zsclient.cpp

// top level includes
#include "zsglobal.h"

// system includes
#include <iostream>
#include <deque>
#include <fcntl.h>
#include <set>
#include <sys/stat.h>
#include <utility>
#include <utime.h>

// library includes
#include <cpr/cpr.h>

extern "C" {
    #include <hashlib/md5.h>
    #include <hashlib/sha1.h>
    #include <hashlib/sha256.h>
    #include <zsync.h>
    #include <zlib.h>
}

// local includes
#include "zsclient.h"
#include "zsutil.h"
extern "C" {
    #include "legacy_http.h"
    #ifdef ZSYNC_STANDALONE
        #include "legacy_progress.h"
    #endif
}

namespace zsync2 {
    class ZSyncClient::Private {
    public:
        // there might be more than one seed file
        // using a set to avoid duplicate entries
        std::set<std::string> seedFiles;
        
        const std::string pathOrUrlToZSyncFile;
        std::string pathToLocalFile;
        std::string pathToStoreZSyncFileInLocally;
        bool zSyncFileStoredLocallyAlready;

        struct zsync_state* zsHandle;

        std::string referer;

        enum State {INITIALIZED=0, RUNNING, VERIFYING, DONE};
        State state;

        long long localUsed;
        long long httpDown;

        std::string cwd;

        off_t remoteFileSizeCache;

        // status message variables
#ifndef ZSYNC_STANDALONE
        std::deque<std::string> statusMessages;
#endif

    public:
        Private(
            std::string pathOrUrlToZSyncFile,
            const std::string& pathToLocalFile,
            const bool overwrite
        ) : pathOrUrlToZSyncFile(std::move(pathOrUrlToZSyncFile)), zsHandle(nullptr), state(INITIALIZED),
                                 localUsed(0), httpDown(0), remoteFileSizeCache(-1),
                                 zSyncFileStoredLocallyAlready(false) {
            // if the local file should be overwritten, we'll instruct
            if (overwrite) {
                this->pathToLocalFile = pathToLocalFile;
            } else {
                this->seedFiles.insert(pathToLocalFile);
            }

            // initialize cwd
            {
                size_t cwdBufSize = 4096;
                auto* cwdBuf = (char*) calloc(4096, sizeof(char));
                cwd = getcwd(cwdBuf, cwdBufSize);
                free(cwdBuf);
            }
        }
        
        ~Private() = default;
        
    public:
        // by default, the messages are pushed into a queue which can be fetched by calling the client's
        // nextStatusMessage()
        // when defining ZSYNC_STANDALONE, the messages are printed out on stderr directly instead
        // TODO: IDEA: why not allow passing an optional "error=true/false" flag?
        void issueStatusMessage(const std::string &message) {
#ifndef ZSYNC_STANDALONE
            statusMessages.push_back(message);
#else
            std::cerr << message << std::endl;
#endif
        }

        double calculateProgress() {
            if(zsHandle == nullptr)
                return 0;

            if (state >= VERIFYING)
                return 1;

            long long zgot, ztot;

            zsync_progress(zsHandle, &zgot, &ztot);
            return (double) zgot / (double) ztot;
        };

        bool setMtime(time_t mtime) {
            struct stat s;
            struct utimbuf u;

            // get access time (shouldn't be modified)
            if (stat(pathToLocalFile.c_str(), &s) != 0) {
                issueStatusMessage("failed to call stat()");
                return false;
            }

            u.actime = s.st_atime;
            u.modtime = mtime;

            if (utime(pathToLocalFile.c_str(), &u) != 0) {
                issueStatusMessage("failed to call utime()");
                return false;
            }

            return true;
        }

        struct zsync_state* readZSyncFile(bool headersOnly = false) {
            struct zsync_state *zs;
            std::FILE* f;

