path: root/src/libstore/download.cc

#include "download.hh"
#include "util.hh"
#include "globals.hh"
#include "hash.hh"
#include "store-api.hh"
#include "archive.hh"
#include "s3.hh"
#include "compression.hh"
#include "pathlocks.hh"
#include "finally.hh"

#ifdef ENABLE_S3
#include <aws/core/client/ClientConfiguration.h>
#endif

#include <unistd.h>
#include <fcntl.h>

#include <curl/curl.h>

#include <algorithm>
#include <cmath>
#include <cstring>
#include <iostream>
#include <queue>
#include <random>
#include <thread>

using namespace std::string_literals;

namespace nix {

struct DownloadSettings : Config
{
    Setting<bool> enableHttp2{this, true, "http2",
        "Whether to enable HTTP/2 support."};

    Setting<std::string> userAgentSuffix{this, "", "user-agent-suffix",
        "String appended to the user agent in HTTP requests."};

    Setting<size_t> httpConnections{this, 25, "http-connections",
        "Number of parallel HTTP connections.",
        {"binary-caches-parallel-connections"}};

    Setting<unsigned long> connectTimeout{this, 0, "connect-timeout",
        "Timeout for connecting to servers during downloads. 0 means use curl's builtin default."};
};

static DownloadSettings downloadSettings;

static GlobalConfig::Register r1(&downloadSettings);

std::string resolveUri(const std::string & uri)
{
    if (uri.compare(0, 8, "channel:") == 0)
        return "https://nixos.org/channels/" + std::string(uri, 8) + "/nixexprs.tar.xz";
    else
        return uri;
}

ref<std::string> decodeContent(const std::string & encoding, ref<std::string> data)
{
    if (encoding == "")
        return data;
    else if (encoding == "br")
        return decompress(encoding, *data);
    else
        throw Error("unsupported Content-Encoding '%s'", encoding);
}

struct CurlDownloader : public Downloader
{
    CURLM * curlm = 0;

    std::random_device rd;
    std::mt19937 mt19937;

    struct DownloadItem : public std::enable_shared_from_this<DownloadItem>
    {
        CurlDownloader & downloader;
        DownloadRequest request;
        DownloadResult result;
        Activity act;
        bool done = false; // whether either the success or failure function has been called
        Callback<DownloadResult> callback;
        CURL * req = 0;
        bool active = false; // whether the handle has been added to the multi object
        std::string status;

        unsigned int attempt = 0;

        /* Don't start this download until the specified time point
           has been reached. */
        std::chrono::steady_clock::time_point embargo;

        struct curl_slist * requestHeaders = 0;

        std::string encoding;

        DownloadItem(CurlDownloader & downloader,
            const DownloadRequest & request,
            Callback<DownloadResult> callback)
            : downloader(downloader)
            , request(request)
            , act(*logger, lvlTalkative, actDownload,
                fmt(request.data ? "uploading '%s'" : "downloading '%s'", request.uri),
                {request.uri}, request.parentAct)
            , callback(callback)
        {
            if (!request.expectedETag.empty())
                requestHeaders = curl_slist_append(requestHeaders, ("If-None-Match: " + request.expectedETag).c_str());
            if (!request.mimeType.empty())
                requestHeaders = curl_slist_append(requestHeaders, ("Content-Type: " + request.mimeType).c_str());
        }

        ~DownloadItem()
        {
            if (req) {
                if (active)
                    curl_multi_remove_handle(downloader.curlm, req);
                curl_easy_cleanup(req);
            }
            if (requestHeaders) curl_slist_free_all(requestHeaders);
            try {
                if (!done)
                    fail(DownloadError(Interrupted, format("download of '%s' was interrupted") % request.uri));
            } catch (...) {
                ignoreException();
            }
        }

        template<class T>
        void fail(const T & e)
        {
            assert(!done);
            done = true;
            callback.rethrow(std::make_exception_ptr(e));
        }

        size_t writeCallback(void * contents, size_t size, size_t nmemb)
        {
            size_t realSize = size * nmemb;
            if (request.dataCallback)
                request.dataCallback((char *) contents, realSize);
            else
                result.data->append((char *) contents, realSize);
            return realSize;
        }

        static size_t writeCallbackWrapper(void * contents, size_t size, size_t nmemb, void * userp)
        {
            return ((DownloadItem *) userp)->writeCallback(contents, size, nmemb);
        }

