diff --git a/src/pip/_internal/metadata/base.py b/src/pip/_internal/metadata/base.py
@@ -57,6 +57,26 @@ def location(self) -> Optional[str]:
         A string value is not necessarily a filesystem path, since distributions
         can be loaded from other sources, e.g. arbitrary zip archives. ``None``
         means the distribution is created in-memory.
+
+        Do not canonicalize this value with e.g. ``pathlib.Path.resolve()``. If
+        this is a symbolic link, we want to preserve the relative path between
+        it and files in the distribution.
+        """
+        raise NotImplementedError()
+
+    @property
+    def info_directory(self) -> Optional[str]:
+        """Location of the .[egg|dist]-info directory.
+
+        Similarly to ``location``, a string value is not necessarily a
+        filesystem path. ``None`` means the distribution is created in-memory.
+
+        For a modern .dist-info installation on disk, this should be something
+        like ``{location}/{raw_name}-{version}.dist-info``.
+
+        Do not canonicalize this value with e.g. ``pathlib.Path.resolve()``. If
+        this is a symbolic link, we want to preserve the relative path between
+        it and other files in the distribution.
         """
         raise NotImplementedError()
 

diff --git a/src/pip/_internal/metadata/pkg_resources.py b/src/pip/_internal/metadata/pkg_resources.py
@@ -48,6 +48,10 @@ def from_wheel(cls, path: str, name: str) -> "Distribution":
     def location(self) -> Optional[str]:
         return self._dist.location
 
+    @property
+    def info_directory(self) -> Optional[str]:
+        return self._dist.egg_info
+
     @property
     def canonical_name(self) -> "NormalizedName":
         return canonicalize_name(self._dist.project_name)

diff --git a/src/pip/_internal/operations/install/legacy.py b/src/pip/_internal/operations/install/legacy.py
@@ -25,6 +25,46 @@ def __init__(self):
         self.parent = sys.exc_info()
 
 
+def write_installed_files_from_setuptools_record(
+    record_lines: List[str],
+    root: Optional[str],
+    req_description: str,
+) -> None:
+    def prepend_root(path):
+        # type: (str) -> str
+        if root is None or not os.path.isabs(path):
+            return path
+        else:
+            return change_root(root, path)
+
+    for line in record_lines:
+        directory = os.path.dirname(line)
+        if directory.endswith('.egg-info'):
+            egg_info_dir = prepend_root(directory)
+            break
+    else:
+        message = (
+            "{} did not indicate that it installed an "
+            ".egg-info directory. Only setup.py projects "
+            "generating .egg-info directories are supported."
+        ).format(req_description)
+        raise InstallationError(message)
+
+    new_lines = []
+    for line in record_lines:
+        filename = line.strip()
+        if os.path.isdir(filename):
+            filename += os.path.sep
+        new_lines.append(
+            os.path.relpath(prepend_root(filename), egg_info_dir)
+        )
+    new_lines.sort()
+    ensure_dir(egg_info_dir)
+    inst_files_path = os.path.join(egg_info_dir, 'installed-files.txt')
+    with open(inst_files_path, 'w') as f:
+        f.write('\n'.join(new_lines) + '\n')
+
+
 def install(
     install_options,  # type: List[str]
     global_options,  # type: Sequence[str]
@@ -88,38 +128,5 @@ def install(
         with open(record_filename) as f:
             record_lines = f.read().splitlines()
 
-    def prepend_root(path):
-        # type: (str) -> str
-        if root is None or not os.path.isabs(path):
-            return path
-        else:
-            return change_root(root, path)
-
-    for line in record_lines:
-        directory = os.path.dirname(line)
-        if directory.endswith('.egg-info'):
-            egg_info_dir = prepend_root(directory)
-            break
-    else:
-        message = (
-            "{} did not indicate that it installed an "
-            ".egg-info directory. Only setup.py projects "
-            "generating .egg-info directories are supported."
-        ).format(req_description)
-        raise InstallationError(message)
-
-    new_lines = []
-    for line in record_lines:
-        filename = line.strip()
-        if os.path.isdir(filename):
-            filename += os.path.sep
-        new_lines.append(
-            os.path.relpath(prepend_root(filename), egg_info_dir)
-        )
-    new_lines.sort()
-    ensure_dir(egg_info_dir)
-    inst_files_path = os.path.join(egg_info_dir, 'installed-files.txt')
-    with open(inst_files_path, 'w') as f:
-        f.write('\n'.join(new_lines) + '\n')
-
+    write_installed_files_from_setuptools_record(record_lines, root, req_description)
     return True
diff --git a/src/pip/_internal/resolution/resolvelib/provider.py b/src/pip/_internal/resolution/resolvelib/provider.py
@@ -143,6 +143,21 @@ def get_preference(
             identifier,
         )
 
+    def _get_constraint(self, identifier: str) -> Constraint:
+        if identifier in self._constraints:
+            return self._constraints[identifier]
+
+        # HACK: Theoratically we should check whether this identifier is a valid
+        # "NAME[EXTRAS]" format, and parse out the name part with packaging or
+        # some regular expression. But since pip's resolver only spits out
+        # three kinds of identifiers: normalized PEP 503 names, normalized names
+        # plus extras, and Requires-Python, we can cheat a bit here.
+        name, open_bracket, _ = identifier.partition("[")
+        if open_bracket and name in self._constraints:
+            return self._constraints[name]
+
+        return Constraint.empty()
+
     def find_matches(
         self,
         identifier: str,
@@ -169,7 +184,7 @@ def _eligible_for_upgrade(name: str) -> bool:
         return self._factory.find_candidates(
             identifier=identifier,
             requirements=requirements,
-            constraint=self._constraints.get(identifier, Constraint.empty()),
+            constraint=self._get_constraint(identifier),
             prefers_installed=(not _eligible_for_upgrade(identifier)),
             incompatibilities=incompatibilities,
         )

diff --git a/src/pip/_vendor/tenacity/__init__.py b/src/pip/_vendor/tenacity/__init__.py
@@ -190,7 +190,7 @@ def __init__(self, last_attempt: "Future") -> None:
         self.last_attempt = last_attempt
         super().__init__(last_attempt)
 
