Correctly parse Git submodule URLs

src/poetry/vcs/git/backend.py

@@ -7,6 +7,7 @@
 from pathlib import Path
 from subprocess import CalledProcessError
 from typing import TYPE_CHECKING
+from urllib.parse import urljoin
 
 from dulwich import porcelain
 from dulwich.client import HTTPUnauthorized
@@ -331,16 +332,24 @@ def _clone_submodules(cls, repo: Repo) -> None:
         repo_root = Path(repo.path)
         modules_config = repo_root.joinpath(".gitmodules")
 
+        # A relative URL by definition starts with ../ or ./
+        relative_submodule_regex = re.compile(r"^\.{1,2}/")
+
         if modules_config.exists():
             config = ConfigFile.from_path(str(modules_config))
 
             url: bytes
             path: bytes
             submodules = parse_submodules(config)
+
             for path, url, name in submodules:
                 path_relative = Path(path.decode("utf-8"))
                 path_absolute = repo_root.joinpath(path_relative)
 
+                url_string = url.decode("utf-8")
+                if relative_submodule_regex.search(url_string):
+                    url_string = urljoin(f"{Git.get_remote_url(repo)}/", url_string)
+
                 source_root = path_absolute.parent
                 source_root.mkdir(parents=True, exist_ok=True)
 
@@ -357,7 +366,7 @@ def _clone_submodules(cls, repo: Repo) -> None:
                         continue
 
                 cls.clone(
-                    url=url.decode("utf-8"),
+                    url=url_string,
                     source_root=source_root,
                     name=path_relative.name,
                     revision=revision,

tests/integration/test_utils_vcs_git.py

@@ -240,6 +240,30 @@ def test_git_clone_clones_submodules(source_url: str) -> None:
     assert len(list(submodule_package_directory.glob("*"))) > 1
 
 
+def test_git_clone_clones_submodules_with_relative_urls(source_url: str) -> None:
+    with Git.clone(url=source_url, branch="relative_submodule") as repo:
+        submodule_package_directory = (
+            Path(repo.path) / "submodules" / "relative-url-submodule"
+        )
+
+    assert submodule_package_directory.exists()
+    assert submodule_package_directory.joinpath("README.md").exists()
+    assert len(list(submodule_package_directory.glob("*"))) > 1
+
+
+def test_git_clone_clones_submodules_with_relative_urls_and_explicit_base(
+    source_url: str,
+) -> None:
+    with Git.clone(url=source_url, branch="relative_submodule") as repo:
+        submodule_package_directory = (
+            Path(repo.path) / "submodules" / "relative-url-submodule-with-base"
+        )
+
+    assert submodule_package_directory.exists()
+    assert submodule_package_directory.joinpath("README.md").exists()
+    assert len(list(submodule_package_directory.glob("*"))) > 1
+
+
 def test_system_git_fallback_on_http_401(
     mocker: MockerFixture,
     source_url: str,