builtins.fetchgit: Support specifying commit hashes

This adds an argument "rev" specififying the Git commit hash. The
existing argument "rev" is renamed to "ref". The default value for
"ref" is "master". When specifying a hash, it's necessary to specify a
ref since we're not cloning the entire repository but only fetching a
specific ref.

Example usage:

  builtins.fetchgit {
    url = https://github.com/NixOS/nixpkgs.git;
    ref = "release-16.03";
    rev = "c1c0484041ab6f9c6858c8ade80a8477c9ae4442";
  };

src/libexpr/primops/fetchgit.cc

@@ -6,13 +6,27 @@
 
 #include <sys/time.h>
 
+#include <regex>
+
 namespace nix {
 
-Path exportGit(ref<Store> store, const std::string & uri, const std::string & rev)
+Path exportGit(ref<Store> store, const std::string & uri,
+    const std::string & ref, const std::string & rev)
 {
     if (!isUri(uri))
         throw EvalError(format("‘%s’ is not a valid URI") % uri);
 
+    if (rev != "") {
+        std::regex revRegex("^[0-9a-fA-F]{40}$");
+        if (!std::regex_match(rev, revRegex))
+            throw Error("invalid Git revision ‘%s’", rev);
+    }
+
+    // FIXME: too restrictive, but better safe than sorry.
+    std::regex refRegex("^[0-9a-zA-Z][0-9a-zA-Z.-]+$");
+    if (!std::regex_match(ref, refRegex))
+        throw Error("invalid Git ref ‘%s’", ref);
+
     Path cacheDir = getCacheDir() + "/nix/git";
 
     if (!pathExists(cacheDir)) {
@@ -22,7 +36,7 @@ Path exportGit(ref<Store> store, const std::string & uri, const std::string & re
 
     //Activity act(*logger, lvlInfo, format("fetching Git repository ‘%s’") % uri);
 
-    std::string localRef = hashString(htSHA256, fmt("%s-%s", uri, rev)).to_string(Base32, false);
+    std::string localRef = hashString(htSHA256, fmt("%s-%s", uri, ref)).to_string(Base32, false);
 
     Path localRefFile = cacheDir + "/refs/heads/" + localRef;
 
@@ -33,7 +47,7 @@ Path exportGit(ref<Store> store, const std::string & uri, const std::string & re
     if (stat(localRefFile.c_str(), &st) != 0 ||
         st.st_mtime < now - settings.tarballTtl)
     {
-        runProgram("git", true, { "-C", cacheDir, "fetch", "--force", uri, rev + ":" + localRef });
+        runProgram("git", true, { "-C", cacheDir, "fetch", "--force", uri, ref + ":" + localRef });
 
         struct timeval times[2];
         times[0].tv_sec = now;
@@ -44,7 +58,9 @@ Path exportGit(ref<Store> store, const std::string & uri, const std::string & re
         utimes(localRefFile.c_str(), times);
     }
 
-    std::string commitHash = chomp(readFile(localRefFile));
+    // FIXME: check whether rev is an ancestor of ref.
+    std::string commitHash =
+        rev != "" ? rev : chomp(readFile(localRefFile));
 
     printTalkative("using revision %s of repo ‘%s’", uri, commitHash);
 
@@ -81,7 +97,8 @@ static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Va
     if (state.restricted) throw Error("‘fetchgit’ is not allowed in restricted mode");
 
     std::string url;
-    std::string rev = "master";
+    std::string ref = "master";
+    std::string rev;
 
     state.forceValue(*args[0]);
 
@@ -95,7 +112,10 @@ static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Va
                 PathSet context;
                 url = state.coerceToString(*attr.pos, *attr.value, context, false, false);
                 if (hasPrefix(url, "/")) url = "file://" + url;
-            } else if (name == "rev")
+            }
+            else if (name == "ref")
+                ref = state.forceStringNoCtx(*attr.value, *attr.pos);
+            else if (name == "rev")
                 rev = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
                 throw EvalError("unsupported argument ‘%s’ to ‘fetchgit’, at %s", attr.name, *attr.pos);
@@ -107,7 +127,7 @@ static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Va
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
-    Path storePath = exportGit(state.store, url, rev);
+    Path storePath = exportGit(state.store, url, ref, rev);
 
     mkString(v, storePath, PathSet({storePath}));
 }

src/libexpr/primops/fetchgit.hh

@@ -8,7 +8,7 @@ namespace nix {
 
 class Store;
 
-Path exportGit(ref<Store> store,
+Path exportGit(ref<Store> store, const std::string & uri,
-    const std::string & uri, const std::string & rev);
+    const std::string & ref, const std::string & rev = "");
 
 }