Prevent optional extensions from crashing editable builds (#3515)

- Don't try to copy the files of an optional extension if its build has
  failed.

3 files changed, 41 insertions, 1 deletions

diff --git a/changelog.d/3515.misc.rst b/changelog.d/3515.misc.rst
new file mode 100644
index 00000000..c884a316
--- /dev/null
+++ b/changelog.d/3515.misc.rst
@@ -0,0 +1,2 @@
+Fixed "inline" file copying for editable installations and

+optional extensions.
\ No newline at end of file
diff --git a/setuptools/command/build_ext.py b/setuptools/command/build_ext.py
index 7ad5a87a..cbfe3ec1 100644
--- a/setuptools/command/build_ext.py
+++ b/setuptools/command/build_ext.py
@@ -104,7 +104,8 @@ class build_ext(_build_ext):
             # Always copy, even if source is older than destination, to ensure
             # that the right extensions for the current Python/platform are
             # used.
-            self.copy_file(regular_file, inplace_file, level=self.verbose)
+            if os.path.exists(regular_file) or not ext.optional:
+                self.copy_file(regular_file, inplace_file, level=self.verbose)
 
             if ext._needs_stub:
                 inplace_stub = self._get_equivalent_stub(ext, inplace_file)
diff --git a/setuptools/tests/test_build_ext.py b/setuptools/tests/test_build_ext.py
index 07ebcaf8..92ce80ef 100644
--- a/setuptools/tests/test_build_ext.py
+++ b/setuptools/tests/test_build_ext.py
@@ -9,10 +9,13 @@ from jaraco import path
 from setuptools.command.build_ext import build_ext, get_abi3_suffix
 from setuptools.dist import Distribution
 from setuptools.extension import Extension
+from setuptools.errors import CompileError
 
 from . import environment
 from .textwrap import DALS
 
+import pytest
+
 
 IS_PYPY = '__pypy__' in sys.builtin_module_names
 
@@ -176,6 +179,40 @@ class TestBuildExt:
         assert example_stub.endswith(".pyc")
 
 
+class TestBuildExtInplace:
+    def get_build_ext_cmd(self, optional: bool, **opts):
+        files = {
+            "eggs.c": "#include missingheader.h\n",
+            ".build": {"lib": {}, "tmp": {}},
+        }
+        path.build(files)
+        extension = Extension('spam.eggs', ['eggs.c'], optional=optional)
+        dist = Distribution(dict(ext_modules=[extension]))
+        dist.script_name = 'setup.py'
+        cmd = build_ext(dist)
+        vars(cmd).update(build_lib=".build/lib", build_temp=".build/tmp", **opts)
+        cmd.ensure_finalized()
+        return cmd
+
+    def test_optional(self, tmpdir_cwd, capsys):
+        """
+        If optional extensions fail to build, setuptools should show the error
+        in the logs but not fail to build
+        """
+        cmd = self.get_build_ext_cmd(optional=True, inplace=True)
+        cmd.run()
+        logs = capsys.readouterr()
+        messages = (logs.out + logs.err)
+        assert 'build_ext: building extension "spam.eggs" failed' in messages
+        # No compile error exception should be raised
+
+    def test_non_optional(self, tmpdir_cwd):
+        # Non-optional extensions should raise an exception
+        cmd = self.get_build_ext_cmd(optional=False, inplace=True)
+        with pytest.raises(CompileError):
+            cmd.run()
+
+
 def test_build_ext_config_handling(tmpdir_cwd):
     files = {
         'setup.py': DALS(