Restrict #egg= fragments to valid PEP 508 names

This should help reduce user confusion about what can go in a URI's
egg fragment.

Fixes pypa#11567.

Signed-off-by: William Woodruff <william@trailofbits.com>

src/pip/_internal/exceptions.py

@@ -658,3 +658,11 @@ def __str__(self) -> str:
             assert self.error is not None
             message_part = f".\n{self.error}\n"
         return f"Configuration file {self.reason}{message_part}"
+
+
+class InvalidEggFragment(InstallationError):
+    """A link's `#egg=` fragment doesn't look like a valid PEP 508 project
+    name."""
+
+    def __init__(self, fragment: str) -> None:
+        super().__init__(f"egg fragment is not a bare project name: {fragment}")

src/pip/_internal/models/link.py

@@ -18,6 +18,7 @@
     Union,
 )
 
+from pip._internal.exceptions import InvalidEggFragment
 from pip._internal.utils.filetypes import WHEEL_EXTENSION
 from pip._internal.utils.hashes import Hashes
 from pip._internal.utils.misc import (
@@ -358,12 +359,25 @@ def url_without_fragment(self) -> str:
 
     _egg_fragment_re = re.compile(r"[#&]egg=([^&]*)")
 
+    # Per PEP 508.
+    _project_name_re = re.compile(
+        r"^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$", re.IGNORECASE
+    )
+
     @property
     def egg_fragment(self) -> Optional[str]:
         match = self._egg_fragment_re.search(self._url)
         if not match:
             return None
-        return match.group(1)
+
+        # The egg fragment must look like a project name, and only
+        # a project name. In particular, it can't contain version constraints
+        # or anything else like that.
+        project_name = match.group(1)
+        if not self._project_name_re.match(project_name):
+            raise InvalidEggFragment(project_name)
+
+        return project_name
 
     _subdirectory_fragment_re = re.compile(r"[#&]subdirectory=([^&]*)")