--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/python3.8/site-packages/pip/_internal/req/req_tracker.py	Mon Jul 27 03:47:31 2020 -0400
@@ -0,0 +1,150 @@
+# The following comment should be removed at some point in the future.
+# mypy: strict-optional=False
+
+from __future__ import absolute_import
+
+import contextlib
+import errno
+import hashlib
+import logging
+import os
+
+from pip._vendor import contextlib2
+
+from pip._internal.utils.temp_dir import TempDirectory
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from types import TracebackType
+    from typing import Dict, Iterator, Optional, Set, Type, Union
+    from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.models.link import Link
+
+logger = logging.getLogger(__name__)
+
+
+@contextlib.contextmanager
+def update_env_context_manager(**changes):
+    # type: (str) -> Iterator[None]
+    target = os.environ
+
+    # Save values from the target and change them.
+    non_existent_marker = object()
+    saved_values = {}  # type: Dict[str, Union[object, str]]
+    for name, new_value in changes.items():
+        try:
+            saved_values[name] = target[name]
+        except KeyError:
+            saved_values[name] = non_existent_marker
+        target[name] = new_value
+
+    try:
+        yield
+    finally:
+        # Restore original values in the target.
+        for name, original_value in saved_values.items():
+            if original_value is non_existent_marker:
+                del target[name]
+            else:
+                assert isinstance(original_value, str)  # for mypy
+                target[name] = original_value
+
+
+@contextlib.contextmanager
+def get_requirement_tracker():
+    # type: () -> Iterator[RequirementTracker]
+    root = os.environ.get('PIP_REQ_TRACKER')
+    with contextlib2.ExitStack() as ctx:
+        if root is None:
+            root = ctx.enter_context(
+                TempDirectory(kind='req-tracker')
+            ).path
+            ctx.enter_context(update_env_context_manager(PIP_REQ_TRACKER=root))
+            logger.debug("Initialized build tracking at %s", root)
+
+        with RequirementTracker(root) as tracker:
+            yield tracker
+
+
+class RequirementTracker(object):
+
+    def __init__(self, root):
+        # type: (str) -> None
+        self._root = root
+        self._entries = set()  # type: Set[InstallRequirement]
+        logger.debug("Created build tracker: %s", self._root)
+
+    def __enter__(self):
+        # type: () -> RequirementTracker
+        logger.debug("Entered build tracker: %s", self._root)
+        return self
+
+    def __exit__(
+        self,
+        exc_type,  # type: Optional[Type[BaseException]]
+        exc_val,  # type: Optional[BaseException]
+        exc_tb  # type: Optional[TracebackType]
+    ):
+        # type: (...) -> None
+        self.cleanup()
+
+    def _entry_path(self, link):
+        # type: (Link) -> str
+        hashed = hashlib.sha224(link.url_without_fragment.encode()).hexdigest()
+        return os.path.join(self._root, hashed)
+
+    def add(self, req):
+        # type: (InstallRequirement) -> None
+        """Add an InstallRequirement to build tracking.
+        """
+
+        # Get the file to write information about this requirement.
+        entry_path = self._entry_path(req.link)
+
+        # Try reading from the file. If it exists and can be read from, a build
+        # is already in progress, so a LookupError is raised.
+        try:
+            with open(entry_path) as fp:
+                contents = fp.read()
+        except IOError as e:
+            # if the error is anything other than "file does not exist", raise.
+            if e.errno != errno.ENOENT:
+                raise
+        else:
+            message = '%s is already being built: %s' % (req.link, contents)
+            raise LookupError(message)
+
+        # If we're here, req should really not be building already.
+        assert req not in self._entries
+
+        # Start tracking this requirement.
+        with open(entry_path, 'w') as fp:
+            fp.write(str(req))
+        self._entries.add(req)
+
+        logger.debug('Added %s to build tracker %r', req, self._root)
+
+    def remove(self, req):
+        # type: (InstallRequirement) -> None
+        """Remove an InstallRequirement from build tracking.
+        """
+
+        # Delete the created file and the corresponding entries.
+        os.unlink(self._entry_path(req.link))
+        self._entries.remove(req)
+
+        logger.debug('Removed %s from build tracker %r', req, self._root)
+
+    def cleanup(self):
+        # type: () -> None
+        for req in set(self._entries):
+            self.remove(req)
+
+        logger.debug("Removed build tracker: %r", self._root)
+
+    @contextlib.contextmanager
+    def track(self, req):
+        # type: (InstallRequirement) -> Iterator[None]
+        self.add(req)
+        yield
+        self.remove(req)