Detect recursively referencing requirements files

Fixes pypa#12653

news/12653.feature.rst

@@ -0,0 +1,2 @@
+Detect recursively referencing requirements files and help users identify
+the source.

src/pip/_internal/req/req_file.py

@@ -324,11 +324,14 @@ def __init__(
     ) -> None:
         self._session = session
         self._line_parser = line_parser
+        self._parsed_files: dict[str, Optional[str]] = {}
 
     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
         yield from self._parse_and_recurse(filename, constraint)
 
     def _parse_and_recurse(
@@ -353,11 +356,25 @@ def _parse_and_recurse(
                 # original file and nested file are paths
                 elif not SCHEME_RE.search(req_path):
                     # do a join so relative paths work
-                    req_path = os.path.join(
-                        os.path.dirname(filename),
-                        req_path,
+                    # and then abspath so that we can identify recursive references
+                    req_path = os.path.abspath(
+                        os.path.join(
+                            os.path.dirname(filename),
+                            req_path,
+                        )
                     )
-
+                if req_path in self._parsed_files.keys():
+                    initial_file = self._parsed_files[req_path]
+                    tail = (
+                        f"and again in {initial_file}"
+                        if initial_file is not None
+                        else ""
+                    )
+                    raise RecursionError(
+                        f"{req_path} recursively references itself in {filename} {tail}"
+                    )
+                # Keeping a track where was each file first included in
+                self._parsed_files[req_path] = filename
                 yield from self._parse_and_recurse(req_path, nested_constraint)
             else:
                 yield line

tests/unit/test_req_file.py

@@ -345,6 +345,34 @@ def test_nested_constraints_file(
         assert reqs[0].name == req_name
         assert reqs[0].constraint
 
+    def test_recursive_requirements_file(
+        self, tmpdir: Path, session: PipSession
+    ) -> None:
+        req_files: list[Path] = []
+        req_file_count = 4
+        for i in range(req_file_count):
+            req_file = tmpdir / f"{i}.txt"
+            req_file.write_text(f"-r {(i+1) % req_file_count}.txt")
+            req_files.append(req_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]}",
+        ):
+            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]}",
+        ):
+            list(parse_requirements(filename=str(req_files[0]), session=session))
+
     def test_options_on_a_requirement_line(self, line_processor: LineProcessor) -> None:
         line = (
             'SomeProject --global-option="yo3" --global-option "yo4" '