Add an HTTP binary cache store

Allowing stuff like

  NIX_REMOTE=https://cache.nixos.org nix-store -qR /nix/store/x1p1gl3a4kkz5ci0nfbayjqlqmczp1kq-geeqie-1.1

or

  NIX_REMOTE=https://cache.nixos.org nix-store --export /nix/store/x1p1gl3a4kkz5ci0nfbayjqlqmczp1kq-geeqie-1.1 | nix-store --import

7 files changed, 100 insertions, 59 deletions

diff --git a/src/libexpr/common-opts.cc b/src/libexpr/common-opts.cc
index 68ab4b5cdcbf..8a7989aac663 100644
--- a/src/libexpr/common-opts.cc
+++ b/src/libexpr/common-opts.cc
@@ -55,7 +55,7 @@ bool parseSearchPathArg(Strings::iterator & i,
 Path lookupFileArg(EvalState & state, string s)
 {
     if (isUri(s))
-        return downloadFileCached(state.store, s, true);
+        return makeDownloader()->downloadCached(state.store, s, true);
     else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
         Path p = s.substr(1, s.size() - 2);
         return state.findFile(p);
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 80ecd44c5981..11dc7bb5ccdf 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -613,7 +613,7 @@ void EvalState::addToSearchPath(const string & s, bool warn)
     }
 
     if (isUri(path))
-        path = downloadFileCached(store, path, true);
+        path = makeDownloader()->downloadCached(store, path, true);
 
     path = absPath(path);
     if (pathExists(path)) {
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 3c899d769253..5bfb95be6849 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1703,7 +1703,7 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
-    Path res = downloadFileCached(state.store, url, unpack);
+    Path res = makeDownloader()->downloadCached(state.store, url, unpack);
     mkString(v, res, PathSet({res}));
 }
 
diff --git a/src/libstore/builtins.cc b/src/libstore/builtins.cc
index a1c4b48bf62e..c22c44f3c7e3 100644
--- a/src/libstore/builtins.cc
+++ b/src/libstore/builtins.cc
@@ -17,9 +17,9 @@ void builtinFetchurl(const BasicDerivation & drv)
     options.verifyTLS = false;
 
     /* Show a progress indicator, even though stderr is not a tty. */
-    options.forceProgress = true;
+    options.showProgress = DownloadOptions::yes;
 
-    auto data = downloadFile(url->second, options);
+    auto data = makeDownloader()->download(url->second, options);
 
     auto out = drv.env.find("out");
     if (out == drv.env.end()) throw Error("attribute ‘url’ missing");
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index e754e82fb27f..4776d0091685 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -18,7 +18,7 @@ double getTime()
     return tv.tv_sec + (tv.tv_usec / 1000000.0);
 }
 
-struct Curl
+struct CurlDownloader : public Downloader
 {
     CURL * curl;
     string data;
@@ -30,37 +30,40 @@ struct Curl
     double prevProgressTime{0}, startTime{0};
     unsigned int moveBack{1};
 
-    static size_t writeCallback(void * contents, size_t size, size_t nmemb, void * userp)
+    size_t writeCallback(void * contents, size_t size, size_t nmemb)
     {
-        Curl & c(* (Curl *) userp);
         size_t realSize = size * nmemb;
-        c.data.append((char *) contents, realSize);
+        data.append((char *) contents, realSize);
         return realSize;
     }
 