        size_t headerCallback(void * contents, size_t size, size_t nmemb)
        {
            size_t realSize = size * nmemb;
            std::string line((char *) contents, realSize);
            printMsg(lvlVomit, format("got header for '%s': %s") % request.uri % trim(line));
            if (line.compare(0, 5, "HTTP/") == 0) { // new response starts
                result.etag = "";
                auto ss = tokenizeString<vector<string>>(line, " ");
                status = ss.size() >= 2 ? ss[1] : "";
                result.data = std::make_shared<std::string>();
                encoding = "";
            } else {
                auto i = line.find(':');
                if (i != string::npos) {
                    string name = toLower(trim(string(line, 0, i)));
                    if (name == "etag") {
                        result.etag = trim(string(line, i + 1));
                        /* Hack to work around a GitHub bug: it sends
                           ETags, but ignores If-None-Match. So if we get
                           the expected ETag on a 200 response, then shut
                           down the connection because we already have the
                           data. */
                        if (result.etag == request.expectedETag && status == "200") {
                            debug(format("shutting down on 200 HTTP response with expected ETag"));
                            return 0;
                        }
                    } else if (name == "content-encoding")
                        encoding = trim(string(line, i + 1));;
                }
            }
            return realSize;
        }

        static size_t headerCallbackWrapper(void * contents, size_t size, size_t nmemb, void * userp)
        {
            return ((DownloadItem *) userp)->headerCallback(contents, size, nmemb);
        }

        int progressCallback(double dltotal, double dlnow)
        {
            try {
              act.progress(dlnow, dltotal);
            } catch (nix::Interrupted &) {
              assert(_isInterrupted);
            }
            return _isInterrupted;
        }

        static int progressCallbackWrapper(void * userp, double dltotal, double dlnow, double ultotal, double ulnow)
        {
            return ((DownloadItem *) userp)->progressCallback(dltotal, dlnow);
        }

        static int debugCallback(CURL * handle, curl_infotype type, char * data, size_t size, void * userptr)
        {
            if (type == CURLINFO_TEXT)
                vomit("curl: %s", chomp(std::string(data, size)));
            return 0;
        }

        size_t readOffset = 0;
        size_t readCallback(char *buffer, size_t size, size_t nitems)
        {
            if (readOffset == request.data->length())
                return 0;
            auto count = std::min(size * nitems, request.data->length() - readOffset);
            assert(count);
            memcpy(buffer, request.data->data() + readOffset, count);
            readOffset += count;
            return count;
        }

        static size_t readCallbackWrapper(char *buffer, size_t size, size_t nitems, void * userp)
        {
            return ((DownloadItem *) userp)->readCallback(buffer, size, nitems);
        }

        long lowSpeedTimeout = 300;

        void init()
        {
            if (!req) req = curl_easy_init();

            curl_easy_reset(req);

            if (verbosity >= lvlVomit) {
                curl_easy_setopt(req, CURLOPT_VERBOSE, 1);
                curl_easy_setopt(req, CURLOPT_DEBUGFUNCTION, DownloadItem::debugCallback);
            }

            curl_easy_setopt(req, CURLOPT_URL, request.uri.c_str());
            curl_easy_setopt(req, CURLOPT_FOLLOWLOCATION, 1L);
            curl_easy_setopt(req, CURLOPT_NOSIGNAL, 1);
            curl_easy_setopt(req, CURLOPT_USERAGENT,
                ("curl/" LIBCURL_VERSION " Nix/" + nixVersion +
                    (downloadSettings.userAgentSuffix != "" ? " " + downloadSettings.userAgentSuffix.get() : "")).c_str());
            #if LIBCURL_VERSION_NUM >= 0x072b00
            curl_easy_setopt(req, CURLOPT_PIPEWAIT, 1);
            #endif
            #if LIBCURL_VERSION_NUM >= 0x072f00
            if (downloadSettings.enableHttp2)
                curl_easy_setopt(req, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2TLS);
            #endif
            curl_easy_setopt(req, CURLOPT_WRITEFUNCTION, DownloadItem::writeCallbackWrapper);
            curl_easy_setopt(req, CURLOPT_WRITEDATA, this);
            curl_easy_setopt(req, CURLOPT_HEADERFUNCTION, DownloadItem::headerCallbackWrapper);
            curl_easy_setopt(req, CURLOPT_HEADERDATA, this);

