Allow URLs in the Nix search path

E.g. to install "hello" from the latest Nixpkgs:

  $ nix-build '<nixpkgs>' -A hello -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz

Or to install a specific version of NixOS:

  $ nixos-rebuild switch -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/63def04891a0abc328b1b0b3a78ec02c58f48583.tar.gz

src/libexpr/download.cc

@@ -1,6 +1,8 @@
 #include "download.hh"
 #include "util.hh"
 #include "globals.hh"
+#include "hash.hh"
+#include "store-api.hh"
 
 #include <curl/curl.h>
 
@@ -134,4 +136,91 @@ DownloadResult downloadFile(string url, string expectedETag)
     return res;
 }
 
+
+Path downloadFileCached(const string & url, bool unpack)
+{
+    Path cacheDir = getEnv("XDG_CACHE_HOME", getEnv("HOME", "") + "/.cache") + "/nix/tarballs";
+    createDirs(cacheDir);
+
+    string urlHash = printHash32(hashString(htSHA256, url));
+
+    Path dataFile = cacheDir + "/" + urlHash + ".info";
+    Path fileLink = cacheDir + "/" + urlHash + "-file";
+
+    Path storePath;
+
+    string expectedETag;
+
+    int ttl = settings.get("tarball-ttl", 60 * 60);
+    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;
+                else if (!ss[1].empty()) {
+                    printMsg(lvlDebug, format("verifying previous ETag ‘%1%’") % ss[1]);
+                    expectedETag = ss[1];
+                }
+            }
+        } else
+            storePath = "";
+    }
+
+    string name;
+    auto p = url.rfind('/');
+    if (p != string::npos) name = string(url, p + 1);
+
+    if (!skip) {
+
+        if (storePath.empty())
+            printMsg(lvlInfo, format("downloading ‘%1%’...") % url);
+        else
+            printMsg(lvlInfo, format("checking ‘%1%’...") % url);
+
+        try {
+            auto res = downloadFile(url, expectedETag);
+
+            if (!res.cached)
+                storePath = store->addTextToStore(name, res.data, PathSet(), false);
+
+            assert(!storePath.empty());
+            replaceSymlink(storePath, fileLink);
+
+            writeFile(dataFile, url + "\n" + res.etag + "\n" + int2String(time(0)) + "\n");
+        } catch (DownloadError & e) {
+            if (storePath.empty()) throw;
+            printMsg(lvlError, format("warning: %1%; using cached result") % e.msg());
+        }
+    }
+
+    if (unpack) {
+        Path unpackedLink = cacheDir + "/" + baseNameOf(storePath) + "-unpacked";
+        Path unpackedStorePath;
+        if (pathExists(unpackedLink)) {
+            unpackedStorePath = readLink(unpackedLink);
+            store->addTempRoot(unpackedStorePath);
+            if (!store->isValidPath(unpackedStorePath))
+                unpackedStorePath = "";
+        }
+        if (unpackedStorePath.empty()) {
+            printMsg(lvlInfo, format("unpacking ‘%1%’...") % url);
+            Path tmpDir = createTempDir();
+            AutoDelete autoDelete(tmpDir, true);
+            runProgram("tar", true, {"xf", storePath, "-C", tmpDir, "--strip-components", "1"}, "");
+            unpackedStorePath = store->addToStore(name, tmpDir, true, htSHA256, defaultPathFilter, false);
+        }
+        replaceSymlink(unpackedStorePath, unpackedLink);
+        return unpackedStorePath;
+    }
+
+    return storePath;
+}
+
+
 }