getNarHash() || tree.storePath == originalRef.input.computeStorePath(*state.store));

    return {std::move(tree), resolvedRef, lockedRef};
}

static void forceTrivialValue(EvalState & state, Value & value, const PosIdx pos)
{
    if (value.isThunk() && value.isTrivial())
        state.forceValue(value, pos);
}


static void expectType(EvalState & state, ValueType type,
    Value & value, const PosIdx pos)
{
    forceTrivialValue(state, value, pos);
    if (value.type() != type)
        throw Error("expected %s but got %s at %s",
            showType(type), showType(value.type()), state.positions[pos]);
}

static std::map<FlakeId, FlakeInput> parseFlakeInputs(
    EvalState & state, Value * value, const PosIdx pos,
    const std::optional<Path> & baseDir, InputPath lockRootPath);

static FlakeInput parseFlakeInput(EvalState & state,
    const std::string & inputName, Value * value, const PosIdx pos,
    const std::optional<Path> & baseDir, InputPath lockRootPath)
{
    expectType(state, nAttrs, *value, pos);

    FlakeInput input;

    auto sInputs = state.symbols.create("inputs");
    auto sUrl = state.symbols.create("url");
    auto sFlake = state.symbols.create("flake");
    auto sFollows = state.symbols.create("follows");

    fetchers::Attrs attrs;
    std::optional<std::string> url;

    for (nix::Attr attr : *(value->attrs)) {
        try {
            if (attr.name == sUrl) {
                expectType(state, nString, *attr.value, attr.pos);
                url = attr.value->string.s;
                attrs.emplace("url", *url);
            } else if (attr.name == sFlake) {
                expectType(state, nBool, *attr.value, attr.pos);
                input.isFlake = attr.value->boolean;
            } else if (attr.name == sInputs) {
                input.overrides = parseFlakeInputs(state, attr.value, attr.pos, baseDir, lockRootPath);
            } else if (attr.name == sFollows) {
                expectType(state, nString, *attr.value, attr.pos);
                auto follows(parseInputPath(attr.value->string.s));
                follows.insert(follows.begin(), lockRootPath.begin(), lockRootPath.end());
                input.follows = follows;
            } else {
                switch (attr.value->type()) {
                    case nString:
                        attrs.emplace(state.symbols[attr.name], attr.value->string.s);
                        break;
                    case nBool:
                        attrs.emplace(state.symbols[attr.name], Explicit<bool> { attr.value->boolean });
                        break;
                    case nInt:
                        attrs.emplace(state.symbols[attr.name], (long unsigned int)attr.value->integer);
                        break;
                    default:
                        throw TypeError("flake input attribute '%s' is %s while a string, Boolean, or integer is expected",
                            state.symbols[attr.name], showType(*attr.value));
                }
            }
        } catch (Error & e) {
            e.addTrace(
                state.positions[attr.pos],
                hintfmt("in flake attribute '%s'", state.symbols[attr.name]));
            throw;
        }
    }

    if (attrs.count("type"))
        try {
            input.ref = FlakeRef::fromAttrs(attrs);
        } catch (Error & e) {
            e.addTrace(state.positions[pos], hintfmt("in flake input"));
            throw;
        }
    else {
        attrs.erase("url");
        if (!attrs.empty())
            throw Error("unexpected flake input attribute '%s', at %s", attrs.begin()->first, state.positions[pos]);
        if (url)
            input.ref = parseFlakeRef(*url, baseDir, true, input.isFlake);
    }

    if (!input.follows && !input.ref)
        input.ref = FlakeRef::fromAttrs({{"type", "indirect"}, {"id", inputName}});

    return input;
}

static std::map<FlakeId, FlakeInput> parseFlakeInputs(
    EvalState & state, Value * value, const PosIdx pos,
    const std::optional<Path> & baseDir, InputPath lockRootPath)
{
    std::map<FlakeId, FlakeInput> inputs;

    expectType(state, nAttrs, *value, pos);

    for (nix::Attr & inputAttr : *(*value).attrs) {
        inputs.emplace(state.symbols[inputAttr.name],
            parseFlakeInput(state,
                state.symbols[inputAttr.name],
                inputAttr.value,
                inputAttr.pos,
                baseDir,
                lockRootPath));
    }

    return inputs;
}

static Flake getFlake(
    EvalState & state,
    const FlakeRef & originalRef,
    bool allowLookup,
    FlakeCache & flakeCache,
    InputPath lockRootPath)
{
    auto [sourceInfo, resolvedRef, lockedRef] = fetchOrSubstituteTree(
        state, originalRef, allowLookup, flakeCache);

