Add a unit test to test the absolute path change

src/pip/_internal/req/req_file.py

@@ -330,8 +330,9 @@ def parse(
         self, filename: str, constraint: bool
     ) -> Generator[ParsedLine, None, None]:
         """Parse a given file, yielding parsed lines."""
-        filename = os.path.abspath(filename)
-        self._parsed_files[filename] = None  # The primary requirements file passed
+        self._parsed_files[os.path.abspath(filename)] = (
+            None  # The primary requirements file passed
+        )
         yield from self._parse_and_recurse(filename, constraint)
 
     def _parse_and_recurse(

tests/unit/test_req_file.py

@@ -9,6 +9,7 @@
 
 import pytest
 
+import pip._internal.exceptions
 import pip._internal.req.req_file  # this will be monkeypatched
 from pip._internal.exceptions import InstallationError, RequirementsFileParseError
 from pip._internal.index.package_finder import PackageFinder
@@ -358,21 +359,54 @@ def test_recursive_requirements_file(
         # When the passed requirements file recursively references itself
         with pytest.raises(
             RecursionError,
-            match=f"{req_files[0]} recursively references itself"
-            f" in {req_files[req_file_count - 1]}",
+            match=(
+                f"{req_files[0]} recursively references itself"
+                f" in {req_files[req_file_count - 1]}"
+            ),
         ):
             list(parse_requirements(filename=str(req_files[0]), session=session))
 
         # When one of other the requirements file recursively references itself
         req_files[req_file_count - 1].write_text(f"-r {req_files[req_file_count - 2]}")
         with pytest.raises(
             RecursionError,
-            match=f"{req_files[req_file_count - 2]} recursively references itself "
-            f"in {req_files[req_file_count - 1]} and again in"
-            f" {req_files[req_file_count - 3]}",
+            match=(
+                f"{req_files[req_file_count - 2]} recursively references itself "
+                f"in {req_files[req_file_count - 1]} and again in"
+                f" {req_files[req_file_count - 3]}"
+            ),
         ):
             list(parse_requirements(filename=str(req_files[0]), session=session))
 
+    def test_recursive_relative_requirements_file(
+        self, monkeypatch: pytest.MonkeyPatch, tmpdir: Path, session: PipSession
+    ) -> None:
+        root_req_file = tmpdir / "root.txt"
+        (tmpdir / "nest" / "nest").mkdir(parents=True)
+        level_1_req_file = tmpdir / "nest" / "level_1.txt"
+        level_2_req_file = tmpdir / "nest" / "nest" / "level_2.txt"
+
+        root_req_file.write_text("-r nest/level_1.txt")
+        level_1_req_file.write_text("-r nest/level_2.txt")
+        level_2_req_file.write_text("-r ../../root.txt")
+
+        with pytest.raises(
+            RecursionError,
+            match=(
+                f"{root_req_file} recursively references itself in {level_2_req_file}"
+            ),
+        ):
+            list(parse_requirements(filename=str(root_req_file), session=session))
+
+        # If we don't use absolute path, it keeps on chaining the filename resulting in
+        # a huge filename, since a != a/b/c/../../
+        monkeypatch.setattr(os.path, "abspath", lambda x: x)
+        with pytest.raises(
+            pip._internal.exceptions.InstallationError,
+            match=r"Could not open requirements file: \[Errno 36\] File name too long:",
+        ):
+            list(parse_requirements(filename=str(root_req_file), session=session))
+
     def test_options_on_a_requirement_line(self, line_processor: LineProcessor) -> None:
         line = (
             'SomeProject --global-option="yo3" --global-option "yo4" '