            // buffer storing the data
            std::vector<char> buffer;

            if (isfile(pathOrUrlToZSyncFile)) {
                f = std::fopen(pathOrUrlToZSyncFile.c_str(), "r");
            } else {
                if (!isUrlAbsolute(pathOrUrlToZSyncFile)) {
                    issueStatusMessage("No such file or directory and not a URL: " + pathOrUrlToZSyncFile);
                    return nullptr;
                }

                auto checkResponseForError = [this](cpr::Response response, unsigned int statusCode) {
                    if (response.status_code != statusCode) {
                        issueStatusMessage("Bad status code " + std::to_string(response.status_code) +
                                           " while trying to download .zsync file!");
                        return false;
                    }
                    return true;
                };

                // implements RFC 3230 (extended by RFC 5843)
                auto verifyInstanceDigest = [this](const cpr::Response verificationResponse, const cpr::Response response, bool& digestFound) {
                    digestFound = false;

                    if (response.status_code != 200) {
                        if (verificationResponse.status_code == 206) {
                            issueStatusMessage("Skipping instance digest verification of partial response");
                            return true;
                        }

                        return false;
                    }

                    for (const auto& header : verificationResponse.header) {
                        if (toLower(header.first) == "digest") {


                            // split by comma to support multiple digests as per RFC 3230
                            for (auto part : split(header.second, ',')) {
                                trim(part);

                                // now split key and value
                                const auto keyval = split(header.second, '=');

                                if (keyval.size() != 2) {
                                    issueStatusMessage("Failed to parse key/value pair: " + part);
                                    return false;
                                }

                                const auto& algorithm = keyval[0];
                                const auto& value = keyval[1];

                                auto rawDigest = base64Decode(value);
                                auto digest = bytesToHex((unsigned char*) rawDigest.data(), (int) rawDigest.size());

                                if (toLower(algorithm) == "md5") {
                                    digestFound = true;

                                    MD5 md5;
                                    md5.add(response.text.data(), response.text.size());
                                    auto calculatedDigest = md5.getHash();

                                    issueStatusMessage("Found MD5 digest: " + digest);

                                    if (digest == calculatedDigest) {
                                        issueStatusMessage("Verified instance digest of redirected .zsync response");
                                    } else {
                                        issueStatusMessage("Failed to verify digest of redirected .zsync response, aborting update");
                                        return false;
                                    }

                                } else if (toLower(algorithm) == "sha") {
                                    digestFound = true;

                                    SHA1 sha1;
                                    sha1.add(response.text.data(), response.text.size());
                                    auto calculatedDigest = sha1.getHash();

                                    issueStatusMessage("Found SHA1 digest: " + digest);

                                    if (digest == calculatedDigest) {
                                        issueStatusMessage("Verified instance digest of redirected .zsync response");
                                    } else {
                                        issueStatusMessage("Failed to verify digest of redirected .zsync response, aborting update");
                                        return false;
                                    }
                                } else if (toLower(algorithm) == "sha-256") {
                                    digestFound = true;

                                    SHA256 sha256;
                                    sha256.add(response.text.data(), response.text.size());
                                    auto calculatedDigest = sha256.getHash();

                                    issueStatusMessage("Found SHA256 digest: " + digest);

                                    if (digest == calculatedDigest) {
                                        issueStatusMessage("Verified instance digest of redirected .zsync response");
                                    } else {
                                        issueStatusMessage("Failed to verify digest of redirected .zsync response, aborting update");
                                        return false;
                                    }
                                } else if (toLower(algorithm) == "sha-512") {
                                    digestFound = false;

                                    issueStatusMessage("Found SHA512 digest: " + digest);
                                    issueStatusMessage("SHA512 instance digests are not supported at the moment");
                                } else {
                                    digestFound = false;
                                    issueStatusMessage("Invalid instance digest type: " + algorithm);
                                    return false;
                                }
                            }

                            // accept only one Digest header
                            break;
                        }
                    }

                    response.header;

                    return true;
                };

                // keep a session to make use of cURL's persistent connections feature
                cpr::Session session;

                session.SetUrl(pathOrUrlToZSyncFile);
                // request so-called Instance Digest (RFC 3230, RFC 5843)
                session.SetHeader(cpr::Header{{"want-digest", "sha-512;q=1, sha-256;q=0.9, sha;q=0.2, md5;q=0.1"}});