    // Guard against symlink attacks.
    auto flakeDir = canonPath(sourceInfo.actualPath + "/" + lockedRef.subdir, true);
    auto flakeFile = canonPath(flakeDir + "/flake.nix", true);
    if (!isInDir(flakeFile, sourceInfo.actualPath))
        throw Error("'flake.nix' file of flake '%s' escapes from '%s'",
            lockedRef, state.store->printStorePath(sourceInfo.storePath));

    Flake flake {
        .originalRef = originalRef,
        .resolvedRef = resolvedRef,
        .lockedRef = lockedRef,
        .sourceInfo = std::make_shared<fetchers::Tree>(std::move(sourceInfo))
    };

    if (!pathExists(flakeFile))
        throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);

    Value vInfo;
    state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack

    expectType(state, nAttrs, vInfo, state.positions.add({flakeFile, foFile}, 0, 0));

    if (auto description = vInfo.attrs->get(state.sDescription)) {
        expectType(state, nString, *description->value, description->pos);
        flake.description = description->value->string.s;
    }

    auto sInputs = state.symbols.create("inputs");

    if (auto inputs = vInfo.attrs->get(sInputs))
        flake.inputs = parseFlakeInputs(state, inputs->value, inputs->pos, flakeDir, lockRootPath);

    auto sOutputs = state.symbols.create("outputs");

    if (auto outputs = vInfo.attrs->get(sOutputs)) {
        expectType(state, nFunction, *outputs->value, outputs->pos);

        if (outputs->value->isLambda() && outputs->value->lambda.fun->hasFormals()) {
            for (auto & formal : outputs->value->lambda.fun->formals->formals) {
                if (formal.name != state.sSelf)
                    flake.inputs.emplace(state.symbols[formal.name], FlakeInput {
                        .ref = parseFlakeRef(state.symbols[formal.name])
                    });
            }
        }

    } else
        throw Error("flake '%s' lacks attribute 'outputs'", lockedRef);

    auto sNixConfig = state.symbols.create("nixConfig");

    if (auto nixConfig = vInfo.attrs->get(sNixConfig)) {
        expectType(state, nAttrs, *nixConfig->value, nixConfig->pos);

        for (auto & setting : *nixConfig->value->attrs) {
            forceTrivialValue(state, *setting.value, setting.pos);
            if (setting.value->type() == nString)
                flake.config.settings.emplace(
                    state.symbols[setting.name],
                    std::string(state.forceStringNoCtx(*setting.value, setting.pos)));
            else if (setting.value->type() == nPath) {
                PathSet emptyContext = {};
                flake.config.settings.emplace(
                    state.symbols[setting.name],
                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true, true) .toOwned());
            }
            else if (setting.value->type() == nInt)
                flake.config.settings.emplace(
                    state.symbols[setting.name],
                    state.forceInt(*setting.value, setting.pos));
            else if (setting.value->type() == nBool)
                flake.config.settings.emplace(
                    state.symbols[setting.name],
                    Explicit<bool> { state.forceBool(*setting.value, setting.pos) });
            else if (setting.value->type() == nList) {
                std::vector<std::string> ss;
                for (auto elem : setting.value->listItems()) {
                    if (elem->type() != nString)
                        throw TypeError("list element in flake configuration setting '%s' is %s while a string is expected",
                            state.symbols[setting.name], showType(*setting.value));
                    ss.emplace_back(state.forceStringNoCtx(*elem, setting.pos));
                }
                flake.config.settings.emplace(state.symbols[setting.name], ss);
            }
            else
                throw TypeError("flake configuration setting '%s' is %s",
                    state.symbols[setting.name], showType(*setting.value));
        }
    }

    for (auto & attr : *vInfo.attrs) {
        if (attr.name != state.sDescription &&
            attr.name != sInputs &&
            attr.name != sOutputs &&
            attr.name != sNixConfig)
            throw Error("flake '%s' has an unsupported attribute '%s', at %s",
                lockedRef, state.symbols[attr.name], state.positions[attr.pos]);
    }

    return flake;
}

Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup, FlakeCache & flakeCache)
{
    return getFlake(state, originalRef, allowLookup, flakeCache, {});
}