-    static size_t headerCallback(void * contents, size_t size, size_t nmemb, void * userp)
+    static size_t writeCallbackWrapper(void * contents, size_t size, size_t nmemb, void * userp)
+    {
+        return ((CurlDownloader *) userp)->writeCallback(contents, size, nmemb);
+    }
+
+    size_t headerCallback(void * contents, size_t size, size_t nmemb)
     {
-        Curl & c(* (Curl *) userp);
         size_t realSize = size * nmemb;
         string line = string((char *) contents, realSize);
         printMsg(lvlVomit, format("got header: %1%") % trim(line));
         if (line.compare(0, 5, "HTTP/") == 0) { // new response starts
-            c.etag = "";
+            etag = "";
             auto ss = tokenizeString<vector<string>>(line, " ");
-            c.status = ss.size() >= 2 ? ss[1] : "";
+            status = ss.size() >= 2 ? ss[1] : "";
         } else {
             auto i = line.find(':');
             if (i != string::npos) {
                 string name = trim(string(line, 0, i));
                 if (name == "ETag") { // FIXME: case
-                    c.etag = trim(string(line, i + 1));
+                    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. */
-                    printMsg(lvlDebug, format("got ETag: %1%") % c.etag);
-                    if (c.etag == c.expectedETag && c.status == "200") {
+                    printMsg(lvlDebug, format("got ETag: %1%") % etag);
+                    if (etag == expectedETag && status == "200") {
                         printMsg(lvlDebug, format("shutting down on 200 HTTP response with expected ETag"));
                         return 0;
                     }
@@ -70,6 +73,11 @@ struct Curl
         return realSize;
     }
 
+    static size_t headerCallbackWrapper(void * contents, size_t size, size_t nmemb, void * userp)
+    {
+        return ((CurlDownloader *) userp)->headerCallback(contents, size, nmemb);
+    }
+
     int progressCallback(double dltotal, double dlnow)
     {
         if (showProgress) {
@@ -88,45 +96,48 @@ struct Curl
         return _isInterrupted;
     }
 
-    static int progressCallback_(void * userp, double dltotal, double dlnow, double ultotal, double ulnow)
+    static int progressCallbackWrapper(void * userp, double dltotal, double dlnow, double ultotal, double ulnow)
     {
-        Curl & c(* (Curl *) userp);
-        return c.progressCallback(dltotal, dlnow);
+        return ((CurlDownloader *) userp)->progressCallback(dltotal, dlnow);
     }
 
-    Curl()
+    CurlDownloader()
     {
         requestHeaders = 0;
 
         curl = curl_easy_init();
-        if (!curl) throw Error("unable to initialize curl");
+        if (!curl) throw nix::Error("unable to initialize curl");
+    }
+
+    ~CurlDownloader()
+    {
+        if (curl) curl_easy_cleanup(curl);
+        if (requestHeaders) curl_slist_free_all(requestHeaders);
+    }
+
+    bool fetch(const string & url, const DownloadOptions & options)
+    {
+        showProgress =
+            options.showProgress == DownloadOptions::yes ||
+            (options.showProgress == DownloadOptions::automatic && isatty(STDERR_FILENO));
+
+        curl_easy_reset(curl);
 
         curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
         curl_easy_setopt(curl, CURLOPT_USERAGENT, ("Nix/" + nixVersion).c_str());
         curl_easy_setopt(curl, CURLOPT_FAILONERROR, 1);
 
-        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeCallback);
-        curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *) &curl);
+        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeCallbackWrapper);
+        curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *) this);
 
-        curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, headerCallback);
-        curl_easy_setopt(curl, CURLOPT_HEADERDATA, (void *) &curl);
+        curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, headerCallbackWrapper);
+        curl_easy_setopt(curl, CURLOPT_HEADERDATA, (void *) this);
 
-        curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, progressCallback_);
-        curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, (void *) &curl);
+        curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, progressCallbackWrapper);
+        curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, (void *) this);
         curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);
 
         curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1);
-    }
-
-    ~Curl()
-    {
-        if (curl) curl_easy_cleanup(curl);
-        if (requestHeaders) curl_slist_free_all(requestHeaders);
-    }
-
-    bool fetch(const string & url, const DownloadOptions & options)
-    {
-        showProgress = options.forceProgress || isatty(STDERR_FILENO);
 
         curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
 
@@ -151,6 +162,9 @@ struct Curl
 
         curl_easy_setopt(curl, CURLOPT_HTTPHEADER, requestHeaders);
 
