import/get: support pre-fetching LFS objects from Git-LFS repos

dvc/fs/__init__.py

@@ -5,6 +5,8 @@ from dvc_http import HTTPFileSystem, HTTPSFileSystem  # noqa: F401
 
 from dvc.config import ConfigError as RepoConfigError
 from dvc.config_schema import SCHEMA, Invalid
+
+# pylint: disable=unused-import
 from dvc_objects.fs import (  # noqa: F401
     LocalFileSystem,
     MemoryFileSystem,
@@ -27,7 +29,7 @@ from dvc_objects.fs.path import Path  # noqa: F401
 
 from .callbacks import Callback
 from .data import DataFileSystem  # noqa: F401
-from .dvc import DVCFileSystem  # noqa: F401
+from .dvc import DVCFileSystem
 from .git import GitFileSystem  # noqa: F401
 
 known_implementations.update(
@@ -47,6 +49,8 @@ known_implementations.update(
 def download(
     fs: "FileSystem", fs_path: str, to: str, jobs: Optional[int] = None
 ) -> int:
+    from dvc.scm import lfs_prefetch
+
     with Callback.as_tqdm_callback(
         desc=f"Downloading {fs.path.name(fs_path)}",
         unit="files",
@@ -70,6 +74,8 @@ def download(
             from_infos = [fs_path]
             to_infos = [to]
 
+        if isinstance(fs, DVCFileSystem):
+            lfs_prefetch(fs, from_infos)
         cb.set_size(len(from_infos))
         jobs = jobs or fs.jobs
         generic.copy(

dvc/scm.py

@@ -24,6 +24,8 @@ from dvc.progress import Tqdm
 if TYPE_CHECKING:
     from scmrepo.progress import GitProgressEvent
 
+    from dvc.fs import FileSystem
+
 
 class SCMError(DvcException):
     """Base class for source control management errors."""
@@ -262,3 +264,30 @@ def iter_revs(
 
     rev_resolver = partial(resolve_rev, scm)
     return group_by(rev_resolver, results)
+
+
+def lfs_prefetch(fs: "FileSystem", paths: List[str]):
+    from scmrepo.git.lfs import fetch as _lfs_fetch
+
+    from dvc.fs.dvc import DVCFileSystem
+    from dvc.fs.git import GitFileSystem
+
+    if isinstance(fs, DVCFileSystem) and isinstance(fs.repo.fs, GitFileSystem):
+        git_fs = fs.repo.fs
+        scm = fs.repo.scm
+        assert isinstance(scm, Git)
+    else:
+        return
+
+    try:
+        if "filter=lfs" not in git_fs.open(".gitattributes").read():
+            return
+    except OSError:
+        return
+    with TqdmGit(desc="Checking for Git-LFS objects") as pbar:
+        _lfs_fetch(
+            scm,
+            [git_fs.rev],
+            include=[(path if path.startswith("/") else f"/{path}") for path in paths],
+            progress=pbar.update_git,
+        )

tests/func/test_scm.py

@@ -3,7 +3,7 @@ import os
 import pytest
 from git import Repo
 
-from dvc.scm import SCM, Git, NoSCM, SCMError
+from dvc.scm import SCM, Git, NoSCM, SCMError, lfs_prefetch
 
 
 def test_init_none(tmp_dir):
@@ -27,3 +27,20 @@ def test_init_sub_dir(tmp_dir):
 
     scm = SCM(os.fspath(subdir))
     assert scm.root_dir == os.fspath(tmp_dir)
+
+
+def test_lfs_prefetch(tmp_dir, dvc, scm, mocker):
+    mock_fetch = mocker.patch("scmrepo.git.lfs.fetch")
+    rev = scm.get_rev()
+
+    with dvc.switch(rev):
+        lfs_prefetch(dvc.dvcfs, ["foo"])
+        mock_fetch.assert_not_called()
+
+    tmp_dir.scm_gen(
+        ".gitattributes", ".lfs filter=lfs diff=lfs merge=lfs -text", commit="init lfs"
+    )
+    rev = scm.get_rev()
+    with dvc.switch(rev):
+        lfs_prefetch(dvc.dvcfs, ["foo"])
+        mock_fetch.assert_called_once()