Add fetchTarball builtin

This function downloads and unpacks the given URL at evaluation
time. This is primarily intended to make it easier to deal with Nix
expressions that have external dependencies. For instance, to fetch
Nixpkgs 14.12:

  with import (fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz) {};

Or to fetch a specific revision:

  with import (fetchTarball https://github.com/NixOS/nixpkgs/archive/2766a4b44ee6eafae03a042801270c7f6b8ed32a.tar.gz) {};

This patch also adds a ‘fetchurl’ builtin that downloads but doesn't
unpack its argument. Not sure if it's useful though.

src/libexpr/primops.cc

@@ -18,6 +18,8 @@
 #include <cstring>
 #include <dlfcn.h>
 
+#include <curl/curl.h>
+
 
 namespace nix {
 
@@ -1479,6 +1481,119 @@ static void prim_compareVersions(EvalState & state, const Pos & pos, Value * * a
 }
 
 
+/*************************************************************
+ * Networking
+ *************************************************************/
+
+
+struct Curl
+{
+    CURL * curl;
+    string data;
+
+    static size_t writeCallback(void * contents, size_t size, size_t nmemb, void * userp)
+    {
+        Curl & c(* (Curl *) userp);
+        size_t realSize = size * nmemb;
+        c.data.append((char *) contents, realSize);
+        return realSize;
+    }
+
+    Curl()
+    {
+        curl = curl_easy_init();
+        if (!curl) throw Error("unable to initialize curl");
+
+        curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
+        curl_easy_setopt(curl, CURLOPT_CAINFO, getEnv("SSL_CERT_FILE", "/etc/ssl/certs/ca-certificates.crt").c_str());
+        curl_easy_setopt(curl, CURLOPT_USERAGENT, ("Nix/" + nixVersion).c_str());
+
+        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeCallback);
+        curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *) &curl);
+    }
+
+    ~Curl()
+    {
+        if (curl) curl_easy_cleanup(curl);
+    }
+
+    string fetch(const string & url)
+    {
+        curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
+
+        data.clear();
+
+        CURLcode res = curl_easy_perform(curl);
+        if (res != CURLE_OK)
+            throw Error(format("unable to download ‘%1%’: %2%")
+                % url % curl_easy_strerror(res));
+
+        return data;
+    }
+};
+
+
+void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
+    const string & who, bool unpack)
+{
+    if (state.restricted) throw Error(format("‘%1%’ is not allowed in restricted mode") % who);
+
+    string url;
+
+    state.forceValue(*args[0]);
+
+    if (args[0]->type == tAttrs) {
+
+        state.forceAttrs(*args[0], pos);
+
+        for (auto & attr : *args[0]->attrs) {
+            string name(attr.name);
+            if (name == "url")
+                url = state.forceStringNoCtx(*attr.value, *attr.pos);
+            else
+                throw EvalError(format("unsupported argument ‘%1%’ to ‘%2%’, at %3%") % attr.name % who % attr.pos);
+        }
+
+        if (url.empty())
+            throw EvalError(format("‘url’ argument required, at %1%") % pos);
+
+    } else
+        url = state.forceStringNoCtx(*args[0], pos);
+
+    // TODO: cache downloads.
+
+    Curl curl;
+    string data = curl.fetch(url);
+
+    string name;
+    string::size_type p = url.rfind('/');
+    if (p != string::npos) name = string(url, p + 1);
+
+    Path storePath = store->addTextToStore(name, data, PathSet(), state.repair);
+
+    if (unpack) {
+        Path tmpDir = createTempDir();
+        AutoDelete autoDelete(tmpDir, true);
+        runProgram("tar", true, {"xf", storePath, "-C", tmpDir, "--strip-components", "1"}, "");
+        storePath = store->addToStore(name, tmpDir, true, htSHA256, defaultPathFilter, state.repair);
+    }
+
+    mkString(v, storePath, singleton<PathSet>(storePath));
+}
+
+
+static void prim_fetchurl(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    fetch(state, pos, args, v, "fetchurl", false);
+}
+
+
+static void prim_fetchTarball(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    fetch(state, pos, args, v, "fetchTarball", true);
+}
+
+
 /*************************************************************
  * Primop registration
  *************************************************************/
@@ -1617,6 +1732,10 @@ void EvalState::createBaseEnv()
     // Derivations
     addPrimOp("derivationStrict", 1, prim_derivationStrict);
 
+    // Networking
+    addPrimOp("__fetchurl", 1, prim_fetchurl);
+    addPrimOp("fetchTarball", 1, prim_fetchTarball);
+
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
     string path = findFile("nix/derivation.nix");