lib.fileset.gitTracked: Allow clones of shallow repositories

The only reason shallow clones are not the default in
`builtins.fetchGit` is that `revCount` can't be provided when cloning a
shallow repository. However, `revCount` isn't used or exposed by
`lib.fileset`. Hence, allowing cloning shallow repositories makes
`gitTracked` more general without any drawbacks.

Co-authored-by: Silvan Mosberger <github@infinisil.com>

lib/fileset/internal.nix

@@ -5,6 +5,7 @@ let
     isAttrs
     isPath
     isString
+    nixVersion
     pathExists
     readDir
     split
@@ -17,6 +18,7 @@ let
     attrNames
     attrValues
     mapAttrs
+    optionalAttrs
     zipAttrsWith
     ;
 
@@ -56,6 +58,7 @@ let
     substring
     stringLength
     hasSuffix
+    versionAtLeast
     ;
 
   inherit (lib.trivial)
@@ -840,6 +843,10 @@ rec {
   # https://github.com/NixOS/nix/commit/55cefd41d63368d4286568e2956afd535cb44018
   _fetchGitSubmodulesMinver = "2.4";
 
+  # Support for `builtins.fetchGit` with `shallow = true` was introduced in 2.4
+  # https://github.com/NixOS/nix/commit/d1165d8791f559352ff6aa7348e1293b2873db1c
+  _fetchGitShallowMinver = "2.4";
+
   # Mirrors the contents of a Nix store path relative to a local path as a file set.
   # Some notes:
   # - The store path is read at evaluation time.
@@ -894,7 +901,17 @@ rec {
           # However a simpler alternative still would be [a builtins.gitLsFiles](https://github.com/NixOS/nix/issues/2944).
           fetchResult = fetchGit ({
             url = path;
-          } // extraFetchGitAttrs);
+          }
+          # In older Nix versions, repositories were always assumed to be deep clones, which made `fetchGit` fail for shallow clones
+          # For newer versions this was fixed, but the `shallow` flag is required.
+          # The only behavioral difference is that for shallow clones, `fetchGit` doesn't return a `revCount`,
+          # which we don't need here, so it's fine to always pass it.
+
+          # Unfortunately this means older Nix versions get a poor error message for shallow repositories, and there's no good way to improve that.
+          # Checking for `.git/shallow` doesn't seem worth it, especially since that's more of an implementation detail,
+          # and would also require more code to handle worktrees where `.git` is a file.
+          // optionalAttrs (versionAtLeast nixVersion _fetchGitShallowMinver) { shallow = true; }
+          // extraFetchGitAttrs);
         in
         # We can identify local working directories by checking for .git,
         # see https://git-scm.com/docs/gitrepository-layout#_description.

lib/fileset/tests.sh

@@ -1439,6 +1439,19 @@ if [[ -n "$fetchGitSupportsSubmodules" ]]; then
 fi
 rm -rf -- *
 
+# shallow = true is not supported on all Nix versions
+# and older versions don't support shallow clones at all
+if [[ "$(nix-instantiate --eval --expr "$prefixExpression (versionAtLeast builtins.nixVersion _fetchGitShallowMinver)")" == true ]]; then
+    createGitRepo full
+    # Extra commit such that there's a commit that won't be in the shallow clone
+    git -C full commit --allow-empty -q -m extra
+    git clone -q --depth 1 "file://${PWD}/full" shallow
+    cd shallow
+    checkGitTracked
+    cd ..
+    rm -rf -- *
+fi
+
 # Go through all stages of Git files
 # See https://www.git-scm.com/book/en/v2/Git-Basics-Recording-Changes-to-the-Repository