            curl_easy_setopt(req, CURLOPT_PROGRESSFUNCTION, progressCallbackWrapper);
            curl_easy_setopt(req, CURLOPT_PROGRESSDATA, this);
            curl_easy_setopt(req, CURLOPT_NOPROGRESS, 0);

            curl_easy_setopt(req, CURLOPT_HTTPHEADER, requestHeaders);

            if (request.head)
                curl_easy_setopt(req, CURLOPT_NOBODY, 1);

            if (request.data) {
                curl_easy_setopt(req, CURLOPT_UPLOAD, 1L);
                curl_easy_setopt(req, CURLOPT_READFUNCTION, readCallbackWrapper);
                curl_easy_setopt(req, CURLOPT_READDATA, this);
                curl_easy_setopt(req, CURLOPT_INFILESIZE_LARGE, (curl_off_t) request.data->length());
            }

            if (request.verifyTLS) {
                if (settings.caFile != "")
                    curl_easy_setopt(req, CURLOPT_CAINFO, settings.caFile.c_str());
            } else {
                curl_easy_setopt(req, CURLOPT_SSL_VERIFYPEER, 0);
                curl_easy_setopt(req, CURLOPT_SSL_VERIFYHOST, 0);
            }

            curl_easy_setopt(req, CURLOPT_CONNECTTIMEOUT, downloadSettings.connectTimeout.get());

            curl_easy_setopt(req, CURLOPT_LOW_SPEED_LIMIT, 1L);
            curl_easy_setopt(req, CURLOPT_LOW_SPEED_TIME, lowSpeedTimeout);

            /* If no file exist in the specified path, curl continues to work
               anyway as if netrc support was disabled. */
            curl_easy_setopt(req, CURLOPT_NETRC_FILE, settings.netrcFile.get().c_str());
            curl_easy_setopt(req, CURLOPT_NETRC, CURL_NETRC_OPTIONAL);

            result.data = std::make_shared<std::string>();
        }

        void finish(CURLcode code)
        {
            long httpStatus = 0;
            curl_easy_getinfo(req, CURLINFO_RESPONSE_CODE, &httpStatus);

            char * effectiveUrlCStr;
            curl_easy_getinfo(req, CURLINFO_EFFECTIVE_URL, &effectiveUrlCStr);
            if (effectiveUrlCStr)
                result.effectiveUrl = effectiveUrlCStr;

            debug("finished %s of '%s'; curl status = %d, HTTP status = %d, body = %d bytes",
                request.verb(), request.uri, code, httpStatus, result.data ? result.data->size() : 0);

            if (code == CURLE_WRITE_ERROR && result.etag == request.expectedETag) {
                code = CURLE_OK;
                httpStatus = 304;
            }

            if (code == CURLE_OK &&
                (httpStatus == 200 || httpStatus == 201 || httpStatus == 204 || httpStatus == 304 || httpStatus == 226 /* FTP */ || httpStatus == 0 /* other protocol */))
            {
                result.cached = httpStatus == 304;
                done = true;

                try {
                    if (request.decompress)
                        result.data = decodeContent(encoding, ref<std::string>(result.data));
                    act.progress(result.data->size(), result.data->size());
                    callback(std::move(result));
                } catch (...) {
                    done = true;
                    callback.rethrow();
                }
            } else {
                // We treat most errors as transient, but won't retry when hopeless
                Error err = Transient;

                if (httpStatus == 404 || httpStatus == 410 || code == CURLE_FILE_COULDNT_READ_FILE) {
                    // The file is definitely not there
                    err = NotFound;
                } else if (httpStatus == 401 || httpStatus == 403 || httpStatus == 407) {
                    // Don't retry on authentication/authorization failures
                    err = Forbidden;
                } else if (httpStatus >= 400 && httpStatus < 500 && httpStatus != 408) {
                    // Most 4xx errors are client errors and are probably not worth retrying:
                    //   * 408 means the server timed out waiting for us, so we try again
                    err = Misc;
                } else if (httpStatus == 501 || httpStatus == 505 || httpStatus == 511) {
                    // Let's treat most 5xx (server) errors as transient, except for a handful:
                    //   * 501 not implemented
                    //   * 505 http version not supported
                    //   * 511 we're behind a captive portal
                    err = Misc;
                } else {
                    // Don't bother retrying on certain cURL errors either
                    switch (code) {
                        case CURLE_FAILED_INIT:
                        case CURLE_URL_MALFORMAT:
                        case CURLE_NOT_BUILT_IN:
                        case CURLE_REMOTE_ACCESS_DENIED:
                        case CURLE_FILE_COULDNT_READ_FILE:
                        case CURLE_FUNCTION_NOT_FOUND:
                        case CURLE_ABORTED_BY_CALLBACK:
                        case CURLE_BAD_FUNCTION_ARGUMENT:
                        case CURLE_INTERFACE_FAILED:
                        case CURLE_UNKNOWN_OPTION:
                        case CURLE_SSL_CACERT_BADFILE:
                            err = Misc;
                            break;
                        default: // Shut up warnings
                            break;
                    }
                }

