scuba: Require relative volume host paths to start with "./" or "../"

This is to avoid ambiguity.

#227 (comment)

docs/configuration.rst

@@ -182,6 +182,7 @@ configuration error.
 
 Volume host paths can be absolute or relative. If a relative path is used, it
 is interpreted as relative to the directory in which ``.scuba.yml`` is found.
+To avoid ambiguity, relative paths must start with ``./`` or ``../``.
 
 Volume host directories which do not already exist are created as the current
 user before creating the container.

scuba/config.py

@@ -344,7 +344,8 @@ def _expand_path(in_str: str, base_dir: Optional[Path] = None) -> Path:
     the host environment.
 
     After environment variable expansion, absolute paths are returned as-is.
-    Relative paths are joined to base_dir, if provided.
+    Relative paths must start with ./ or ../ and are joined to base_dir, if
+    provided.
 
     Args:
       in_str: Input path as a string.
@@ -357,13 +358,14 @@ def _expand_path(in_str: str, base_dir: Optional[Path] = None) -> Path:
       ValueError: If base_dir is provided but not absolute.
       ConfigError: If a referenced environment variable is not set.
       ConfigError: An environment variable reference could not be parsed.
+      ConfigError: A relative path does not start with "./" or "../".
       ConfigError: A relative path is given when base_dir is not provided.
     """
     if base_dir is not None and not base_dir.is_absolute():
         raise ValueError(f"base_dir is not absolute: {base_dir}")
 
     try:
-        output = expand_env_vars(in_str)
+        path_str = expand_env_vars(in_str)
     except KeyError as ke:
         # pylint: disable=raise-missing-from
         raise ConfigError(
@@ -374,11 +376,18 @@ def _expand_path(in_str: str, base_dir: Optional[Path] = None) -> Path:
             f"Unable to expand string '{in_str}' due to parsing errors"
         ) from ve
 
-    path = Path(output)
+    path = Path(path_str)
 
     if not path.is_absolute():
         if base_dir is None:
             raise ConfigError(f"Relative path not allowed: {path}")
+
+        # Make sure it starts with ./ or ../
+        # We have to use the original string input since Path() will remove ./
+        valid_prefixes = ("./", "../")
+        if not any(path_str.startswith(pfx) for pfx in valid_prefixes):
+            raise ConfigError(f"Relative path must start with {' or '.join(valid_prefixes)}: {path}")
+
         path = base_dir / path
 
     assert path.is_absolute()

tests/test_config.py

@@ -1012,18 +1012,17 @@ def test_volumes_with_invalid_env_vars(self, monkeypatch) -> None:
 
     def test_volumes_hostpath_rel(self, monkeypatch, in_tmp_path) -> None:
         """volume hostpath can be relative to scuba_root (top dir)"""
-        monkeypatch.setenv("RELVAR", "spam/eggs")
+        monkeypatch.setenv("RELVAR", "./spam/eggs")
 
         with open(".scuba.yml", "w") as f:
             f.write(
                 r"""
                 image: na
                 volumes:
-                  /one: foo             # simple form, naked dir name
-                  /two: ./foo/bar       # simple form, dotted path
-                  /three:               # complex form
+                  /bar: ./foo/bar       # simple form, dotted path
+                  /scp:                 # complex form
                     hostpath: ./snap/crackle/pop
-                  /four: $RELVAR        # simple form, relative path in variable
+                  /relvar: $RELVAR      # simple form, relative path in variable
                 """
             )
 
@@ -1038,10 +1037,34 @@ def test_volumes_hostpath_rel(self, monkeypatch, in_tmp_path) -> None:
         assert found_rel == subdir
         assert config.volumes is not None
 
-        assert_vol(config.volumes, "/one", in_tmp_path / "foo")
-        assert_vol(config.volumes, "/two", in_tmp_path / "foo" / "bar")
-        assert_vol(config.volumes, "/three", in_tmp_path / "snap" / "crackle" / "pop")
-        assert_vol(config.volumes, "/four", in_tmp_path / "spam" / "eggs")
+        assert_vol(config.volumes, "/bar", in_tmp_path / "foo" / "bar")
+        assert_vol(config.volumes, "/scp", in_tmp_path / "snap" / "crackle" / "pop")
+        assert_vol(config.volumes, "/relvar", in_tmp_path / "spam" / "eggs")
+
+    def test_volumes_hostpath_rel_requires_dot_simple(self, monkeypatch, in_tmp_path) -> None:
+        """relaitve volume hostpath (simple form) must start with ./ or ../"""
+        with open(".scuba.yml", "w") as f:
+            f.write(
+                r"""
+                image: na
+                volumes:
+                  /one: foo  # Forbidden
+                """
+            )
+        self._invalid_config("Relative path must start with ./ or ../")
+
+    def test_volumes_hostpath_rel_requires_dot_complex(self, monkeypatch, in_tmp_path) -> None:
+        """relaitve volume hostpath (complex form) must start with ./ or ../"""
+        with open(".scuba.yml", "w") as f:
+            f.write(
+                r"""
+                image: na
+                volumes:
+                  /one:
+                    hostpath: foo  # Forbidden
+                """
+            )
+        self._invalid_config("Relative path must start with ./ or ../")
 
     def test_volumes_contpath_rel(self, monkeypatch, in_tmp_path) -> None:
         with open(".scuba.yml", "w") as f:

tests/test_main.py

@@ -1081,7 +1081,7 @@ def test_volumes_host_path_rel(self) -> None:
                 f"""
                 image: {DOCKER_IMAGE}
                 volumes:
-                  /userdir: {userdir}
+                  /userdir: ./{userdir}
                 """
             )