Flake getFlake(EvalState & state, const FlakeRef & originalRef, bool allowLookup)
{
    FlakeCache flakeCache;
    return getFlake(state, originalRef, allowLookup, flakeCache);
}

/* Compute an in-memory lock file for the specified top-level flake,
   and optionally write it to file, if the flake is writable. */
LockedFlake lockFlake(
    EvalState & state,
    const FlakeRef & topRef,
    const LockFlags & lockFlags)
{
    settings.requireExperimentalFeature(Xp::Flakes);

    FlakeCache flakeCache;

    auto useRegistries = lockFlags.useRegistries.value_or(fetchSettings.useRegistries);

    auto flake = getFlake(state, topRef, useRegistries, flakeCache);

    if (lockFlags.applyNixConfig) {
        flake.config.apply();
        state.store->setOptions();
    }

    try {

        // FIXME: symlink attack
        auto oldLockFile = LockFile::read(
            flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir + "/flake.lock");

        debug("old lock file: %s", oldLockFile);

        // FIXME: check whether all overrides are used.
        std::map<InputPath, FlakeInput> overrides;
        std::set<InputPath> overridesUsed, updatesUsed;

        for (auto & i : lockFlags.inputOverrides)
            overrides.insert_or_assign(i.first, FlakeInput { .ref = i.second });

        LockFile newLockFile;

        std::vector<FlakeRef> parents;

        std::function<void(
            const InputPath & inputPathPrefix,
            const FlakeInputs & flakeInputs
            )>
            checkFollowsDeclarations;

        checkFollowsDeclarations = [&](
            const InputPath & inputPathPrefix,
            const FlakeInputs & flakeInputs
        ) {
            for (auto [inputPath, inputOverride] : overrides) {
                auto inputPath2(inputPath);
                auto follow = inputPath2.back();
                inputPath2.pop_back();
                if (inputPath2 == inputPathPrefix
                    && flakeInputs.find(follow) == flakeInputs.end()
                ) {
                    std::string root;
                    for (auto & element : inputPath2) {
                        root.append(element);
                        if (element != inputPath2.back()) {
                            root.append(".inputs.");
                        }
                    }
                    warn(
                        "%s has a `follows'-declaration for a non-existant input %s!",
                        root,
                        follow
                    );
                }
            }
        };

        std::function<void(
            const FlakeInputs & flakeInputs,
            std::shared_ptr<Node> node,
            const InputPath & inputPathPrefix,
            std::shared_ptr<const Node> oldNode,
            const InputPath & lockRootPath,
            const Path & parentPath,
            bool trustLock)>
            computeLocks;

        computeLocks = [&](
            const FlakeInputs & flakeInputs,
            std::shared_ptr<Node> node,
            const InputPath & inputPathPrefix,
            std::shared_ptr<const Node> oldNode,
            const InputPath & lockRootPath,
            const Path & parentPath,
            bool trustLock)
        {
            debug("computing lock file node '%s'", printInputPath(inputPathPrefix));

            checkFollowsDeclarations(inputPathPrefix, flakeInputs);

            /* Get the overrides (i.e. attributes of the form
               'inputs.nixops.inputs.nixpkgs.url = ...'). */
            for (auto & [id, input] : flakeInputs) {
                for (auto & [idOverride, inputOverride] : input.overrides) {
                    auto inputPath(inputPathPrefix);
                    inputPath.push_back(id);
                    inputPath.push_back(idOverride);
                    overrides.insert_or_assign(inputPath, inputOverride);
                }
            }

            /* Go over the flake inputs, resolve/fetch them if
               necessary (i.e. if they're new or the flakeref changed
               from what's in the lock file). */
            for (auto & [id, input2] : flakeInputs) {
                auto inputPath(inputPathPrefix);
                inputPath.push_back(id);
                auto inputPathS = printInputPath(inputPath);
                debug("computing input '%s'", inputPathS);

                try {

                    /* Do we have an override for this input from one of the
                       ancestors? */
                    auto i = overrides.find(inputPath);
                    bool hasOverride = i != overrides.end();
                    if (hasOverride) {
                        overridesUsed.insert(inputPath);
                        // Respect the “flakeness” of the input even if we
                        // override it
                        i->second.isFlake = input2.isFlake;
                    }
                    auto & input = hasOverride ? i->second : input2;

                    /* Resolve 'follows' later (since it may refer to an input
                       path we haven't processed yet. */
                    if (input.follows) {
                        InputPath target;

