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, 95 insertions, 0 deletions

diff --git a/tests/util/test_check_dependencies.py b/tests/util/test_check_dependencies.py
new file mode 100644
index 0000000000..3c07252252
--- /dev/null
+++ b/tests/util/test_check_dependencies.py
@@ -0,0 +1,95 @@
+from contextlib import contextmanager
+from typing import Generator, Optional
+from unittest.mock import patch
+
+from synapse.util.check_dependencies import (
+    DependencyException,
+    check_requirements,
+    metadata,
+)
+
+from tests.unittest import TestCase
+
+
+class DummyDistribution(metadata.Distribution):
+    def __init__(self, version: str):
+        self._version = version
+
+    @property
+    def version(self):
+        return self._version
+
+    def locate_file(self, path):
+        raise NotImplementedError()
+
+    def read_text(self, filename):
+        raise NotImplementedError()
+
+
+old = DummyDistribution("0.1.2")
+new = DummyDistribution("1.2.3")
+
+# could probably use stdlib TestCase --- no need for twisted here
+
+
+class TestDependencyChecker(TestCase):
+    @contextmanager
+    def mock_installed_package(
+        self, distribution: Optional[DummyDistribution]
+    ) -> Generator[None, None, None]:
+        """Pretend that looking up any distribution yields the given `distribution`."""
+
+        def mock_distribution(name: str):
+            if distribution is None:
+                raise metadata.PackageNotFoundError
+            else:
+                return distribution
+
+        with patch(
+            "synapse.util.check_dependencies.metadata.distribution",
+            mock_distribution,
+        ):
+            yield
+
+    def test_mandatory_dependency(self) -> None:
+        """Complain if a required package is missing or old."""
+        with patch(
+            "synapse.util.check_dependencies.metadata.requires",
+            return_value=["dummypkg >= 1"],
+        ):
+            with self.mock_installed_package(None):
+                self.assertRaises(DependencyException, check_requirements)
+            with self.mock_installed_package(old):
+                self.assertRaises(DependencyException, check_requirements)
+            with self.mock_installed_package(new):
+                # should not raise
+                check_requirements()
+
+    def test_generic_check_of_optional_dependency(self) -> None:
+        """Complain if an optional package is old."""
+        with patch(
+            "synapse.util.check_dependencies.metadata.requires",
+            return_value=["dummypkg >= 1; extra == 'cool-extra'"],
+        ):
+            with self.mock_installed_package(None):
+                # should not raise
+                check_requirements()
+            with self.mock_installed_package(old):
+                self.assertRaises(DependencyException, check_requirements)
+            with self.mock_installed_package(new):
+                # should not raise
+                check_requirements()
+
+    def test_check_for_extra_dependencies(self) -> None:
+        """Complain if a package required for an extra is missing or old."""
+        with patch(
+            "synapse.util.check_dependencies.metadata.requires",
+            return_value=["dummypkg >= 1; extra == 'cool-extra'"],
+        ), patch("synapse.util.check_dependencies.EXTRAS", {"cool-extra"}):
+            with self.mock_installed_package(None):
+                self.assertRaises(DependencyException, check_requirements, "cool-extra")
+            with self.mock_installed_package(old):
+                self.assertRaises(DependencyException, check_requirements, "cool-extra")
+            with self.mock_installed_package(new):
+                # should not raise
+                check_requirements()