1 files changed, 127 insertions, 0 deletions
diff --git a/synapse/util/check_dependencies.py b/synapse/util/check_dependencies.py
new file mode 100644
index 0000000000..3a1f6b3c75
--- /dev/null
+++ b/synapse/util/check_dependencies.py
@@ -0,0 +1,127 @@
+import logging
+from typing import Iterable, NamedTuple, Optional
+
+from packaging.requirements import Requirement
+
+DISTRIBUTION_NAME = "matrix-synapse"
+
+try:
+    from importlib import metadata
+except ImportError:
+    import importlib_metadata as metadata  # type: ignore[no-redef]
+
+
+class DependencyException(Exception):
+    @property
+    def message(self) -> str:
+        return "\n".join(
+            [
+                "Missing Requirements: %s" % (", ".join(self.dependencies),),
+                "To install run:",
+                "    pip install --upgrade --force %s" % (" ".join(self.dependencies),),
+                "",
+            ]
+        )
+
+    @property
+    def dependencies(self) -> Iterable[str]:
+        for i in self.args[0]:
+            yield '"' + i + '"'
+
+
+EXTRAS = set(metadata.metadata(DISTRIBUTION_NAME).get_all("Provides-Extra"))
+
+
+class Dependency(NamedTuple):
+    requirement: Requirement
+    must_be_installed: bool
+
+
+def _generic_dependencies() -> Iterable[Dependency]:
+    """Yield pairs (requirement, must_be_installed)."""
+    requirements = metadata.requires(DISTRIBUTION_NAME)
+    assert requirements is not None
+    for raw_requirement in requirements:
+        req = Requirement(raw_requirement)
+        # https://packaging.pypa.io/en/latest/markers.html#usage notes that
+        #   > Evaluating an extra marker with no environment is an error
+        # so we pass in a dummy empty extra value here.
+        must_be_installed = req.marker is None or req.marker.evaluate({"extra": ""})
+        yield Dependency(req, must_be_installed)
+
+
+def _dependencies_for_extra(extra: str) -> Iterable[Dependency]:
+    """Yield additional dependencies needed for a given `extra`."""
+    requirements = metadata.requires(DISTRIBUTION_NAME)
+    assert requirements is not None
+    for raw_requirement in requirements:
+        req = Requirement(raw_requirement)
+        # Exclude mandatory deps by only selecting deps needed with this extra.
+        if (
+            req.marker is not None
+            and req.marker.evaluate({"extra": extra})
+            and not req.marker.evaluate({"extra": ""})
+        ):
+            yield Dependency(req, True)
+
+
+def _not_installed(requirement: Requirement, extra: Optional[str] = None) -> str:
+    if extra:
+        return f"Need {requirement.name} for {extra}, but it is not installed"
+    else:
+        return f"Need {requirement.name}, but it is not installed"
+
+
+def _incorrect_version(
+    requirement: Requirement, got: str, extra: Optional[str] = None
+) -> str:
+    if extra:
+        return f"Need {requirement} for {extra}, but got {requirement.name}=={got}"
+    else:
+        return f"Need {requirement}, but got {requirement.name}=={got}"
+
+
+def check_requirements(extra: Optional[str] = None) -> None:
+    """Check Synapse's dependencies are present and correctly versioned.
+
+    If provided, `extra` must be the name of an pacakging extra (e.g. "saml2" in
+    `pip install matrix-synapse[saml2]`).
+
+    If `extra` is None, this function checks that
+    - all mandatory dependencies are installed and correctly versioned, and
+    - each optional dependency that's installed is correctly versioned.
+
+    If `extra` is not None, this function checks that
+    - the dependencies needed for that extra are installed and correctly versioned.
+
+    :raises DependencyException: if a dependency is missing or incorrectly versioned.
+    :raises ValueError: if this extra does not exist.
+    """
+    # First work out which dependencies are required, and which are optional.
+    if extra is None:
+        dependencies = _generic_dependencies()
+    elif extra in EXTRAS:
+        dependencies = _dependencies_for_extra(extra)
+    else:
+        raise ValueError(f"Synapse does not provide the feature '{extra}'")
+
+    deps_unfulfilled = []
+    errors = []
+
+    for (requirement, must_be_installed) in dependencies:
+        try:
+            dist: metadata.Distribution = metadata.distribution(requirement.name)
+        except metadata.PackageNotFoundError:
+            if must_be_installed:
+                deps_unfulfilled.append(requirement.name)
+                errors.append(_not_installed(requirement, extra))
+        else:
+            if not requirement.specifier.contains(dist.version):
+                deps_unfulfilled.append(requirement.name)
+                errors.append(_incorrect_version(requirement, dist.version, extra))
+
+    if deps_unfulfilled:
+        for err in errors:
+            logging.error(err)
+
+        raise DependencyException(deps_unfulfilled)