pytest test sharding (#493)

Adds test sharding support to the pytest runner.

I took some inspiration from
https://github.com/caseyduquettesc/rules_python_pytest . In particular,
I used it as a reference for which Bazel environment variables need to
be pulled in the shim.

The pytest plugin is a derivative work of
https://github.com/AdamGleave/pytest-shard . That dependency hasn't been
updated in a long time, and the sharding was based off hashing which
often results in unbalanced shards or errors from empty shards. I
vendored the code into the repo, modified it to use a plain round robin
strategy for selection, and converted the code to be class based so it
can be used with `pytest.main()`.

### Test plan

I added an example to the examples directory and I tested it with my own
repository here:
https://github.com/vinnybod/bazel-examples/blob/python-test-sharding/test-shard-python/BUILD.bazel

Author: vinnybod

examples/pytest/BUILD.bazel

@@ -58,3 +58,18 @@ py_test(
         "@pypi_pytest//:pkg",
     ],
 )
+
+py_test(
+    name = "sharding_test",
+    srcs = [
+        "__test__.py",
+        "sharding_test.py",
+    ],
+    imports = ["../.."],
+    main = "__test__.py",
+    package_collisions = "warning",
+    shard_count = 2,
+    deps = [
+        "__test__",
+    ],
+)

examples/pytest/sharding_test.py

@@ -0,0 +1,5 @@
+def test_shard_one():
+    assert True
+
+def test_shard_two():
+    assert True

examples/virtual_deps/BUILD.bazel

@@ -69,6 +69,7 @@ py_test(
     ),
     deps = [
         requirement("pytest"),
+        "__test__",
         ":greet",
     ],
 )

py/private/py_pytest_main.bzl

@@ -88,6 +88,6 @@ def py_pytest_main(name, py_library = default_py_library, deps = [], data = [],
         srcs = [test_main],
         tags = tags,
         visibility = visibility,
-        deps = deps,
+        deps = deps + [Label("//py/private/pytest_shard")],
         data = data,
     )

py/private/pytest.py.tmpl

@@ -14,6 +14,7 @@
 
 import sys
 import os
+from pathlib import Path
 from typing import List
 
 import pytest
@@ -33,12 +34,15 @@ if "COVERAGE_MANIFEST" in os.environ:
         print("WARNING: python coverage setup failed. Do you need to include the 'coverage' library as a dependency of py_pytest_main?", e)
         pass
 
+from pytest_shard import ShardPlugin
+
 if __name__ == "__main__":
     # Change to the directory where we need to run the test or execute a no-op
     $$CHDIR$$
 
     os.environ["ENV"] = "testing"
 
+    plugins = []
     args = [
         "--verbose",
         "--ignore=external/",
@@ -55,6 +59,20 @@ if __name__ == "__main__":
         if suite_name:
             args.extend(["-o", f"junit_suite_name={suite_name}"])
 
+    test_shard_index = os.environ.get("TEST_SHARD_INDEX")
+    test_total_shards = os.environ.get("TEST_TOTAL_SHARDS")
+    test_shard_status_file = os.environ.get("TEST_SHARD_STATUS_FILE")
+    if (
+        all([test_shard_index, test_total_shards, test_shard_status_file])
+        and int(test_total_shards) > 1
+    ):
+        args.extend([
+            f"--shard-id={test_shard_index}",
+            f"--num-shards={test_total_shards}",
+        ])
+        Path(test_shard_status_file).touch()
+        plugins.append(ShardPlugin())
+
     test_filter = os.environ.get("TESTBRIDGE_TEST_ONLY")
     if test_filter is not None:
         args.append(f"-k={test_filter}")
@@ -67,7 +85,7 @@ if __name__ == "__main__":
     if len(cli_args) > 0:
         args.extend(cli_args)
 
-    exit_code = pytest.main(args)
+    exit_code = pytest.main(args, plugins=plugins)
 
     if exit_code != 0:
         print("Pytest exit code: " + str(exit_code), file=sys.stderr)

py/private/pytest_shard/BUILD.bazel

@@ -0,0 +1,11 @@
+load("@aspect_rules_py//py:defs.bzl", "py_library")
+
+py_library(
+    name = "pytest_shard",
+    srcs = [
+        "__init__.py",
+        ":pytest_shard.py",
+    ],
+    imports = ["."],
+    visibility = ["//visibility:public"],
+)

py/private/pytest_shard/LICENSE

@@ -0,0 +1,7 @@
+Copyright 2019 Adam Gleave
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

py/private/pytest_shard/README.md

@@ -0,0 +1,9 @@
+# pytest-shard
+
+This is a fork of [pytest-shard](https://github.com/AdamGleave/pytest-shard) @ [4610a08](https://github.com/AdamGleave/pytest-shard/commit/64610a08dac6b0511b6d51cf895d0e1040d162ad)
+
+## Changes
+
+- The pytest hooks were moved into a class `ShardPlugin`, so that they can be loaded via `pytest.main`
+- The sharding strategy was changed to a simple round-robin strategy
+  - The hash-bashed strategy was causing unbalanced or empty shards with small test sets

py/private/pytest_shard/__init__.py

@@ -0,0 +1,64 @@
+from typing import Iterable, List, Sequence
+
+from _pytest import nodes  # for type checking only
+
+
+def positive_int(x) -> int:
+    x = int(x)
+    if x < 0:
+        raise ValueError(f"Argument {x} must be positive")
+    return x
+
+
+def filter_items_by_shard(
+    items: Iterable[nodes.Node], shard_id: int, num_shards: int
+) -> Sequence[nodes.Node]:
+    """Computes `items` that should be tested in `shard_id` out of `num_shards` total shards."""
+    shards = [i % num_shards for i in range(len(items))]
+
+    new_items = []
+    for shard, item in zip(shards, items):
+        if shard == shard_id:
+            new_items.append(item)
+    return new_items
+
+
+class ShardPlugin:
+    @staticmethod
+    def pytest_addoption(parser):
+        """Add pytest-shard specific configuration parameters."""
+        group = parser.getgroup("shard")
+        group.addoption(
+            "--shard-id",
+            dest="shard_id",
+            type=positive_int,
+            default=0,
+            help="Number of this shard.",
+        )
+        group.addoption(
+            "--num-shards",
+            dest="num_shards",
+            type=positive_int,
+            default=1,
+            help="Total number of shards.",
+        )
+
+    @staticmethod
+    def pytest_report_collectionfinish(config, items: Sequence[nodes.Node]) -> str:
+        """Log how many and, if verbose, which items are tested in this shard."""
+        msg = f"Running {len(items)} items in this shard"
+        if config.option.verbose > 0 and config.getoption("num_shards") > 1:
+            msg += ": " + ", ".join([item.nodeid for item in items])
+        return msg
+
+    @staticmethod
+    def pytest_collection_modifyitems(config, items: List[nodes.Node]):
+        """Mutate the collection to consist of just items to be tested in this shard."""
+        shard_id = config.getoption("shard_id")
+        shard_total = config.getoption("num_shards")
+        if shard_id >= shard_total:
+            raise ValueError(
+                "shard_num = f{shard_num} must be less than shard_total = f{shard_total}"
+            )
+
+        items[:] = filter_items_by_shard(items, shard_id, shard_total)