From 7e35e914c1aa24957107c666c76f1d834ebae90a Mon Sep 17 00:00:00 2001
From: Daiderd Jordan <daiderd@gmail.com>
Date: Fri, 14 Dec 2018 20:07:23 +0100
Subject: [PATCH 1/2] fetchGit: allow fetching explicit refs

Trying to fetch refs that are not in refs/heads currently fails because
it looks for refs/heads/refs/foo instead of refs/foo.

eg.

	builtins.fetchGit {
	  url = https://github.com/NixOS/nixpkgs.git;
	  ref = "refs/pull/1024/head;
	}
---
 src/libexpr/primops/fetchGit.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index b46d2f258..588b0fa4d 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -94,7 +94,11 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
         runProgram("git", true, { "init", "--bare", cacheDir });
     }
 
-    Path localRefFile = cacheDir + "/refs/heads/" + *ref;
+    Path localRefFile;
+    if (ref->compare(0, 5, "refs/") == 0)
+        localRefFile = cacheDir + "/" + *ref;
+    else
+        localRefFile = cacheDir + "/refs/heads/" + *ref;
 
     bool doFetch;
     time_t now = time(0);

From c8205a3413217ccf8a6a1f7e064b06a5b86c3253 Mon Sep 17 00:00:00 2001
From: Graham Christensen <graham@grahamc.com>
Date: Tue, 2 Jul 2019 09:04:04 -0400
Subject: [PATCH 2/2] builtins.fetchGit: document absolute ref support

---
 doc/manual/expressions/builtins.xml        | 15 +++++++++++++++
 doc/manual/release-notes/release-notes.xml |  1 +
 doc/manual/release-notes/rl-2.3.xml        | 22 ++++++++++++++++++++++
 3 files changed, 38 insertions(+)
 create mode 100644 doc/manual/release-notes/rl-2.3.xml

diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index 412622714..dcd7aaf24 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -419,6 +419,13 @@ stdenv.mkDerivation { … }
               This is often a branch or tag name. Defaults to
               <literal>HEAD</literal>.
             </para>
+
+            <para>
+              By default, the <varname>ref</varname> value is prefixed
+              with <literal>refs/heads/</literal>. As of Nix 2.3.0
+              Nix will not prefix <literal>refs/heads/</literal> if
+              <varname>ref</varname> starts with <literal>refs/</literal>.
+            </para>
           </listitem>
         </varlistentry>
       </variablelist>
@@ -432,6 +439,14 @@ stdenv.mkDerivation { … }
 }</programlisting>
       </example>
 
+      <example>
+        <title>Fetching an arbitrary ref</title>
+        <programlisting>builtins.fetchGit {
+  url = "https://gitub.com/NixOS/nix.git";
+  ref = "refs/heads/0.5-release";
+}</programlisting>
+      </example>
+
       <example>
         <title>Fetching a repository's specific commit on an arbitrary branch</title>
         <para>
diff --git a/doc/manual/release-notes/release-notes.xml b/doc/manual/release-notes/release-notes.xml
index e8ff586fa..2655d68e3 100644
--- a/doc/manual/release-notes/release-notes.xml
+++ b/doc/manual/release-notes/release-notes.xml
@@ -12,6 +12,7 @@
 </partintro>
 -->
 
+<xi:include href="rl-2.3.xml" />
 <xi:include href="rl-2.2.xml" />
 <xi:include href="rl-2.1.xml" />
 <xi:include href="rl-2.0.xml" />
diff --git a/doc/manual/release-notes/rl-2.3.xml b/doc/manual/release-notes/rl-2.3.xml
new file mode 100644
index 000000000..428213b36
--- /dev/null
+++ b/doc/manual/release-notes/rl-2.3.xml
@@ -0,0 +1,22 @@
+<section xmlns="http://docbook.org/ns/docbook"
+      xmlns:xlink="http://www.w3.org/1999/xlink"
+      xmlns:xi="http://www.w3.org/2001/XInclude"
+      version="5.0"
+      xml:id="ssec-relnotes-2.3">
+
+<title>Release 2.3 (????-??-??)</title>
+
+<para>This release contains the following changes:</para>
+
+<itemizedlist>
+
+  <listitem>
+    <para><function>builtins.fetchGit</function>'s <varname>ref</varname>
+    argument now allows specifying an absolute remote ref.
+    Nix will automatically prefix <varname>ref</varname> with
+    <literal>refs/heads</literal> only if <varname>ref</varname> doesn't
+    already begin with <literal>refs/</literal>.
+    </para>
+  </listitem>
+</itemizedlist>
+</section>