ubuntu .gitattributes workaround

git-diff-dvc.sh

@@ -141,9 +141,37 @@ init_tmppath() {
 
 prefix0=a
 prefix1=b
-tmppath0=$(init_tmppath "$path0" $prefix0)
-tmppath1=$(init_tmppath "$path1" $prefix1)
+relpath0=$(init_tmppath "$path0" $prefix0)
+relpath1=$(init_tmppath "$path1" $prefix1)
+
+# We essentially want to run:
+#
+# ```bash
+# GIT_DIR="$orig_dir/.git" GIT_WORK_TREE="$orig_dir" git diff --no-index --ext-diff "$relpath0" "$relpath1"
+# ```
+#
+# - `$GIT_DIR` picks up configs from the original repo.
+# - `$GIT_WORK_TREE` should pick up `.gitattributes` (in the root of the Git repo, not contained in `.git` / `$GIT_DIR`).
+#
+# However, on Ubuntu, something about setting the latter results in the correct diff driver not being selected (even though e.g. `git check-attr diff $relpath1` will detect it, with the env vars above set). On MacOS it seems to work as expected.
+#
+# As a workaround, we manually ask `check-attr diff` for each file's "type", and write them to `.gitattributes` in the current directory (`$tmpdir`).
+#
+# TODO: would creating `$tmpdir` in the Git worktree make this unnecessary?
+
+get_attr_type() {
+    if [ -z "$1" ]; then return 0; fi
+    if [ "$1" == /dev/null ]; then return 0; fi
+    # if [ "${1:0:1}" == "/" ]; then return 0; fi
+    type="$(GIT_WORK_TREE="$orig_dir" git check-attr diff "$1" | sed -E 's/.*: diff: (.*)/\1/')"
+    if [ "$type" != "unset" ] && [ "$type" != "unspecified" ]; then
+	echo "$1 diff=$type" >> .gitattributes
+    fi
+}
+
+get_attr_type "$relpath0"
+get_attr_type "$relpath1"
 
 set +e
-GIT_DIR="$orig_dir/.git" git diff --no-index --ext-diff "$tmppath0" "$tmppath1"
+GIT_DIR="$orig_dir/.git" git diff --no-index --ext-diff "$relpath0" "$relpath1"
 exit 0