                // if interested in headers only, download 1 kiB chunks until end of zsync header is found
                if (headersOnly && zSyncFileStoredLocallyAlready) {
                    static const auto chunkSize = 1024;
                    unsigned long currentChunk = 0;

                    // download a chunk at a time
                    while (true) {
                        std::ostringstream bytes;
                        bytes << "bytes=" << currentChunk << "-" << currentChunk + chunkSize - 1;
                        session.SetHeader(cpr::Header{{"range", bytes.str()}});

                        session.SetRedirect(false);
                        auto verificationResponse = session.Get();

//                        bool digestVerified;
//                        if (!verifyInstanceDigest(verificationResponse, digestVerified))
//                            return nullptr;

                        session.SetRedirect(true);
                        auto response = session.Get();

                        // expect a range response
                        if (!checkResponseForError(response, 206))
                            return nullptr;

                        std::copy(response.text.begin(), response.text.end(), std::back_inserter(buffer));

                        // check whether double newline is contained, which marks the last request
                        if (response.text.find("\n\n") || response.text.find("\r\n\r\n"))
                            break;

                        currentChunk += chunkSize;
                    }
                } else {
                    session.SetRedirect(false);
                    auto verificationResponse = session.Get();

                    session.SetRedirect(true);
                    auto response = session.Get();

                    bool digestVerified;
                    if (!verifyInstanceDigest(verificationResponse, response, digestVerified))
                        return nullptr;

                    // expecting a 200 response
                    if (!checkResponseForError(response, 200))
                        return nullptr;

                    std::copy(response.text.begin(), response.text.end(), std::back_inserter(buffer));
                }

                // might have been redirected to another URL
                // therefore, store final URL of response as referer in case relative URLs will have to be resolved
                referer = pathOrUrlToZSyncFile;

                // open buffer as file
                f = fmemopen(buffer.data(), buffer.size(), "r");
            }

            if ((zs = zsync_begin(f, (headersOnly ? 1 : 0), (cwd.empty() ? nullptr : cwd.c_str()))) == nullptr) {
                issueStatusMessage("Failed to parse .zsync file!");
                return nullptr;
            }

            // store copy of .zsync file locally, if specified
            if (!pathToStoreZSyncFileInLocally.empty() && !zSyncFileStoredLocallyAlready) {
                std::ofstream ofs(pathToStoreZSyncFileInLocally);
                auto error = errno;

                if (!ofs) {
                    issueStatusMessage(
                        "Warning: could not store copy of .zsync file in path: " +
                        std::string(strerror(error))
                    );
                }

                std::ostringstream oss;
                oss << "Storing copy of .zsync file in " << pathToStoreZSyncFileInLocally << ", as requested";
                issueStatusMessage(oss.str());

                // make sure we start at the beginning of the file
                rewind(f);

                // prepare buffer
                constexpr size_t bufsize = 4096;
                std::vector<char> copyBuf;
                copyBuf.resize(bufsize);

                // read buffers and write data to file
                size_t bytesRead = 0;
                while ((bytesRead = fread(&copyBuf[0], sizeof(char), bufsize, f)) > 0) {
                    ofs.write(copyBuf.data(), bytesRead);
                }

                // reset file offset
                rewind(f);

                zSyncFileStoredLocallyAlready = true;
            }

            if (fclose(f) != 0) {
                issueStatusMessage("fclose() call failed!");
                return nullptr;
            }

            return zs;
        }

        // TODO: verify functionality
        bool populatePathToLocalFileFromZSyncFile(struct zsync_state* zs) {
            // don't overwrite path
            if (!pathToLocalFile.empty())
                return true;

            auto* p = zsync_filename(zs);

            std::string newPath;

            if (p) {
                std::string zsFilename = p;
                free(p);
                p = nullptr;

