Add support for gathering all requirements needed to build a wheel fo…

…r a project.

pex/build_system/__init__.py

@@ -10,3 +10,4 @@
 #
 # See: https://peps.python.org/pep-0517/#source-trees
 DEFAULT_BUILD_BACKEND = "setuptools.build_meta:__legacy__"
+DEFAULT_BUILD_REQUIRES = ("setuptools",)

pex/build_system/pep_517.py

@@ -10,10 +10,11 @@
 
 from pex import third_party
 from pex.build_system import DEFAULT_BUILD_BACKEND
-from pex.build_system.pep_518 import BuildSystem, load_build_system
+from pex.build_system.pep_518 import BuildSystem, load_build_system, load_build_system_table
 from pex.common import safe_mkdtemp
 from pex.dist_metadata import DistMetadata, Distribution, MetadataType
 from pex.jobs import Job, SpawnedJob
+from pex.orderedset import OrderedSet
 from pex.pip.version import PipVersion, PipVersionValue
 from pex.resolve.resolvers import Resolver
 from pex.result import Error, try_
@@ -22,7 +23,7 @@
 from pex.typing import TYPE_CHECKING, cast
 
 if TYPE_CHECKING:
-    from typing import Any, Dict, Iterable, List, Mapping, Optional, Set, Text, Union
+    from typing import Any, Dict, Iterable, List, Mapping, Optional, Set, Text, Tuple, Union
 
 _DEFAULT_BUILD_SYSTEMS = {}  # type: Dict[PipVersionValue, BuildSystem]
 
@@ -248,15 +249,16 @@ def build_sdist(
     return os.path.join(dist_dir, sdist_relpath)
 
 
-def spawn_prepare_metadata(
+def get_requires_for_build_wheel(
     project_directory,  # type: str
     target,  # type: Target
     resolver,  # type: Resolver
     pip_version=None,  # type: Optional[PipVersionValue]
 ):
-    # type: (...) -> SpawnedJob[DistMetadata]
+    # type: (...) -> Tuple[str, ...]
 
-    extra_requirements = []
+    build_system_table = try_(load_build_system_table(project_directory))
+    requires = OrderedSet(build_system_table.requires)
     spawned_job = try_(
         _invoke_build_hook(
             project_directory,
@@ -267,11 +269,24 @@ def spawn_prepare_metadata(
         )
     )
     try:
-        extra_requirements.extend(spawned_job.await_result())
+        requires.update(spawned_job.await_result())
     except Job.Error as e:
         if e.exitcode != _HOOK_UNAVAILABLE_EXIT_CODE:
             raise e
+    return tuple(requires)
+
 
+def spawn_prepare_metadata(
+    project_directory,  # type: str
+    target,  # type: Target
+    resolver,  # type: Resolver
+    pip_version=None,  # type: Optional[PipVersionValue]
+):
+    # type: (...) -> SpawnedJob[DistMetadata]
+
+    extra_requirements = get_requires_for_build_wheel(
+        project_directory, target, resolver, pip_version=pip_version
+    )
     build_dir = os.path.join(safe_mkdtemp(), "build")
     os.mkdir(build_dir)
     spawned_job = try_(

pex/build_system/pep_518.py

@@ -7,7 +7,7 @@
 import subprocess
 
 from pex import toml
-from pex.build_system import DEFAULT_BUILD_BACKEND
+from pex.build_system import DEFAULT_BUILD_BACKEND, DEFAULT_BUILD_REQUIRES
 from pex.common import REPRODUCIBLE_BUILDS_ENV, CopyMode
 from pex.dist_metadata import Distribution
 from pex.interpreter import PythonInterpreter
@@ -159,6 +159,25 @@ def create(
     env = attr.ib()  # type: Mapping[str, str]
 
 
+def _maybe_load_build_system_table(project_directory):
+    # type: (str) -> Union[Optional[BuildSystemTable], Error]
+
+    # The interface is spec'd here: https://peps.python.org/pep-0518/
+    pyproject_toml = os.path.join(project_directory, "pyproject.toml")
+    if not os.path.isfile(pyproject_toml):
+        return None
+    return _read_build_system_table(pyproject_toml)
+
+
+def load_build_system_table(project_directory):
+    # type: (str) -> Union[BuildSystemTable, Error]
+
+    maybe_build_system_table_or_error = _maybe_load_build_system_table(project_directory)
+    if maybe_build_system_table_or_error is not None:
+        return maybe_build_system_table_or_error
+    return BuildSystemTable(requires=DEFAULT_BUILD_REQUIRES, build_backend=DEFAULT_BUILD_BACKEND)
+
+
 def load_build_system(
     target,  # type: Target
     resolver,  # type: Resolver
@@ -167,12 +186,7 @@ def load_build_system(
 ):
     # type: (...) -> Union[Optional[BuildSystem], Error]
 
-    # The interface is spec'd here: https://peps.python.org/pep-0518/
-    pyproject_toml = os.path.join(project_directory, "pyproject.toml")
-    if not os.path.isfile(pyproject_toml):
-        return None
-
-    maybe_build_system_table_or_error = _read_build_system_table(pyproject_toml)
+    maybe_build_system_table_or_error = _maybe_load_build_system_table(project_directory)
     if not isinstance(maybe_build_system_table_or_error, BuildSystemTable):
         return maybe_build_system_table_or_error
     build_system_table = maybe_build_system_table_or_error