                attempt++;

                auto exc =
                    code == CURLE_ABORTED_BY_CALLBACK && _isInterrupted
                    ? DownloadError(Interrupted, fmt("%s of '%s' was interrupted", request.verb(), request.uri))
                    : httpStatus != 0
                    ? DownloadError(err,
                        fmt("unable to %s '%s': HTTP error %d",
                            request.verb(), request.uri, httpStatus)
                        + (code == CURLE_OK ? "" : fmt(" (curl error: %s)", curl_easy_strerror(code)))
                        )
                    : DownloadError(err,
                        fmt("unable to %s '%s': %s (%d)",
                            request.verb(), request.uri, curl_easy_strerror(code), code));

                /* If this is a transient error, then maybe retry the
                   download after a while. */
                if (err == Transient && attempt < request.tries) {
                    int ms = request.baseRetryTimeMs * std::pow(2.0f, attempt - 1 + std::uniform_real_distribution<>(0.0, 0.5)(downloader.mt19937));
                    printError(format("warning: %s; retrying in %d ms") % exc.what() % ms);
                    embargo = std::chrono::steady_clock::now() + std::chrono::milliseconds(ms);
                    downloader.enqueueItem(shared_from_this());
                }
                else
                    fail(exc);
            }
        }
    };

    struct State
    {
        struct EmbargoComparator {
            bool operator() (const std::shared_ptr<DownloadItem> & i1, const std::shared_ptr<DownloadItem> & i2) {
                return i1->embargo > i2->embargo;
            }
        };
        bool quit = false;
        std::priority_queue<std::shared_ptr<DownloadItem>, std::vector<std::shared_ptr<DownloadItem>>, EmbargoComparator> incoming;
    };

    Sync<State> state_;

    /* We can't use a std::condition_variable to wake up the curl
       thread, because it only monitors file descriptors. So use a
       pipe instead. */
    Pipe wakeupPipe;

    std::thread workerThread;

    CurlDownloader()
        : mt19937(rd())
    {
        static std::once_flag globalInit;
        std::call_once(globalInit, curl_global_init, CURL_GLOBAL_ALL);

        curlm = curl_multi_init();

        #if LIBCURL_VERSION_NUM >= 0x072b00 // Multiplex requires >= 7.43.0
        curl_multi_setopt(curlm, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);
        #endif
        #if LIBCURL_VERSION_NUM >= 0x071e00 // Max connections requires >= 7.30.0
        curl_multi_setopt(curlm, CURLMOPT_MAX_TOTAL_CONNECTIONS,
            downloadSettings.httpConnections.get());
        #endif

        wakeupPipe.create();
        fcntl(wakeupPipe.readSide.get(), F_SETFL, O_NONBLOCK);

        workerThread = std::thread([&]() { workerThreadEntry(); });
    }

    ~CurlDownloader()
    {
        stopWorkerThread();

        workerThread.join();

        if (curlm) curl_multi_cleanup(curlm);
    }

    void stopWorkerThread()
    {
        /* Signal the worker thread to exit. */
        {
            auto state(state_.lock());
            state->quit = true;
        }
        writeFull(wakeupPipe.writeSide.get(), " ", false);
    }

    void workerThreadMain()
    {
        /* Cause this thread to be notified on SIGINT. */
        auto callback = createInterruptCallback([&]() {
            stopWorkerThread();
        });

        std::map<CURL *, std::shared_ptr<DownloadItem>> items;

        bool quit = false;

        std::chrono::steady_clock::time_point nextWakeup;

        while (!quit) {
            checkInterrupt();