                // check that the filename does not contain any directory prefixes
                // that might be abused to create files in arbitrary locations, and could be used to put malicious files
                // in system locations in case ZSync2 is called with administrative permissions
                if (zsFilename.find('/') != std::string::npos) {
                    issueStatusMessage("rejected filename specified in " + pathOrUrlToZSyncFile + ", contained path component");
                    return false;
                }

                std::string filenamePrefix = pathPrefix(pathToLocalFile);

                if (zsFilename.substr(0, filenamePrefix.length()) == filenamePrefix)
                    newPath = zsFilename;

                if (!filenamePrefix.empty() && newPath.empty()) {
                    issueStatusMessage("Rejected filename specified in " + pathOrUrlToZSyncFile +
                                       " - prefix " + filenamePrefix + " is different from filename " + zsFilename);
                }
            }

            if (newPath.empty()) {
                newPath = pathPrefix(pathToLocalFile);
                if (newPath.empty())
                    newPath = "zsync-download";
            }

            pathToLocalFile = newPath;
            return true;
        }

        // open a gz-compressed file using zlib
        // the function produces a transparent wrapper for zlib, so that normal file operations (fread, fseek etc.) can
        // be used without noticing any difference
        std::FILE* openGzFile(const std::string &filePath) {
            // open file using zlib
            auto f = gzopen(filePath.c_str(), "r");

            // if file couldn't be opened, behave like normal fopen()
            if (f == nullptr)
                return nullptr;

#if defined(APPIMAGEUPDATE_LINUX)
            // map file functions to zlib functions
            // NOTE: fopencookie is a Linux-only solution
            // for other platforms, this will have to be adapted (e.g., using funopen() on BSD)
            cookie_io_functions_t iofuncs = {
                [](void* gzf, char* buf, size_t count) { return (ssize_t) gzread((gzFile) gzf, buf, (unsigned) count); },
                [](void* gzf, const char* buf, size_t count) { return (ssize_t) gzwrite((gzFile) gzf, buf, (unsigned) count); },
                [](void* gzf, off64_t* offset, int whence) { return (int) gzseek((gzFile) gzf, (unsigned) *offset, whence); },
                [](void* gzf) { return gzclose((gzFile) gzf); },
            };
            return fopencookie(f, "r", iofuncs);
#else
#error TODO: implement openGzFile() for this platform!
#endif
        }

        bool readSeedFile(const std::string &pathToSeedFile) {
            std::FILE* f;

            // check whether to decompress this file
            if (zsync_hint_decompress(zsHandle) && pathToSeedFile.length() > 3 && endsWith(pathToSeedFile, ".gz")) {
                f = openGzFile(pathToSeedFile);

                if (!f) {
                    issueStatusMessage("Failed to open gzip compressed file " + pathToSeedFile);
                    return false;
                }
            } else {
                f = fopen(pathToSeedFile.c_str(), "r");

                if (!f) {
                    issueStatusMessage("Failed to open file " + pathToSeedFile);
                    return false;
                }
            }

            zsync_submit_source_file(zsHandle, f, false);

            if (fclose(f) != 0) {
                issueStatusMessage("fclose() on file handle failed!");
                return false;
            }

            return true;
        }

        bool verifyDownloadedFile(std::string tempFilePath) {
            state = VERIFYING;

            auto r = zsync_complete(zsHandle);

            switch (r) {
                case -1:
                    issueStatusMessage("aborting, download available in " + tempFilePath);
                    return false;
                case 0:
                    issueStatusMessage("no recognized checksum found");
                    break;
                case 1:
                    issueStatusMessage("checksum matches OK");
                    break;
                default:
                    issueStatusMessage("verification failed: unrecognized error code");
                    return false;
            }

            return true;
        }

        bool makeUrlAbsolute(const std::string& base, const std::string& relative, std::string& final) {
            if (isUrlAbsolute(relative)) {
                final = relative;
                return true;
            }

            if (base.empty())
                return false;

            // check if URL is relative to root, not current "directory"
            if (relative.front() == '/') {
                // find end of scheme
                auto schemePos = base.find("://");

                // if there's no scheme prefix, one can't make the relative URL absolute
                if (schemePos == std::string::npos)
                    return false;