+        if (options.head)
+            curl_easy_setopt(curl, CURLOPT_NOBODY, 1);
+
         if (showProgress) {
             std::cerr << (format("downloading ‘%1%’... ") % url);
             std::cerr.flush();
@@ -163,34 +177,46 @@ struct Curl
             std::cerr << "\n";
         checkInterrupt();
         if (res == CURLE_WRITE_ERROR && etag == options.expectedETag) return false;
-        if (res != CURLE_OK)
-            throw DownloadError(format("unable to download ‘%1%’: %2% (%3%)")
+
+        long httpStatus = -1;
+        if (res == CURLE_HTTP_RETURNED_ERROR)
+            curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &httpStatus);
+
+        if (res != CURLE_OK) {
+            long httpStatus = 0;
+            Error err =
+                httpStatus == 404 ? NotFound :
+                httpStatus == 403 ? Forbidden : Misc;
+            throw DownloadError(err, format("unable to download ‘%1%’: %2% (%3%)")
                 % url % curl_easy_strerror(res) % res);
+        }
 
-        long httpStatus = 0;
-        curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &httpStatus);
         if (httpStatus == 304) return false;
 
         return true;
     }
+
+    DownloadResult download(string url, const DownloadOptions & options) override
+    {
+        DownloadResult res;
+        if (fetch(url, options)) {
+            res.cached = false;
+            res.data = data;
+        } else
+            res.cached = true;
+        res.etag = etag;
+        return res;
+    }
 };
 
 
-DownloadResult downloadFile(string url, const DownloadOptions & options)
+ref<Downloader> makeDownloader()
 {
-    DownloadResult res;
-    Curl curl;
-    if (curl.fetch(url, options)) {
-        res.cached = false;
-        res.data = curl.data;
-    } else
-        res.cached = true;
-    res.etag = curl.etag;
-    return res;
+    return make_ref<CurlDownloader>();
 }
 
 
-Path downloadFileCached(ref<Store> store, const string & url, bool unpack)
+Path Downloader::downloadCached(ref<Store> store, const string & url, bool unpack)
 {
     Path cacheDir = getEnv("XDG_CACHE_HOME", getEnv("HOME", "") + "/.cache") + "/nix/tarballs";
     createDirs(cacheDir);
@@ -234,7 +260,7 @@ Path downloadFileCached(ref<Store> store, const string & url, bool unpack)
         try {
             DownloadOptions options;
             options.expectedETag = expectedETag;
-            auto res = downloadFile(url, options);
+            auto res = download(url, options);
 
             if (!res.cached)
                 storePath = store->addTextToStore(name, res.data, PathSet(), false);
diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index 7aec8de73e48..5dd2d2c82dec 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -10,7 +10,8 @@ struct DownloadOptions
 {
     string expectedETag;
     bool verifyTLS{true};
-    bool forceProgress{false};
+    enum { yes, no, automatic } showProgress{yes};
+    bool head{false};
 };
 
 struct DownloadResult
@@ -21,11 +22,25 @@ struct DownloadResult
 
 class Store;
 
-DownloadResult downloadFile(string url, const DownloadOptions & options);
+struct Downloader
+{
+    virtual DownloadResult download(string url, const DownloadOptions & options) = 0;
+
+    Path downloadCached(ref<Store> store, const string & url, bool unpack);
+
+    enum Error { NotFound, Forbidden, Misc };
+};
 
-Path downloadFileCached(ref<Store> store, const string & url, bool unpack);
+ref<Downloader> makeDownloader();
 
-MakeError(DownloadError, Error)
+class DownloadError : public Error
+{
+public:
+    Downloader::Error error;
+    DownloadError(Downloader::Error error, const FormatOrString & fs)
+        : Error(fs), error(error)
+    { }
+};
 
 bool isUri(const string & s);
 
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index c0c05a60bd78..c65961a15720 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -158,7 +158,7 @@ int main(int argc, char * * argv)
             auto actualUri = resolveMirrorUri(state, uri);
 
             /* Download the file. */
-            auto result = downloadFile(actualUri, DownloadOptions());
+            auto result = makeDownloader()->download(actualUri, DownloadOptions());
 
             AutoDelete tmpDir(createTempDir(), true);
             Path tmpFile = (Path) tmpDir + "/tmp";