            /* Let curl do its thing. */
            int running;
            CURLMcode mc = curl_multi_perform(curlm, &running);
            if (mc != CURLM_OK)
                throw nix::Error(format("unexpected error from curl_multi_perform(): %s") % curl_multi_strerror(mc));

            /* Set the promises of any finished requests. */
            CURLMsg * msg;
            int left;
            while ((msg = curl_multi_info_read(curlm, &left))) {
                if (msg->msg == CURLMSG_DONE) {
                    auto i = items.find(msg->easy_handle);
                    assert(i != items.end());
                    i->second->finish(msg->data.result);
                    curl_multi_remove_handle(curlm, i->second->req);
                    i->second->active = false;
                    items.erase(i);
                }
            }

            /* Wait for activity, including wakeup events. */
            int numfds = 0;
            struct curl_waitfd extraFDs[1];
            extraFDs[0].fd = wakeupPipe.readSide.get();
            extraFDs[0].events = CURL_WAIT_POLLIN;
            extraFDs[0].revents = 0;
            long maxSleepTimeMs = items.empty() ? 10000 : 100;
            auto sleepTimeMs =
                nextWakeup != std::chrono::steady_clock::time_point()
                ? std::max(0, (int) std::chrono::duration_cast<std::chrono::milliseconds>(nextWakeup - std::chrono::steady_clock::now()).count())
                : maxSleepTimeMs;
            vomit("download thread waiting for %d ms", sleepTimeMs);
            mc = curl_multi_wait(curlm, extraFDs, 1, sleepTimeMs, &numfds);
            if (mc != CURLM_OK)
                throw nix::Error(format("unexpected error from curl_multi_wait(): %s") % curl_multi_strerror(mc));

            nextWakeup = std::chrono::steady_clock::time_point();

            /* Add new curl requests from the incoming requests queue,
               except for requests that are embargoed (waiting for a
               retry timeout to expire). */
            if (extraFDs[0].revents & CURL_WAIT_POLLIN) {
                char buf[1024];
                auto res = read(extraFDs[0].fd, buf, sizeof(buf));
                if (res == -1 && errno != EINTR)
                    throw SysError("reading curl wakeup socket");
            }

            std::vector<std::shared_ptr<DownloadItem>> incoming;
            auto now = std::chrono::steady_clock::now();

            {
                auto state(state_.lock());
                while (!state->incoming.empty()) {
                    auto item = state->incoming.top();
                    if (item->embargo <= now) {
                        incoming.push_back(item);
                        state->incoming.pop();
                    } else {
                        if (nextWakeup == std::chrono::steady_clock::time_point()
                            || item->embargo < nextWakeup)
                            nextWakeup = item->embargo;
                        break;
                    }
                }
                quit = state->quit;
            }

            for (auto & item : incoming) {
                debug("starting %s of %s", item->request.verb(), item->request.uri);
                item->init();
                curl_multi_add_handle(curlm, item->req);
                item->active = true;
                items[item->req] = item;
            }
        }

        debug("download thread shutting down");
    }

    void workerThreadEntry()
    {
        try {
            workerThreadMain();
        } catch (nix::Interrupted & e) {
        } catch (std::exception & e) {
            printError(format("unexpected error in download thread: %s") % e.what());
        }

        {
            auto state(state_.lock());
            while (!state->incoming.empty()) state->incoming.pop();
            state->quit = true;
        }
    }

    void enqueueItem(std::shared_ptr<DownloadItem> item)
    {
        if (item->request.data
            && !hasPrefix(item->request.uri, "http://")
            && !hasPrefix(item->request.uri, "https://"))
            throw nix::Error("uploading to '%s' is not supported", item->request.uri);

        {
            auto state(state_.lock());
            if (state->quit)
                throw nix::Error("cannot enqueue download request because the download thread is shutting down");
            state->incoming.push(item);
        }
        writeFull(wakeupPipe.writeSide.get(), " ");
    }

