Use importlib.metadata to read requirements (#12088)

* Pull runtime dep checks into their own module
* Reimplement `check_requirements` using `importlib`

I've tried to make this clearer. We start by working out which of
Synapse's requirements we need to be installed here and now. I was
surprised that there wasn't an easier way to see which packages were
installed by a given extra.

I've pulled out the error messages into functions that deal with "is
this for an extra or not". And I've rearranged the loop over two
different sets of requirements into one loop with a "must be instaled"
flag.

I hope you agree that this is clearer.

* Test cases

1 files changed, 3 insertions, 104 deletions

diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index f43fbb5842..8f48a33936 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -17,14 +17,7 @@
 
 import itertools
 import logging
-from typing import List, Set
-
-from pkg_resources import (
-    DistributionNotFound,
-    Requirement,
-    VersionConflict,
-    get_provider,
-)
+from typing import Set
 
 logger = logging.getLogger(__name__)
 
@@ -90,6 +83,8 @@ REQUIREMENTS = [
     # ijson 3.1.4 fixes a bug with "." in property names
     "ijson>=3.1.4",
     "matrix-common~=1.1.0",
+    # For runtime introspection of our dependencies
+    "packaging~=21.3",
 ]
 
 CONDITIONAL_REQUIREMENTS = {
@@ -144,102 +139,6 @@ def list_requirements():
     return list(set(REQUIREMENTS) | ALL_OPTIONAL_REQUIREMENTS)
 
 
-class DependencyException(Exception):
-    @property
-    def message(self):
-        return "\n".join(
-            [
-                "Missing Requirements: %s" % (", ".join(self.dependencies),),
-                "To install run:",
-                "    pip install --upgrade --force %s" % (" ".join(self.dependencies),),
-                "",
-            ]
-        )
-
-    @property
-    def dependencies(self):
-        for i in self.args[0]:
-            yield '"' + i + '"'
-
-
-def check_requirements(for_feature=None):
-    deps_needed = []
-    errors = []
-
-    if for_feature:
-        reqs = CONDITIONAL_REQUIREMENTS[for_feature]
-    else:
-        reqs = REQUIREMENTS
-
-    for dependency in reqs:
-        try:
-            _check_requirement(dependency)
-        except VersionConflict as e:
-            deps_needed.append(dependency)
-            errors.append(
-                "Needed %s, got %s==%s"
-                % (
-                    dependency,
-                    e.dist.project_name,  # type: ignore[attr-defined] # noqa
-                    e.dist.version,  # type: ignore[attr-defined] # noqa
-                )
-            )
-        except DistributionNotFound:
-            deps_needed.append(dependency)
-            if for_feature:
-                errors.append(
-                    "Needed %s for the '%s' feature but it was not installed"
-                    % (dependency, for_feature)
-                )
-            else:
-                errors.append("Needed %s but it was not installed" % (dependency,))
-
-    if not for_feature:
-        # Check the optional dependencies are up to date. We allow them to not be
-        # installed.
-        OPTS: List[str] = sum(CONDITIONAL_REQUIREMENTS.values(), [])
-
-        for dependency in OPTS:
-            try:
-                _check_requirement(dependency)
-            except VersionConflict as e:
-                deps_needed.append(dependency)
-                errors.append(
-                    "Needed optional %s, got %s==%s"
-                    % (
-                        dependency,
-                        e.dist.project_name,  # type: ignore[attr-defined] # noqa
-                        e.dist.version,  # type: ignore[attr-defined] # noqa
-                    )
-                )
-            except DistributionNotFound:
-                # If it's not found, we don't care
-                pass
-
-    if deps_needed:
-        for err in errors:
-            logging.error(err)
-
-        raise DependencyException(deps_needed)
-
-
-def _check_requirement(dependency_string):
-    """Parses a dependency string, and checks if the specified requirement is installed
-
-    Raises:
-        VersionConflict if the requirement is installed, but with the the wrong version
-        DistributionNotFound if nothing is found to provide the requirement
-    """
-    req = Requirement.parse(dependency_string)
-
-    # first check if the markers specify that this requirement needs installing
-    if req.marker is not None and not req.marker.evaluate():
-        # not required for this environment
-        return
-
-    get_provider(req)
-
-
 if __name__ == "__main__":
     import sys