                // search for next slash
                auto pos = base.find('/', schemePos + 1);

                //
                if (pos == std::string::npos)
                    pos = base.length();

                final = base.substr(0, pos - 1) + relative;
            } else {
                // find "directory" in relative path
                auto pos = relative.find_last_of('?');

                if (pos == std::string::npos)
                    pos = relative.find_last_of('#');

                if (pos == std::string::npos)
                    pos = base.length();

                auto slashPos = base.find_last_of('/');
                if (slashPos == std::string::npos)
                    return false;

                final = base.substr(0, slashPos + 1) + relative;
            }

            return true;
        }

        int fetchRemainingBlocksHttp(const std::string &url, int urlType) {
            // use static const int instead of a define
            static const auto BUFFERSIZE = 8192;

            int ret = 0;

            struct range_fetch* rf;
            struct zsync_receiver* zr;

            // URL might be relative -- we need an absolute URL to do a fetch
            std::string absoluteUrl;

            if (!makeUrlAbsolute(referer, url, absoluteUrl)) {
                issueStatusMessage("URL '" + url + "' from .zsync file is relative, which cannot be resolved without "
                                   "knowing the URL to the .zsync file (you're most likely trying to use a .zsync "
                                   "file you downloaded from the internet). Without knowing the original URL, it is "
                                   "impossible to resolve the URL from the .zsync file. Please specify a URL with the "
                                   "-u flag, or edit and fix the lines in the .zsync file directly.");
                return -1;
            }

            // follow redirections of the URL before passing it to libzsync to avoid unnecessary redirects for
            // multiple range requests
            std::string redirectedUrl;
            if (!resolveRedirections(absoluteUrl, redirectedUrl)) {
                issueStatusMessage("Failed to resolve redirection.");
                return -1;
            }

            /* Start a range fetch and a zsync receiver */
            rf = range_fetch_start(redirectedUrl.c_str());
            if (rf == nullptr)
                return -1;

            zr = zsync_begin_receive(zsHandle, urlType);
            if (zr == nullptr) {
                range_fetch_end(rf);
                return -1;
            }

            issueStatusMessage("Downloading from " + redirectedUrl);

            /* Create a read buffer */
            std::vector<unsigned char> buffer;
            try {
                buffer.reserve(BUFFERSIZE);
            } catch (std::bad_alloc& e) {
                // finish available data, then re-throw
                zsync_end_receive(zr);
                range_fetch_end(rf);
                throw;
            }

            {   /* Get a set of byte ranges that we need to complete the target */
                int nrange;
                auto* zbyterange = zsync_needed_byte_ranges(zsHandle, &nrange, urlType);
                if (zbyterange == nullptr)
                    return 1;
                if (nrange == 0)
                    return 0;

                for(int i = 0; i < 2 * nrange; i++){
                    auto beginbyte = zbyterange[i];
                    i++;
                    auto endbyte = zbyterange[i];
                    off_t single_range[2] = {beginbyte, endbyte};
                    /* And give that to the range fetcher */
                    /* Only one range at a time because Akamai can't handle more than one range per request */
                    range_fetch_addranges(rf, single_range, 1);

                    {
                        int len;
                        off_t zoffset;

                        #ifdef ZSYNC_STANDALONE
                        struct progress p = { 0, 0, 0, 0 };

                        /* Set up progress display to run during the fetch */
                        fputc('\n', stderr);
                        do_progress(&p, (float) calculateProgress() * 100.0f, range_fetch_bytes_down(rf));
                        #endif

                        /* Loop while we're receiving data, until we're done or there is an error */
                        while (!ret
                               && (len = get_range_block(rf, &zoffset, buffer.data(), BUFFERSIZE)) > 0) {
                            /* Pass received data to the zsync receiver, which writes it to the
                             * appropriate location in the target file */
                            if (zsync_receive_data(zr, buffer.data(), zoffset, len) != 0)
                                ret = 1;