    void enqueueDownload(const DownloadRequest & request,
        Callback<DownloadResult> callback) override
    {
        /* Ugly hack to support s3:// URIs. */
        if (hasPrefix(request.uri, "s3://")) {
            // FIXME: do this on a worker thread
            try {
#ifdef ENABLE_S3
                S3Helper s3Helper("", Aws::Region::US_EAST_1); // FIXME: make configurable
                auto slash = request.uri.find('/', 5);
                if (slash == std::string::npos)
                    throw nix::Error("bad S3 URI '%s'", request.uri);
                std::string bucketName(request.uri, 5, slash - 5);
                std::string key(request.uri, slash + 1);
                // FIXME: implement ETag
                auto s3Res = s3Helper.getObject(bucketName, key);
                DownloadResult res;
                if (!s3Res.data)
                    throw DownloadError(NotFound, fmt("S3 object '%s' does not exist", request.uri));
                res.data = s3Res.data;
                callback(std::move(res));
#else
                throw nix::Error("cannot download '%s' because Nix is not built with S3 support", request.uri);
#endif
            } catch (...) { callback.rethrow(); }
            return;
        }

        enqueueItem(std::make_shared<DownloadItem>(*this, request, callback));
    }
};

ref<Downloader> getDownloader()
{
    static ref<Downloader> downloader = makeDownloader();
    return downloader;
}

ref<Downloader> makeDownloader()
{
    return make_ref<CurlDownloader>();
}

std::future<DownloadResult> Downloader::enqueueDownload(const DownloadRequest & request)
{
    auto promise = std::make_shared<std::promise<DownloadResult>>();
    enqueueDownload(request,
        {[promise](std::future<DownloadResult> fut) {
            try {
                promise->set_value(fut.get());
            } catch (...) {
                promise->set_exception(std::current_exception());
            }
        }});
    return promise->get_future();
}

DownloadResult Downloader::download(const DownloadRequest & request)
{
    return enqueueDownload(request).get();
}

void Downloader::download(DownloadRequest && request, Sink & sink)
{
    /* Note: we can't call 'sink' via request.dataCallback, because
       that would cause the sink to execute on the downloader
       thread. If 'sink' is a coroutine, this will fail. Also, if the
       sink is expensive (e.g. one that does decompression and writing
       to the Nix store), it would stall the download thread too much.
       Therefore we use a buffer to communicate data between the
       download thread and the calling thread. */

    struct State {
        bool quit = false;
        std::exception_ptr exc;
        std::string data;
        std::condition_variable avail, request;
    };

    auto _state = std::make_shared<Sync<State>>();

    /* In case of an exception, wake up the download thread. FIXME:
       abort the download request. */
    Finally finally([&]() {
        auto state(_state->lock());
        state->quit = true;
        state->request.notify_one();
    });

    request.dataCallback = [_state](char * buf, size_t len) {

        auto state(_state->lock());

        if (state->quit) return;

        /* If the buffer is full, then go to sleep until the calling
           thread wakes us up (i.e. when it has removed data from the
           buffer). Note: this does stall the download thread. */
        while (state->data.size() > 1024 * 1024) {
            if (state->quit) return;
            debug("download buffer is full; going to sleep");
            state.wait(state->request);
        }

        /* Append data to the buffer and wake up the calling
           thread. */
        state->data.append(buf, len);
        state->avail.notify_one();
    };

    enqueueDownload(request,
        {[_state](std::future<DownloadResult> fut) {
            auto state(_state->lock());
            state->quit = true;
            try {
                fut.get();
            } catch (...) {
                state->exc = std::current_exception();
            }
            state->avail.notify_one();
            state->request.notify_one();
        }});

    auto state(_state->lock());

    while (true) {
        checkInterrupt();

        if (state->quit) {
            if (state->exc) std::rethrow_exception(state->exc);
            break;
        }

        /* If no data is available, then wait for the download thread
           to wake us up. */
        if (state->data.empty())
            state.wait(state->avail);

        /* If data is available, then flush it to the sink and wake up
           the download thread if it's blocked on a full buffer. */
        if (!state->data.empty()) {
            sink((unsigned char *) state->data.data(), state->data.size());
            state->data.clear();
            state->request.notify_one();
        }
    }
}

Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpack, string name, const Hash & expectedHash, string * effectiveUrl, int ttl)
{
    auto url = resolveUri(url_);

    if (name == "") {
        auto p = url.rfind('/');
        if (p != string::npos) name = string(url, p + 1);
    }

    Path expectedStorePath;
    if (expectedHash) {
        expectedStorePath = store->makeFixedOutputPath(unpack, expectedHash, name);
        if (store->isValidPath(expectedStorePath))
            return store->toRealPath(expectedStorePath);
    }

