about summary refs log tree commit diff
path: root/src/libstore/download.cc
blob: 7773d903265b500e0a39b74be3d13462b37d7936 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
#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;
}

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)
            , finalSink([this](const unsigned char * data, size_t len) {
                if (this->request.dataCallback)
                    this->request.dataCallback((char *) data, len);
                else
                    this->result.data->append((char *) data, len);
              })
        {
            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();
            }
        }

        void failEx(std::exception_ptr ex)
        {
            assert(!done);
            done = true;
            callback.rethrow(ex);
        }

        template<class T>
        void fail(const T & e)
        {
            failEx(std::make_exception_ptr(e));
        }

        LambdaSink finalSink;
        std::shared_ptr<CompressionSink> decompressionSink;

        std::exception_ptr writeException;

        size_t writeCallback(void * contents, size_t size, size_t nmemb)
        {
            try {
                size_t realSize = size * nmemb;
                result.bodySize += realSize;

                if (!decompressionSink)
                    decompressionSink = makeDecompressionSink(encoding, finalSink);

                (*decompressionSink)((unsigned char *) contents, realSize);

                return realSize;
            } catch (...) {
                writeException = std::current_exception();
                return 0;
            }
        }

        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>();
                result.bodySize = 0;
                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_MAXREDIRS, 10);
            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>();
            result.bodySize = 0;
        }

        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.bodySize);

            if (decompressionSink)
                decompressionSink->finish();

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

            if (writeException)
                failEx(writeException);

            else 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 {
                    act.progress(result.bodySize, result.bodySize);
                    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:
                        case CURLE_TOO_MANY_REDIRECTS:
                        case CURLE_WRITE_ERROR:
                            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). We don't wait forever to prevent stalling the
           download thread. (Hopefully sleeping will throttle the
           sender.) */
        if (state->data.size() > 1024 * 1024) {
            debug("download buffer is full; going to sleep");
            state.wait_for(state->request, std::chrono::seconds(10));
        }

        /* 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();
        }});

    while (true) {
        checkInterrupt();

        std::string chunk;

        /* Grab data if available, otherwise wait for the download
           thread to wake us up. */
        {
            auto state(_state->lock());

            while (state->data.empty()) {

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

                state.wait(state->avail);
            }

            chunk = std::move(state->data);

            state->request.notify_one();
        }

        /* Flush the data to the sink and wake up the download thread
           if it's blocked on a full buffer. We don't hold the state
           lock while doing this to prevent blocking the download
           thread if sink() takes a long time. */
        sink((unsigned char *) chunk.data(), chunk.size());
    }
}

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";
}


}