                            #ifdef ZSYNC_STANDALONE
                            /* Maintain progress display */
                            do_progress(&p, (float) calculateProgress() * 100.0f,
                                        range_fetch_bytes_down(rf));
                            #endif

                            // Needed in case next call returns len=0 and we need to signal where the EOF was.
                            zoffset += len;
                        }

                        /* If error, we need to flag that to our caller */
                        if (len < 0) {
                            fprintf(stdout, "%d returned\n", len);
                            ret = -1;
                        }
                        else{    /* Else, let the zsync receiver know that we're at EOF; there
                         *could be data in its buffer that it can use or needs to process */
                            zsync_receive_data(zr, nullptr, zoffset, 0);
                        }

                        #ifdef ZSYNC_STANDALONE
                        end_progress(&p, zsync_status(zsHandle) >= 2 ? 2 : len == 0 ? 1 : 0);
                        #endif
                    }

                }

                free(zbyterange);

            }

            /* Clean up */
            httpDown += range_fetch_bytes_down(rf);
            zsync_end_receive(zr);
            range_fetch_end(rf);
            return ret;
        }

        bool fetchRemainingBlocks() {
            int n = 0, utype = 0;
            const auto* url = zsync_get_urls(zsHandle, &n, &utype);

            // copy value for later use
            int okUrls = n;

            if (!url) {
                issueStatusMessage("no URLs available from zsync?");
                return false;
            }

            int* status;
            status = static_cast<int*>(calloc(n, sizeof *status));

            while (zsync_status(zsHandle) < 2 && okUrls) {
                int attempt = rand() % n;

                if (!status[attempt]) {
                    const std::string tryurl = url[attempt];

                    auto result = fetchRemainingBlocksHttp(tryurl, utype);

                    if (result != 0) {
                        issueStatusMessage("failed to retrieve from " + tryurl + ", status " + std::to_string(result));
                        return false;
                    }
                }
            }

            return true;
        }

        void applyCwdToPathToLocalFile() {
            if (strncmp(pathToLocalFile.c_str(), "/", 1) == 0)
                return;

            auto oldPath = pathToLocalFile;
            pathToLocalFile = cwd;
            if (!endsWith(pathToLocalFile, "/"))
                pathToLocalFile += "/";
            pathToLocalFile += oldPath;
        }

        bool run() {
            // exit if run has been called before
            if (state != INITIALIZED) {
                issueStatusMessage("Could not start client: running/done already!");
                return false;
            }

            state = RUNNING;

            /**** step 1: read .zsync file ****/
            if ((zsHandle = readZSyncFile()) == nullptr) {
                issueStatusMessage("Reading and/or parsing .zsync file failed!");
                state = DONE;
                return false;
            }

            // check whether path was explicitly passed
            // otherwise, use the one defined in the .zsync file
            if (!populatePathToLocalFileFromZSyncFile(zsHandle)) {
                state = DONE;
                return false;
            }

            // make sure new file will be created in the same directory as the original file
            applyCwdToPathToLocalFile();

            // calculate path to temporary file
            auto tempFilePath = pathToLocalFile + ".part";

            {
                /**** step 2: read in available data from seed files and fill in existing data into target file ****/
                if (isfile(pathToLocalFile)) {
                    issueStatusMessage(pathToLocalFile + " found, using as seed file");
                    seedFiles.insert(pathToLocalFile);
                }

                // if the temporary file exists, it's likely left over from a previous attempt that got interrupted
                // due to how zsync works, one can't just "resume" from this file like a normal HTTP client would do (also,
                // the server file might have changed in the meantime), but one can certainly make use of it as a seed file
                if (isfile(tempFilePath)) {
                    issueStatusMessage(tempFilePath + " found, using as seed file");
                    seedFiles.insert(tempFilePath);
                }

                issueStatusMessage("Target file: " + pathToLocalFile);

                // try to make use of any seed file provided
                for (const auto &seedFile : seedFiles) {
                    // exit loop if file is complete
                    if (zsync_status(zsHandle) >= 2) {
                        state = DONE;
                        break;
                    }

                    issueStatusMessage("Reading seed file: " + seedFile);
                    if(!readSeedFile(seedFile)) {
                        state = DONE;
                        return false;
                    }
                }