    Path cacheDir = getCacheDir() + "/nix/tarballs";
    createDirs(cacheDir);

    string urlHash = hashString(htSHA256, name + std::string("\0"s) + url).to_string(Base32, false);

    Path dataFile = cacheDir + "/" + urlHash + ".info";
    Path fileLink = cacheDir + "/" + urlHash + "-file";

    PathLocks lock({fileLink}, fmt("waiting for lock on '%1%'...", fileLink));

    Path storePath;

    string expectedETag;

    bool skip = false;

    if (pathExists(fileLink) && pathExists(dataFile)) {
        storePath = readLink(fileLink);
        store->addTempRoot(storePath);
        if (store->isValidPath(storePath)) {
            auto ss = tokenizeString<vector<string>>(readFile(dataFile), "\n");
            if (ss.size() >= 3 && ss[0] == url) {
                time_t lastChecked;
                if (string2Int(ss[2], lastChecked) && lastChecked + ttl >= time(0)) {
                    skip = true;
                    if (effectiveUrl)
                        *effectiveUrl = url_;
                } else if (!ss[1].empty()) {
                    debug(format("verifying previous ETag '%1%'") % ss[1]);
                    expectedETag = ss[1];
                }
            }
        } else
            storePath = "";
    }

    if (!skip) {

        try {
            DownloadRequest request(url);
            request.expectedETag = expectedETag;
            auto res = download(request);
            if (effectiveUrl)
                *effectiveUrl = res.effectiveUrl;

            if (!res.cached) {
                ValidPathInfo info;
                StringSink sink;
                dumpString(*res.data, sink);
                Hash hash = hashString(expectedHash ? expectedHash.type : htSHA256, *res.data);
                info.path = store->makeFixedOutputPath(false, hash, name);
                info.narHash = hashString(htSHA256, *sink.s);
                info.narSize = sink.s->size();
                info.ca = makeFixedOutputCA(false, hash);
                store->addToStore(info, sink.s, NoRepair, NoCheckSigs);
                storePath = info.path;
            }

            assert(!storePath.empty());
            replaceSymlink(storePath, fileLink);

            writeFile(dataFile, url + "\n" + res.etag + "\n" + std::to_string(time(0)) + "\n");
        } catch (DownloadError & e) {
            if (storePath.empty()) throw;
            printError(format("warning: %1%; using cached result") % e.msg());
        }
    }

    if (unpack) {
        Path unpackedLink = cacheDir + "/" + baseNameOf(storePath) + "-unpacked";
        PathLocks lock2({unpackedLink}, fmt("waiting for lock on '%1%'...", unpackedLink));
        Path unpackedStorePath;
        if (pathExists(unpackedLink)) {
            unpackedStorePath = readLink(unpackedLink);
            store->addTempRoot(unpackedStorePath);
            if (!store->isValidPath(unpackedStorePath))
                unpackedStorePath = "";
        }
        if (unpackedStorePath.empty()) {
            printInfo(format("unpacking '%1%'...") % url);
            Path tmpDir = createTempDir();
            AutoDelete autoDelete(tmpDir, true);
            // FIXME: this requires GNU tar for decompression.
            runProgram("tar", true, {"xf", store->toRealPath(storePath), "-C", tmpDir, "--strip-components", "1"});
            unpackedStorePath = store->addToStore(name, tmpDir, true, htSHA256, defaultPathFilter, NoRepair);
        }
        replaceSymlink(unpackedStorePath, unpackedLink);
        storePath = unpackedStorePath;
    }

    if (expectedStorePath != "" && storePath != expectedStorePath) {
        Hash gotHash = unpack
            ? hashPath(expectedHash.type, store->toRealPath(storePath)).first
            : hashFile(expectedHash.type, store->toRealPath(storePath));
        throw nix::Error("hash mismatch in file downloaded from '%s': got hash '%s' instead of the expected hash '%s'",
            url, gotHash.to_string(), expectedHash.to_string());
    }

    return store->toRealPath(storePath);
}


bool isUri(const string & s)
{
    if (s.compare(0, 8, "channel:") == 0) return true;
    size_t pos = s.find("://");
    if (pos == string::npos) return false;
    string scheme(s, 0, pos);
    return scheme == "http" || scheme == "https" || scheme == "file" || scheme == "channel" || scheme == "git" || scheme == "s3" || scheme == "ssh";
}


}