fetchgit{,hub}: add tag argument

It's become a common pattern to use `rev = "refs/tags/${version}"` rather than
just `rev = version` to ensure that the tag gets fetched rather than a branch
that has the same name. This is done using boilerplate though, so let's add a
simple abstraction for fetching tags instead.

pkgs/build-support/fetchgit/default.nix

@@ -11,7 +11,10 @@
   in "${if matched == null then base else builtins.head matched}${appendShort}";
 in
 lib.makeOverridable (lib.fetchers.withNormalizedHash { } (
-{ url, rev ? "HEAD", leaveDotGit ? deepClone
+{ url
+, tag ? null
+, rev ? if tag != null then tag else "HEAD" # FIXME fetching HEAD by default is problematic at best
+, leaveDotGit ? deepClone
 , outputHash ? lib.fakeHash, outputHashAlgo ? null
 , fetchSubmodules ? true, deepClone ? false
 , branchName ? null

pkgs/build-support/fetchgithub/default.nix

@@ -1,7 +1,10 @@
 { lib, fetchgit, fetchzip }:
 
 lib.makeOverridable (
-{ owner, repo, rev, name ? "source"
+{ owner, repo
+, tag ? null
+, rev ? if tag != null then "refs/tags/${tag}" else null
+, name ? "source"
 , fetchSubmodules ? false, leaveDotGit ? null
 , deepClone ? false, private ? false, forceFetchGit ? false
 , fetchLFS ? false
@@ -11,6 +14,8 @@ lib.makeOverridable (
 , ... # For hash agility
 }@args:
 
+assert (lib.assertMsg (rev != null) "You must provide `fetchFromGitHub with a `rev` or `tag`.");
+
 let
 
   position = (if args.meta.description or null != null
@@ -24,7 +29,7 @@ let
     # to indicate where derivation originates, similar to make-derivation.nix's mkDerivation
     position = "${position.file}:${toString position.line}";
   };
-  passthruAttrs = removeAttrs args [ "owner" "repo" "rev" "fetchSubmodules" "forceFetchGit" "private" "githubBase" "varPrefix" ];
+  passthruAttrs = removeAttrs args [ "owner" "repo" "tag" "rev" "fetchSubmodules" "forceFetchGit" "private" "githubBase" "varPrefix" ];
   varBase = "NIX${lib.optionalString (varPrefix != null) "_${varPrefix}"}_GITHUB_PRIVATE_";
   useFetchGit = fetchSubmodules || (leaveDotGit == true) || deepClone || forceFetchGit || fetchLFS || (sparseCheckout != []);
   # We prefer fetchzip in cases we don't need submodules as the hash
@@ -53,7 +58,7 @@ let
 
   fetcherArgs = (if useFetchGit
     then {
-      inherit rev deepClone fetchSubmodules sparseCheckout fetchLFS; url = gitRepoUrl;
+      inherit tag rev deepClone fetchSubmodules sparseCheckout fetchLFS; url = gitRepoUrl;
     } // lib.optionalAttrs (leaveDotGit != null) { inherit leaveDotGit; }
     else {
       url = "${baseUrl}/archive/${rev}.tar.gz";