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 has so far been done using boilerplate though, so
let's add a simple abstraction to fetch a tag instead.

doc/build-helpers/fetchers.chapter.md

@@ -755,6 +755,9 @@ Used with Subversion. Expects `url` to a Subversion directory, `rev`, and `hash`
 
 Used with Git. Expects `url` to a Git repo, `rev`, and `hash`. `rev` in this case can be full the git commit id (SHA1 hash) or a tag name like `refs/tags/v1.0`.
 
+If you want to fetch a tag you should pass the `tag` parameter instead of `rev` which has the same effect as setting `rev = "refs/tags"/${version}"`.
+This is safer than just setting `rev = version` w.r.t. possible branch and tag name conflicts.
+
 Additionally, the following optional arguments can be given:
 
 *`fetchSubmodules`* (Boolean)
@@ -833,7 +836,7 @@ A number of fetcher functions wrap part of `fetchurl` and `fetchzip`. They are m
 
 ## `fetchFromGitHub` {#fetchfromgithub}
 
-`fetchFromGitHub` expects four arguments. `owner` is a string corresponding to the GitHub user or organization that controls this repository. `repo` corresponds to the name of the software repository. These are located at the top of every GitHub HTML page as `owner`/`repo`. `rev` corresponds to the Git commit hash or tag (e.g `v1.0`) that will be downloaded from Git. Finally, `hash` corresponds to the hash of the extracted directory. Again, other hash algorithms are also available, but `hash` is currently preferred.
+`fetchFromGitHub` expects four arguments. `owner` is a string corresponding to the GitHub user or organization that controls this repository. `repo` corresponds to the name of the software repository. These are located at the top of every GitHub HTML page as `owner`/`repo`. `rev` corresponds to the Git commit hash or tag (e.g `v1.0`) that will be downloaded from Git. To fetch a tag, you should prefer to use the `tag` parameter however which achieves this in a safer way. Finally, `hash` corresponds to the hash of the extracted directory. Again, other hash algorithms are also available, but `hash` is currently preferred.
 
 To use a different GitHub instance, use `githubBase` (defaults to `"github.com"`).
 

pkgs/build-support/fetchgit/default.nix

@@ -13,7 +13,10 @@ in
 lib.makeOverridable (lib.fetchers.withNormalizedHash { } (
 # NOTE Please document parameter additions or changes in
 #   doc/build-helpers/fetchers.chapter.md
-{ 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";