Forward warnings from build-backend to build-frontend

This makes it possible for build-frontends to surface this information
to end users.

src/pyproject_hooks/__init__.py

@@ -2,6 +2,7 @@
 """
 
 from ._impl import (
+    BuildBackendWarning,
     BackendInvalid,
     BackendUnavailable,
     BuildBackendHookCaller,
@@ -13,6 +14,7 @@
 
 __version__ = "1.0.0"
 __all__ = [
+    "BuildBackendWarning",
     "BackendUnavailable",
     "BackendInvalid",
     "HookMissing",

src/pyproject_hooks/_impl.py

@@ -6,6 +6,7 @@
 from os.path import abspath
 from os.path import join as pjoin
 from subprocess import STDOUT, check_call, check_output
+import warnings
 
 from ._in_process import _in_proc_script_path
 
@@ -20,6 +21,10 @@ def read_json(path):
         return json.load(f)
 
 
+class BuildBackendWarning(UserWarning):
+    """Will be emitted for every UserWarning emitted by the hook process."""
+
+
 class BackendUnavailable(Exception):
     """Will be raised if the backend cannot be imported in the hook process."""
 
@@ -343,4 +348,12 @@ def _call_hook(self, hook_name, kwargs):
                 )
             if data.get("hook_missing"):
                 raise HookMissing(data.get("missing_hook_name") or hook_name)
+
+            for w in data.get("warnings", []):
+                warnings.warn_explicit(
+                    message=w["message"],
+                    category=BuildBackendWarning,
+                    filename=w["filename"],
+                    lineno=w["lineno"],
+                )
             return data["return_val"]

src/pyproject_hooks/_in_process/_in_process.py

@@ -22,6 +22,7 @@
 from glob import glob
 from importlib import import_module
 from os.path import join as pjoin
+import warnings
 
 # This file is run as a script, and `import wrappers` is not zip-safe, so we
 # include write_json() and read_json() from wrappers.py.
@@ -336,22 +337,33 @@ def main():
 
     hook_input = read_json(pjoin(control_dir, "input.json"))
 
-    json_out = {"unsupported": False, "return_val": None}
-    try:
-        json_out["return_val"] = hook(**hook_input["kwargs"])
-    except BackendUnavailable as e:
-        json_out["no_backend"] = True
-        json_out["traceback"] = e.traceback
-    except BackendInvalid as e:
-        json_out["backend_invalid"] = True
-        json_out["backend_error"] = e.message
-    except GotUnsupportedOperation as e:
-        json_out["unsupported"] = True
-        json_out["traceback"] = e.traceback
-    except HookMissing as e:
-        json_out["hook_missing"] = True
-        json_out["missing_hook_name"] = e.hook_name or hook_name
-
+    with warnings.catch_warnings(record=True) as captured_warnings:
+        json_out = {"unsupported": False, "return_val": None}
+        try:
+            json_out["return_val"] = hook(**hook_input["kwargs"])
+        except BackendUnavailable as e:
+            json_out["no_backend"] = True
+            json_out["traceback"] = e.traceback
+        except BackendInvalid as e:
+            json_out["backend_invalid"] = True
+            json_out["backend_error"] = e.message
+        except GotUnsupportedOperation as e:
+            json_out["unsupported"] = True
+            json_out["traceback"] = e.traceback
+        except HookMissing as e:
+            json_out["hook_missing"] = True
+            json_out["missing_hook_name"] = e.hook_name or hook_name
+
+    json_out["warnings"] = [
+        {
+            "message": str(w.message),
+            "filename": w.filename,
+            "lineno": w.lineno,
+        }
+        for w in captured_warnings
+        if isinstance(w.category, type) and issubclass(w.category, UserWarning)
+    ]
+    print(json_out)
     write_json(json_out, pjoin(control_dir, "output.json"), indent=2)
 
 

tests/samples/buildsys_pkgs/buildsys_warnings.py

@@ -0,0 +1,9 @@
+"""Test "backend" defining nothing other than a hook that logs a warning.
+"""
+
+import warnings
+
+
+def get_requires_for_build_wheel(config_settings):
+    warnings.warn("this is my example warning")
+    return []

tests/samples/pkg-with-warnings/pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["eg_buildsys"]
+build-backend = "buildsys_warnings"

tests/test_call_hooks.py

@@ -1,6 +1,7 @@
 import json
 import os
 import tarfile
+import warnings
 import zipfile
 from os.path import abspath, dirname
 from os.path import join as pjoin
@@ -12,6 +13,7 @@
 
 from pyproject_hooks import (
     BackendUnavailable,
+    BuildBackendWarning,
     BuildBackendHookCaller,
     UnsupportedOperation,
     default_subprocess_runner,
@@ -226,3 +228,10 @@ def test__supported_features(pkg, expected):
     with modified_env({"PYTHONPATH": BUILDSYS_PKGS}):
         res = hooks._supported_features()
     assert res == expected
+
+
+def test_warnings():
+    hooks = get_hooks("pkg-with-warnings")
+    with modified_env({"PYTHONPATH": BUILDSYS_PKGS}):
+        with pytest.warns(BuildBackendWarning, match="this is my example warning"):
+            hooks.get_requires_for_build_wheel({})