Add explicit allRefs = true; argument to fetchGit

Sometimes it's necessary to fetch a git repository at a revision and
it's unknown which ref contains the revision in question. An example
would be a Cargo.lock which only provides the URL and the revision when
using a git repository as build input.

However it's considered a bad practice to perform a full checkout of a
repository since this may take a lot of time and can eat up a lot of
disk space. This patch makes a full checkout explicit by adding an
`allRefs` argument to `builtins.fetchGit` which fetches all refs if
explicitly set to true.

Closes NixOS#2409

doc/manual/expressions/builtins.xml

@@ -432,6 +432,14 @@ stdenv.mkDerivation { … }
             </para>
           </listitem>
         </varlistentry>
+        <varlistentry>
+         <term>allRefs</term>
+         <listitem>
+          <para>
+           If set to <literal>true</literal>, it fetches all <literal>refs</literal> of a git repository.
+          </para>
+         </listitem>
+        </varlistentry>
       </variablelist>
 
       <example>

src/libfetchers/git.cc

@@ -58,12 +58,13 @@ struct GitInputScheme : InputScheme
         if (maybeGetStrAttr(attrs, "type") != "git") return {};
 
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow" && name != "submodules" && name != "lastModified" && name != "revCount" && name != "narHash")
+            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow" && name != "submodules" && name != "lastModified" && name != "revCount" && name != "narHash" && name != "allRefs")
                 throw Error("unsupported Git input attribute '%s'", name);
 
         parseURL(getStrAttr(attrs, "url"));
         maybeGetBoolAttr(attrs, "shallow");
         maybeGetBoolAttr(attrs, "submodules");
+        maybeGetBoolAttr(attrs, "allRefs");
 
         if (auto ref = maybeGetStrAttr(attrs, "ref")) {
             if (std::regex_search(*ref, badGitRefRegex))
@@ -168,10 +169,12 @@ struct GitInputScheme : InputScheme
 
         bool shallow = maybeGetBoolAttr(input.attrs, "shallow").value_or(false);
         bool submodules = maybeGetBoolAttr(input.attrs, "submodules").value_or(false);
+        bool allRefs = maybeGetBoolAttr(input.attrs, "allRefs").value_or(false);
 
         std::string cacheType = "git";
         if (shallow) cacheType += "-shallow";
         if (submodules) cacheType += "-submodules";
+        if (allRefs) cacheType += "-all-refs";
 
         auto getImmutableAttrs = [&]()
         {
@@ -337,11 +340,15 @@ struct GitInputScheme : InputScheme
                     }
                 }
             } else {
-                /* If the local ref is older than ‘tarball-ttl’ seconds, do a
-                   git fetch to update the local ref to the remote ref. */
-                struct stat st;
-                doFetch = stat(localRefFile.c_str(), &st) != 0 ||
-                    (uint64_t) st.st_mtime + settings.tarballTtl <= (uint64_t) now;
+                if (allRefs) {
+                    doFetch = true;
+                } else {
+                    /* If the local ref is older than ‘tarball-ttl’ seconds, do a
+                       git fetch to update the local ref to the remote ref. */
+                    struct stat st;
+                    doFetch = stat(localRefFile.c_str(), &st) != 0 ||
+                        (uint64_t) st.st_mtime + settings.tarballTtl <= (uint64_t) now;
+                }
             }
 
             if (doFetch) {
@@ -351,9 +358,11 @@ struct GitInputScheme : InputScheme
                 // we're using --quiet for now. Should process its stderr.
                 try {
                     auto ref = input.getRef();
-                    auto fetchRef = ref->compare(0, 5, "refs/") == 0
-                        ? *ref
-                        : "refs/heads/" + *ref;
+                    auto fetchRef = allRefs
+                        ? "refs/*"
+                        : ref->compare(0, 5, "refs/") == 0
+                            ? *ref
+                            : "refs/heads/" + *ref;
                     runProgram("git", true, { "-C", repoDir, "fetch", "--quiet", "--force", "--", actualUrl, fmt("%s:%s", fetchRef, fetchRef) });
                 } catch (Error & e) {
                     if (!pathExists(localRefFile)) throw;