Support Git repos in the Nix path

E.g.

  $ nix-build -I nixpkgs=git://github.com/NixOS/nixpkgs '<nixpkgs>' -A hello

This is not extremely useful yet because you can't specify a
branch/revision.

src/libexpr/parser.y

@@ -520,9 +520,10 @@ formal
 #include <fcntl.h>
 #include <unistd.h>
 
-#include <eval.hh>
-#include <download.hh>
-#include <store-api.hh>
+#include "eval.hh"
+#include "download.hh"
+#include "store-api.hh"
+#include "primops/fetchgit.hh"
 
 
 namespace nix {
@@ -657,7 +658,11 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
 
     if (isUri(elem.second)) {
         try {
-            res = { true, makeDownloader()->downloadCached(store, elem.second, true) };
+            if (hasPrefix(elem.second, "git://") || hasSuffix(elem.second, ".git"))
+                // FIXME: support specifying revision/branch
+                res = { true, exportGit(store, elem.second, "master") };
+            else
+                res = { true, makeDownloader()->downloadCached(store, elem.second, true) };
         } catch (DownloadError & e) {
             printMsg(lvlError, format("warning: Nix search path entry ‘%1%’ cannot be downloaded, ignoring") % elem.second);
             res = { false, "" };

src/libexpr/primops/fetchgit.cc

@@ -5,6 +5,43 @@
 
 namespace nix {
 
+Path exportGit(ref<Store> store, const std::string & uri, const std::string & rev)
+{
+    if (!isUri(uri))
+        throw EvalError(format("‘%s’ is not a valid URI") % uri);
+
+    Path cacheDir = getCacheDir() + "/nix/git";
+
+    if (!pathExists(cacheDir)) {
+        createDirs(cacheDir);
+        runProgram("git", true, { "init", "--bare", cacheDir });
+    }
+
+    Activity act(*logger, lvlInfo, format("fetching Git repository ‘%s’") % uri);
+
+    std::string localRef = "pid-" + std::to_string(getpid());
+    Path localRefFile = cacheDir + "/refs/heads/" + localRef;
+
+    runProgram("git", true, { "-C", cacheDir, "fetch", uri, rev + ":" + localRef });
+
+    std::string commitHash = chomp(readFile(localRefFile));
+
+    unlink(localRefFile.c_str());
+
+    debug(format("got revision ‘%s’") % commitHash);
+
+    // FIXME: should pipe this, or find some better way to extract a
+    // revision.
+    auto tar = runProgram("git", true, { "-C", cacheDir, "archive", commitHash });
+
+    Path tmpDir = createTempDir();
+    AutoDelete delTmpDir(tmpDir, true);
+
+    runProgram("tar", true, { "x", "-C", tmpDir }, tar);
+
+    return store->addToStore("git-export", tmpDir);
+}
+
 static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     // FIXME: cut&paste from fetch().
@@ -35,39 +72,7 @@ static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Va
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
-    if (!isUri(url))
-        throw EvalError(format("‘%s’ is not a valid URI, at %s") % url % pos);
-
-    Path cacheDir = getCacheDir() + "/nix/git";
-
-    if (!pathExists(cacheDir)) {
-        createDirs(cacheDir);
-        runProgram("git", true, { "init", "--bare", cacheDir });
-    }
-
-    Activity act(*logger, lvlInfo, format("fetching Git repository ‘%s’") % url);
-
-    std::string localRef = "pid-" + std::to_string(getpid());
-    Path localRefFile = cacheDir + "/refs/heads/" + localRef;
-
-    runProgram("git", true, { "-C", cacheDir, "fetch", url, rev + ":" + localRef });
-
-    std::string commitHash = chomp(readFile(localRefFile));
-
-    unlink(localRefFile.c_str());
-
-    debug(format("got revision ‘%s’") % commitHash);
-
-    // FIXME: should pipe this, or find some better way to extract a
-    // revision.
-    auto tar = runProgram("git", true, { "-C", cacheDir, "archive", commitHash });
-
-    Path tmpDir = createTempDir();
-    AutoDelete delTmpDir(tmpDir, true);
-
-    runProgram("tar", true, { "x", "-C", tmpDir }, tar);
-
-    Path storePath = state.store->addToStore("git-export", tmpDir);
+    Path storePath = exportGit(state.store, url, rev);
 
     mkString(v, storePath, PathSet({storePath}));
 }

src/libexpr/primops/fetchgit.hh

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <string>
+
+#include "ref.hh"
+
+namespace nix {
+
+class Store;
+
+Path exportGit(ref<Store> store,
+    const std::string & uri, const std::string & rev);
+
+}

src/libutil/util.cc

@@ -1082,6 +1082,12 @@ bool statusOk(int status)
 }
 
 
+bool hasPrefix(const string & s, const string & suffix)
+{
+    return s.compare(0, suffix.size(), suffix) == 0;
+}
+
+
 bool hasSuffix(const string & s, const string & suffix)
 {
     return s.size() >= suffix.size() && string(s, s.size() - suffix.size()) == suffix;

src/libutil/util.hh

@@ -333,6 +333,10 @@ template<class N> bool string2Float(const string & s, N & n)
 }
 
 
+/* Return true iff `s' starts with `prefix'. */
+bool hasPrefix(const string & s, const string & prefix);
+
+
 /* Return true iff `s' ends in `suffix'. */
 bool hasSuffix(const string & s, const string & suffix);