                        target.insert(target.end(), input.follows->begin(), input.follows->end());

                        debug("input '%s' follows '%s'", inputPathS, printInputPath(target));
                        node->inputs.insert_or_assign(id, target);
                        continue;
                    }

                    assert(input.ref);

                    /* Do we have an entry in the existing lock file? And we
                       don't have a --update-input flag for this input? */
                    std::shared_ptr<LockedNode> oldLock;

                    updatesUsed.insert(inputPath);

                    if (oldNode && !lockFlags.inputUpdates.count(inputPath))
                        if (auto oldLock2 = get(oldNode->inputs, id))
                            if (auto oldLock3 = std::get_if<0>(&*oldLock2))
                                oldLock = *oldLock3;

                    if (oldLock
                        && oldLock->originalRef == *input.ref
                        && !hasOverride)
                    {
                        debug("keeping existing input '%s'", inputPathS);

                        /* Copy the input from the old lock since its flakeref
                           didn't change and there is no override from a
                           higher level flake. */
                        auto childNode = std::make_shared<LockedNode>(
                            oldLock->lockedRef, oldLock->originalRef, oldLock->isFlake);

                        node->inputs.insert_or_assign(id, childNode);

                        /* If we have an --update-input flag for an input
                           of this input, then we must fetch the flake to
                           update it. */
                        auto lb = lockFlags.inputUpdates.lower_bound(inputPath);

                        auto mustRefetch =
                            lb != lockFlags.inputUpdates.end()
                            && lb->size() > inputPath.size()
                            && std::equal(inputPath.begin(), inputPath.end(), lb->begin());

                        FlakeInputs fakeInputs;

                        if (!mustRefetch) {
                            /* No need to fetch this flake, we can be
                               lazy. However there may be new overrides on the
                               inputs of this flake, so we need to check
                               those. */
                            for (auto & i : oldLock->inputs) {
                                if (auto lockedNode = std::get_if<0>(&i.second)) {
                                    fakeInputs.emplace(i.first, FlakeInput {
                                        .ref = (*lockedNode)->originalRef,
                                        .isFlake = (*lockedNode)->isFlake,
                                    });
                                } else if (auto follows = std::get_if<1>(&i.second)) {
                                    if (! trustLock) {
                                        // It is possible that the flake has changed,
                                        // so we must confirm all the follows that are in the lockfile are also in the flake.
                                        auto overridePath(inputPath);
                                        overridePath.push_back(i.first);
                                        auto o = overrides.find(overridePath);
                                        // If the override disappeared, we have to refetch the flake,
                                        // since some of the inputs may not be present in the lockfile.
                                        if (o == overrides.end()) {
                                            mustRefetch = true;
                                            // There's no point populating the rest of the fake inputs,
                                            // since we'll refetch the flake anyways.
                                            break;
                                        }
                                    }
                                    auto absoluteFollows(lockRootPath);
                                    absoluteFollows.insert(absoluteFollows.end(), follows->begin(), follows->end());
                                    fakeInputs.emplace(i.first, FlakeInput {
                                        .follows = absoluteFollows,
                                    });
                                }
                            }
                        }

                        auto localPath(parentPath);
                        // If this input is a path, recurse it down.
                        // This allows us to resolve path inputs relative to the current flake.
                        if ((*input.ref).input.getType() == "path")
                            localPath = absPath(*input.ref->input.getSourcePath(), parentPath);
                        computeLocks(
                            mustRefetch
                            ? getFlake(state, oldLock->lockedRef, false, flakeCache, inputPath).inputs
                            : fakeInputs,
                            childNode, inputPath, oldLock, lockRootPath, parentPath, !mustRefetch);

                    } else {
                        /* We need to create a new lock file entry. So fetch
                           this input. */
                        debug("creating new input '%s'", inputPathS);

                        if (!lockFlags.allowMutable && !input.ref->input.isLocked())
                            throw Error("cannot update flake input '%s' in pure mode", inputPathS);

                        /* Note: in case of an --override-input, we use
                            the *original* ref (input2.ref) for the
                            "original" field, rather than the
                            override. This ensures that the override isn't
                            nuked the next time we update the lock
                            file. That is, overrides are sticky unless you
                            use --no-write-lock-file. */
                        auto ref = input2.ref ? *input2.ref : *input.ref;

                        if (input.isFlake) {
                            Path localPath = parentPath;
                            FlakeRef localRef = *input.ref;