-    def reraise(self) -> t.NoReturn:
+    def reraise(self) -> "t.NoReturn":
         if self.last_attempt.failed:
             raise self.last_attempt.result()
         raise self

diff --git a/tests/functional/test_new_resolver.py b/tests/functional/test_new_resolver.py
@@ -2009,3 +2009,21 @@ def test_new_resolver_file_url_normalize(script, format_dep, format_input):
         format_input(lib_a), lib_b,
     )
     script.assert_installed(lib_a="1", lib_b="1")
+
+
+def test_new_resolver_dont_backtrack_on_extra_if_base_constrained(script):
+    create_basic_wheel_for_package(script, "dep", "1.0")
+    create_basic_wheel_for_package(script, "pkg", "1.0", extras={"ext": ["dep"]})
+    create_basic_wheel_for_package(script, "pkg", "2.0", extras={"ext": ["dep"]})
+    constraints_file = script.scratch_path / "constraints.txt"
+    constraints_file.write_text("pkg==1.0")
+
+    result = script.pip(
+        "install",
+        "--no-cache-dir", "--no-index",
+        "--find-links", script.scratch_path,
+        "--constraint", constraints_file,
+        "pkg[ext]",
+    )
+    assert "pkg-2.0" not in result.stdout, "Should not try 2.0 due to constraint"
+    script.assert_installed(pkg="1.0", dep="1.0")
diff --git a/tests/functional/test_show.py b/tests/functional/test_show.py
@@ -1,10 +1,12 @@
 import os
 import re
 
-import pytest
-
 from pip import __version__
 from pip._internal.commands.show import search_packages_info
+from pip._internal.operations.install.legacy import (
+    write_installed_files_from_setuptools_record,
+)
+from pip._internal.utils.unpacking import untar_file
 from tests.lib import create_test_package_with_setup
 
 
@@ -41,7 +43,7 @@ def test_show_with_files_not_found(script, data):
 
 def test_show_with_files_from_wheel(script, data):
     """
-    Test that a wheel's files can be listed
+    Test that a wheel's files can be listed.
     """
     wheel_file = data.packages.joinpath('simple.dist-0.1-py2.py3-none-any.whl')
     script.pip('install', '--no-index', wheel_file)
@@ -50,18 +52,36 @@ def test_show_with_files_from_wheel(script, data):
     assert 'Name: simple.dist' in lines
     assert 'Cannot locate RECORD or installed-files.txt' not in lines[6], lines[6]
     assert re.search(r"Files:\n(  .+\n)+", result.stdout)
+    assert f"  simpledist{os.sep}__init__.py" in lines[6:]
 
 
-@pytest.mark.network
-def test_show_with_all_files(script):
+def test_show_with_files_from_legacy(tmp_path, script, data):
     """
-    Test listing all files in the show command.
+    Test listing files in the show command (legacy installed-files.txt).
     """
-    script.pip('install', 'initools==0.2')
-    result = script.pip('show', '--files', 'initools')
+    # Since 'pip install' now always tries to build a wheel from sdist, it
+    # cannot properly generate a setup. The legacy code path is basically
+    # 'setup.py install' plus installed-files.txt, which we manually generate.
+    source_dir = tmp_path.joinpath("unpacked-sdist")
+    setuptools_record = tmp_path.joinpath("installed-record.txt")
+    untar_file(data.packages.joinpath("simple-1.0.tar.gz"), str(source_dir))
+    script.run(
+        "python", "setup.py", "install",
+        "--single-version-externally-managed",
+        "--record", str(setuptools_record),
+        cwd=source_dir,
+    )
+    write_installed_files_from_setuptools_record(
+        setuptools_record.read_text().splitlines(),
+        root=None,
+        req_description="simple==1.0",
+    )
+
+    result = script.pip('show', '--files', 'simple')
     lines = result.stdout.splitlines()
     assert 'Cannot locate RECORD or installed-files.txt' not in lines[6], lines[6]
     assert re.search(r"Files:\n(  .+\n)+", result.stdout)
+    assert f"  simple{os.sep}__init__.py" in lines[6:]
 
 
 def test_missing_argument(script):

diff --git a/tools/vendoring/patches/tenacity.patch b/tools/vendoring/patches/tenacity.patch
@@ -1,5 +1,5 @@
 diff --git a/src/pip/_vendor/tenacity/__init__.py b/src/pip/_vendor/tenacity/__init__.py
-index 88c28d2d6..f984eec4e 100644
+index 88c28d2d6..086ad46e1 100644
 --- a/src/pip/_vendor/tenacity/__init__.py
 +++ b/src/pip/_vendor/tenacity/__init__.py
 @@ -76,10 +76,12 @@ from .after import after_nothing  # noqa
@@ -19,3 +19,16 @@ index 88c28d2d6..f984eec4e 100644
 
  if t.TYPE_CHECKING:
      import types
+
+--- a/src/pip/_vendor/tenacity/__init__.py
++++ b/src/pip/_vendor/tenacity/__init__.py
+@@ -190,7 +190,7 @@ class RetryError(Exception):
+         self.last_attempt = last_attempt
+         super().__init__(last_attempt)
+
+-    def reraise(self) -> t.NoReturn:
++    def reraise(self) -> "t.NoReturn":
+         if self.last_attempt.failed:
+             raise self.last_attempt.result()
+         raise self
+