                // first, store current value
                zsync_progress(zsHandle, &localUsed, nullptr);
                // now, show how far that got us
                issueStatusMessage("Usable data from seed files: " + std::to_string(calculateProgress() * 100.0f) + "%");
            }

            // libzsync has been writing to a randomly-named temp file so far -
            // because we didn't want to overwrite the .part from previous runs. Now
            // we've read any previous .part, we can replace it with our new
            // in-progress run (which should be a superset of the old .part - unless
            // the content changed, in which case it still contains anything relevant
            // from the old .part).
            issueStatusMessage("Renaming temp file");
            if (zsync_rename_file(zsHandle, tempFilePath.c_str()) != 0) {
                state = DONE;
                return false;
            }

            // step 3: fetch remaining blocks via the URLs from the .zsync
            issueStatusMessage("Fetching remaining blocks");
            if(!fetchRemainingBlocks()) {
                state = DONE;
                return false;
            }

            // step 4: verify download
            issueStatusMessage("Verifying downloaded file");
            if(!verifyDownloadedFile(tempFilePath)) {
                state = DONE;
                return false;
            }

            // Get any mtime that we is suggested to set for the file, and then shut
            // down the zsync_state as we are done on the file transfer. Getting the
            // current name of the file at the same time.
            auto mtime = zsync_mtime(zsHandle);
            tempFilePath = zsync_end(zsHandle);
            zsHandle = nullptr;

            // step 5: replace original file by completed .part file
            if (!pathToLocalFile.empty()) {
                bool ok = true;
                std::string oldFileBackup = pathToLocalFile + ".zs-old";

                if (isfile(pathToLocalFile)) {
                    // remove previous update
                    // error check unnecessary -- failures will be checked for in the next part
                    unlink(oldFileBackup.c_str());

                    // copy permissions from old file
                    mode_t newPerms;
                    auto errCode = getPerms(pathToLocalFile, newPerms);

                    if (errCode != 0) {
                        std::ostringstream oss;
                        oss << "Failed to copy permissions to new file: " << strerror(errCode);
                        issueStatusMessage(oss.str());
                    } else {
                        chmod(tempFilePath.c_str(), newPerms);
                    }

                    if (link(pathToLocalFile.c_str(), oldFileBackup.c_str()) != 0) {
                        issueStatusMessage("Unable to backup " + pathToLocalFile + " to " + oldFileBackup);
                        ok = false;
                    } else if (errno == EPERM) {
                        int error = errno;
                        issueStatusMessage("Unable to backup " + pathToLocalFile + " to " + oldFileBackup +
                                           ": " + strerror(error));
                        ok = false;
                    } else if (rename(pathToLocalFile.c_str(), oldFileBackup.c_str()) != 0) {
                        issueStatusMessage("Unable to back up " + pathToLocalFile + " to old file " + oldFileBackup +
                                           " - completed download left in " + tempFilePath);
                        // prevent overwrite of old file below
                        ok = false;
                    }
                }

                if (ok) {
                    if (rename(tempFilePath.c_str(), pathToLocalFile.c_str()) == 0) {
                        // success, setting mtime
                        if (mtime != -1)
                            setMtime(mtime);
                    } else {
                        int error = errno;
                        std::ostringstream ss;
                        ss << "Unable to move " << oldFileBackup << " to final file " << pathToLocalFile
                           << ": " << strerror(error)
                           << " - completed download left in " + tempFilePath;
                        issueStatusMessage(ss.str());
                    }
                }
            } else {
                issueStatusMessage("No filename specified for download - completed download left in " + tempFilePath);
            }

            // final stats and cleanup
            issueStatusMessage("used " + std::to_string(localUsed) + " local, fetched " + std::to_string(httpDown));
            state = DONE;
            return true;
        }

        bool checkForChanges(bool& updateAvailable, const unsigned int method) {
            struct zsync_state *zs;