                            // If this input is a path, recurse it down.
                            // This allows us to resolve path inputs relative to the current flake.
                            if (localRef.input.getType() == "path")
                                localPath = absPath(*input.ref->input.getSourcePath(), parentPath);

                            auto inputFlake = getFlake(state, localRef, useRegistries, flakeCache, inputPath);

                            auto childNode = std::make_shared<LockedNode>(inputFlake.lockedRef, ref);

                            node->inputs.insert_or_assign(id, childNode);

                            /* Guard against circular flake imports. */
                            for (auto & parent : parents)
                                if (parent == *input.ref)
                                    throw Error("found circular import of flake '%s'", parent);
                            parents.push_back(*input.ref);
                            Finally cleanup([&]() { parents.pop_back(); });

                            /* Recursively process the inputs of this
                               flake. Also, unless we already have this flake
                               in the top-level lock file, use this flake's
                               own lock file. */
                            computeLocks(
                                inputFlake.inputs, childNode, inputPath,
                                oldLock
                                ? std::dynamic_pointer_cast<const Node>(oldLock)
                                : LockFile::read(
                                    inputFlake.sourceInfo->actualPath + "/" + inputFlake.lockedRef.subdir + "/flake.lock").root,
                                oldLock ? lockRootPath : inputPath, localPath, false);
                        }

                        else {
                            auto [sourceInfo, resolvedRef, lockedRef] = fetchOrSubstituteTree(
                                state, *input.ref, useRegistries, flakeCache);
                            node->inputs.insert_or_assign(id,
                                std::make_shared<LockedNode>(lockedRef, ref, false));
                        }
                    }

                } catch (Error & e) {
                    e.addTrace({}, "while updating the flake input '%s'", inputPathS);
                    throw;
                }
            }
        };

        // Bring in the current ref for relative path resolution if we have it
        auto parentPath = canonPath(flake.sourceInfo->actualPath + "/" + flake.lockedRef.subdir, true);

        computeLocks(
            flake.inputs, newLockFile.root, {},
            lockFlags.recreateLockFile ? nullptr : oldLockFile.root, {}, parentPath, false);

        for (auto & i : lockFlags.inputOverrides)
            if (!overridesUsed.count(i.first))
                warn("the flag '--override-input %s %s' does not match any input",
                    printInputPath(i.first), i.second);

        for (auto & i : lockFlags.inputUpdates)
            if (!updatesUsed.count(i))
                warn("the flag '--update-input %s' does not match any input", printInputPath(i));

        /* Check 'follows' inputs. */
        newLockFile.check();

        debug("new lock file: %s", newLockFile);

        /* Check whether we need to / can write the new lock file. */
        if (!(newLockFile == oldLockFile)) {

            auto diff = LockFile::diff(oldLockFile, newLockFile);

            if (lockFlags.writeLockFile) {
                if (auto sourcePath = topRef.input.getSourcePath()) {
                    if (!newLockFile.isImmutable()) {
                        if (fetchSettings.warnDirty)
                            warn("will not write lock file of flake '%s' because it has a mutable input", topRef);
                    } else {
                        if (!lockFlags.updateLockFile)
                            throw Error("flake '%s' requires lock file changes but they're not allowed due to '--no-update-lock-file'", topRef);

                        auto relPath = (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock";

                        auto path = *sourcePath + "/" + relPath;

                        bool lockFileExists = pathExists(path);

                        if (lockFileExists) {
                            auto s = chomp(diff);
                            if (s.empty())
                                warn("updating lock file '%s'", path);
                            else
                                warn("updating lock file '%s':\n%s", path, s);
                        } else
                            warn("creating lock file '%s'", path);

                        newLockFile.write(path);

                        std::optional<std::string> commitMessage = std::nullopt;
                        if (lockFlags.commitLockFile) {
                            std::string cm;

                            cm = fetchSettings.commitLockFileSummary.get();

                            if (cm == "") {
                                cm = fmt("%s: %s", relPath, lockFileExists ? "Update" : "Add");
                            }

                            cm += "\n\nFlake lock file updates:\n\n";
                            cm += filterANSIEscapes(diff, true);
                            commitMessage = cm;
                        }

                        topRef.input.markChangedFile(
                            (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock",
                            commitMessage);

