lix/src/libstore/download.cc
Peter Simons 040acdcee2
Merge pull request #1707 from dtzWill/fix/issue-1692
ignore "interrupted" exception in progress callback
2018-04-21 16:51:19 +02:00

756 lines
28 KiB
C++

#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"
#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 {
double getTime()
{
struct timeval tv;
gettimeofday(&tv, 0);
return tv.tv_sec + (tv.tv_usec / 1000000.0);
}
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;
bool enableHttp2;
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
std::function<void(const DownloadResult &)> success;
std::function<void(std::exception_ptr exc)> failure;
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)
: downloader(downloader)
, request(request)
, act(*logger, lvlTalkative, actDownload, fmt("downloading '%s'", request.uri), {request.uri}, request.parentAct)
{
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;
callFailure(failure, std::make_exception_ptr(e));
}
size_t writeCallback(void * contents, size_t size, size_t nmemb)
{
size_t realSize = size * nmemb;
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;
int 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 int 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 +
(settings.userAgentSuffix != "" ? " " + settings.userAgentSuffix.get() : "")).c_str());
#if LIBCURL_VERSION_NUM >= 0x072b00
curl_easy_setopt(req, CURLOPT_PIPEWAIT, 1);
#endif
#if LIBCURL_VERSION_NUM >= 0x072f00
if (downloader.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, settings.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(format("finished download of '%s'; curl status = %d, HTTP status = %d, body = %d bytes")
% 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));
callSuccess(success, failure, const_cast<const DownloadResult &>(result));
act.progress(result.data->size(), result.data->size());
} catch (...) {
done = true;
callFailure(failure, std::current_exception());
}
} else {
// We treat most errors as transient, but won't retry when hopeless
Error err = Transient;
if (httpStatus == 404 || 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, format("download of '%s' was interrupted") % request.uri)
: httpStatus != 0
? DownloadError(err, format("unable to download '%s': HTTP error %d (curl error: %s)") % request.uri % httpStatus % curl_easy_strerror(code))
: DownloadError(err, format("unable to download '%s': %s (%d)") % 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,
settings.binaryCachesParallelConnections.get());
#endif
enableHttp2 = settings.enableHttp2;
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;
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())
: 10000;
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(format("starting download of %s") % 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)
{
{
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,
std::function<void(const DownloadResult &)> success,
std::function<void(std::exception_ptr exc)> failure) override
{
/* Ugly hack to support s3:// URIs. */
if (hasPrefix(request.uri, "s3://")) {
// FIXME: do this on a worker thread
sync2async<DownloadResult>(success, failure, [&]() -> DownloadResult {
#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;
return res;
#else
throw nix::Error("cannot download '%s' because Nix is not built with S3 support", request.uri);
#endif
});
return;
}
auto item = std::make_shared<DownloadItem>(*this, request);
item->success = success;
item->failure = failure;
enqueueItem(item);
}
};
ref<Downloader> getDownloader()
{
static std::shared_ptr<Downloader> downloader;
static std::once_flag downloaderCreated;
std::call_once(downloaderCreated, [&]() { downloader = makeDownloader(); });
return ref<Downloader>(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](const DownloadResult & result) { promise->set_value(result); },
[promise](std::exception_ptr exc) { promise->set_exception(exc); });
return promise->get_future();
}
DownloadResult Downloader::download(const DownloadRequest & request)
{
return enqueueDownload(request).get();
}
Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpack, string name, const Hash & expectedHash, string * effectiveUrl)
{
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;
int ttl = settings.tarballTtl;
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': expected hash '%s', got '%s'",
url, expectedHash.to_string(), gotHash.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";
}
}