build: add circular dependency protection

Fixes  pypa#511.

src/build/__init__.py

@@ -176,6 +176,13 @@ def check_dependency(
         import importlib_metadata
 
     req = packaging.requirements.Requirement(req_string)
+    normalised_req_string = str(req)
+
+    # ``Requirement`` doesn't implement ``__eq__`` so we cannot compare reqs for
+    # equality directly but the string representation is stable.
+    if normalised_req_string in ancestral_req_strings:
+        # cyclical dependency, already checked.
+        return
 
     if req.marker:
         extras = frozenset(('',)).union(parent_extras)
@@ -190,15 +197,15 @@ def check_dependency(
         dist = importlib_metadata.distribution(req.name)  # type: ignore[no-untyped-call]
     except importlib_metadata.PackageNotFoundError:
         # dependency is not installed in the environment.
-        yield ancestral_req_strings + (req_string,)
+        yield ancestral_req_strings + (normalised_req_string,)
     else:
         if req.specifier and not req.specifier.contains(dist.version, prereleases=True):
             # the installed version is incompatible.
-            yield ancestral_req_strings + (req_string,)
+            yield ancestral_req_strings + (normalised_req_string,)
         elif dist.requires:
             for other_req_string in dist.requires:
                 # yields transitive dependencies that are not satisfied.
-                yield from check_dependency(other_req_string, ancestral_req_strings + (req_string,), req.extras)
+                yield from check_dependency(other_req_string, ancestral_req_strings + (normalised_req_string,), req.extras)
 
 
 def _find_typo(dictionary: Mapping[str, str], expected: str) -> None:

tests/test_projectbuilder.py

@@ -133,24 +133,24 @@ def read_text(self, filename):
         ('extras_dep[extra_without_associated_deps]', None),
         (
             'extras_dep[extra_with_unmet_deps]',
-            ('extras_dep[extra_with_unmet_deps]', "unmet_dep; extra == 'extra_with_unmet_deps'"),
+            ('extras_dep[extra_with_unmet_deps]', 'unmet_dep; extra == "extra_with_unmet_deps"'),
         ),
         (
             'extras_dep[recursive_extra_with_unmet_deps]',
             (
                 'extras_dep[recursive_extra_with_unmet_deps]',
-                "recursive_dep; extra == 'recursive_extra_with_unmet_deps'",
+                'recursive_dep; extra == "recursive_extra_with_unmet_deps"',
                 'recursive_unmet_dep',
             ),
         ),
         ('extras_dep[extra_with_met_deps]', None),
         ('missing_dep; python_version>"10"', None),
         ('missing_dep; python_version<="1"', None),
-        ('missing_dep; python_version>="1"', ('missing_dep; python_version>="1"',)),
+        ('missing_dep; python_version>="1"', ('missing_dep; python_version >= "1"',)),
         ('extras_dep == 1.0.0', None),
-        ('extras_dep == 2.0.0', ('extras_dep == 2.0.0',)),
+        ('extras_dep == 2.0.0', ('extras_dep==2.0.0',)),
         ('extras_dep[extra_without_associated_deps] == 1.0.0', None),
-        ('extras_dep[extra_without_associated_deps] == 2.0.0', ('extras_dep[extra_without_associated_deps] == 2.0.0',)),
+        ('extras_dep[extra_without_associated_deps] == 2.0.0', ('extras_dep[extra_without_associated_deps]==2.0.0',)),
         ('prerelease_dep >= 1.0.0', None),
         ('circular_dep', None),
     ],
@@ -259,12 +259,12 @@ def test_check_dependencies(mocker, package_test_flit):
     builder._hook.get_requires_for_build_wheel.side_effect = copy.copy(side_effects)
 
     # requires = []
-    assert builder.check_dependencies('sdist') == {('flit_core >=2,<3',)}
-    assert builder.check_dependencies('wheel') == {('flit_core >=2,<3',)}
+    assert builder.check_dependencies('sdist') == {('flit_core<3,>=2',)}
+    assert builder.check_dependencies('wheel') == {('flit_core<3,>=2',)}
 
     # requires = ['something']
-    assert builder.check_dependencies('sdist') == {('flit_core >=2,<3',), ('something',)}
-    assert builder.check_dependencies('wheel') == {('flit_core >=2,<3',), ('something',)}
+    assert builder.check_dependencies('sdist') == {('flit_core<3,>=2',), ('something',)}
+    assert builder.check_dependencies('wheel') == {('flit_core<3,>=2',), ('something',)}
 
     # BackendUnavailable
     with pytest.raises(build.BuildBackendException):