            // now, read zsync file
            if ((zs = readZSyncFile(true)) == nullptr) {
                issueStatusMessage("Reading and/or parsing .zsync file failed!");
                return false;
            }

            // make sure pathToLocalFile is set
            if (!populatePathToLocalFileFromZSyncFile(zs)) {
                issueStatusMessage("Failed to read filename from .zsync file!");
                return false;
            }

            // check whether file exists at all, because if not, a full download is required
            if (!isfile(pathToLocalFile)) {
                issueStatusMessage("Cannot find file " + pathToLocalFile);
                updateAvailable = true;
                return true;
            }

            switch (method) {
                case 0: {
                    const auto fh = open(pathToLocalFile.c_str(), O_RDONLY);

                    if (fh < 0) {
                        issueStatusMessage("Error opening file " + pathToLocalFile);
                        return false;
                    }

                    auto rv = zsync_sha1(zs, fh);

                    switch(rv) {
                        case -1:
                            updateAvailable = true;
                            break;
                        case 1:
                            updateAvailable = false;
                            break;
                        default:
                            // unknown/invalid return value
                            close(fh);
                            return false;
                    }

                    close(fh);
                    break;
                }
                case 1: {
                    struct stat appImageStat;
                    if (stat(pathToLocalFile.c_str(), &appImageStat) != 0) {
                        return false;
                    }

                    updateAvailable = (zsync_mtime(zs) > appImageStat.st_mtime);
                    break;
                }
                default: {
                    issueStatusMessage("Unknown update method: " + std::to_string(method));
                    return false;
                }
            }

            return true;
        }

        bool setCwd(const std::string& path) {
            // cwd can be changed until the update is started
            if (state > INITIALIZED)
                return false;

            // make path absolute
            char* realPath;
            if ((realPath = realpath(path.c_str(), nullptr)) == nullptr)
                return false;

            cwd = realPath;
            free(realPath);
            return true;
        }

        bool remoteFileSize(off_t& fileSize) {
            if (remoteFileSizeCache < 0) {
                if (zsHandle == nullptr)
                    return false;

                remoteFileSizeCache = zsync_filelen(zsHandle);
            }

            if (remoteFileSizeCache < 0)
                return false;

            fileSize = remoteFileSizeCache;
            return true;
        }
    };

    ZSyncClient::ZSyncClient(const std::string pathOrUrlToZSyncFile, const std::string pathToLocalFile, bool overwrite) {
        d = new Private(pathOrUrlToZSyncFile, pathToLocalFile, overwrite);
    }
    ZSyncClient::~ZSyncClient() {
        delete d;
    };

#ifndef ZSYNC_STANDALONE
    bool ZSyncClient::nextStatusMessage(std::string &message) {
        if (!d->statusMessages.empty()) {
            message = d->statusMessages.front();
            d->statusMessages.pop_front();
            return true;
        }

        return false;
    }
#endif

    double ZSyncClient::progress() {
        return d->calculateProgress();
    }

    bool ZSyncClient::run() {
        auto result = d->run();

        // make sure to change state to DONE unless result shows an error
        if (result)
            d->state = d->DONE;

        return result;
    }

    bool ZSyncClient::checkForChanges(bool& updateAvailable, const unsigned int method) {
        return d->checkForChanges(updateAvailable, method);
    }

    void ZSyncClient::addSeedFile(const std::string &path) {
        d->seedFiles.insert(path);
    }

    bool ZSyncClient::pathToNewFile(std::string& path) {
        if (d->state <= d->RUNNING)
            return false;

        if (d->pathToLocalFile.empty())
            return false;

        path = d->pathToLocalFile;
        return true;
    }

    bool ZSyncClient::setCwd(const std::string& path) {
        return d->setCwd(path);
    }

    bool ZSyncClient::remoteFileSize(off_t& fileSize) {
        return d->remoteFileSize(fileSize);
    }

    void ZSyncClient::storeZSyncFileInPath(const std::string& path) {
        d->pathToStoreZSyncFileInLocally = path;
    }
}