                        /* Rewriting the lockfile changed the top-level
                           repo, so we should re-read it. FIXME: we could
                           also just clear the 'rev' field... */
                        auto prevLockedRef = flake.lockedRef;
                        FlakeCache dummyCache;
                        flake = getFlake(state, topRef, useRegistries, dummyCache);

                        if (lockFlags.commitLockFile &&
                            flake.lockedRef.input.getRev() &&
                            prevLockedRef.input.getRev() != flake.lockedRef.input.getRev())
                            warn("committed new revision '%s'", flake.lockedRef.input.getRev()->gitRev());

                        /* Make sure that we picked up the change,
                           i.e. the tree should usually be dirty
                           now. Corner case: we could have reverted from a
                           dirty to a clean tree! */
                        if (flake.lockedRef.input == prevLockedRef.input
                            && !flake.lockedRef.input.isLocked())
                            throw Error("'%s' did not change after I updated its 'flake.lock' file; is 'flake.lock' under version control?", flake.originalRef);
                    }
                } else
                    throw Error("cannot write modified lock file of flake '%s' (use '--no-write-lock-file' to ignore)", topRef);
            } 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) };

    } catch (Error & e) {
        e.addTrace({}, "while updating the lock file of flake '%s'", flake.lockedRef.to_string());
        throw;
    }
}

void callFlake(EvalState & state,
    const LockedFlake & lockedFlake,
    Value & vRes)
{
    auto vLocks = state.allocValue();
    auto vRootSrc = state.allocValue();
    auto vRootSubdir = state.allocValue();
    auto vTmp1 = state.allocValue();
    auto vTmp2 = state.allocValue();

    vLocks->mkString(lockedFlake.lockFile.to_string());

    emitTreeAttrs(
        state,
        *lockedFlake.flake.sourceInfo,
        lockedFlake.flake.lockedRef.input,
        *vRootSrc,
        false,
        lockedFlake.flake.forceDirty);

    vRootSubdir->mkString(lockedFlake.flake.lockedRef.subdir);

    if (!state.vCallFlake) {
        state.vCallFlake = allocRootValue(state.allocValue());
        state.eval(state.parseExprFromString(
            #include "call-flake.nix.gen.hh"
            , "/"), **state.vCallFlake);
    }

    state.callFunction(**state.vCallFlake, *vLocks, *vTmp1, noPos);
    state.callFunction(*vTmp1, *vRootSrc, *vTmp2, noPos);
    state.callFunction(*vTmp2, *vRootSubdir, vRes, noPos);
}

static void prim_getFlake(EvalState & state, const PosIdx pos, Value * * args, Value & v)
{
    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos));
    auto flakeRef = parseFlakeRef(flakeRefS, {}, true);
    if (evalSettings.pureEval && !flakeRef.input.isLocked())
        throw Error("cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)", flakeRefS, state.positions[pos]);

    callFlake(state,
        lockFlake(state, flakeRef,
            LockFlags {
                .updateLockFile = false,
                .writeLockFile = false,
                .useRegistries = !evalSettings.pureEval && fetchSettings.useRegistries,
                .allowMutable  = !evalSettings.pureEval,
            }),
        v);
}

static RegisterPrimOp r2({
    .name =  "__getFlake",
    .args = {"args"},
    .doc = R"(
      Fetch a flake from a flake reference, and return its output attributes and some metadata. For example:

      ```nix
      (builtins.getFlake "nix/55bc52401966fbffa525c574c14f67b00bc4fb3a").packages.x86_64-linux.nix
      ```

      Unless impure evaluation is allowed (`--impure`), the flake reference
      must be "locked", e.g. contain a Git revision or content hash. An
      example of an unlocked usage is:

      ```nix
      (builtins.getFlake "github:edolstra/dwarffs").rev
      ```

      This function is only available if you enable the experimental feature
      `flakes`.
    )",
    .fun = prim_getFlake,
    .experimentalFeature = Xp::Flakes,
});

}

Fingerprint LockedFlake::getFingerprint() const
{
    // FIXME: as an optimization, if the flake contains a lock file
    // and we haven't changed it, then it's sufficient to use
    // flake.sourceInfo.storePath for the fingerprint.
    return hashString(htSHA256,
        fmt("%s;%s;%d;%d;%s",
            flake.sourceInfo->storePath.to_string(),
            flake.lockedRef.subdir,
            flake.lockedRef.input.getRevCount().value_or(0),
            flake.lockedRef.input.getLastModified().value_or(0),
            lockFile));
}

Flake::~Flake() { }

}