locker: handle cyclic dependencies during walk

Resolves: #3213

poetry/packages/locker.py

@@ -226,45 +226,43 @@ class Locker(object):
         next_level_dependencies = []
 
         for requirement in dependencies:
+            key = (requirement.name, requirement.pretty_constraint)
             locked_package = cls.__get_locked_package(requirement, packages_by_name)
 
             if locked_package:
-                for require in locked_package.requires:
-                    if require.marker.is_empty():
-                        require.marker = requirement.marker
-                    else:
-                        require.marker = require.marker.intersect(requirement.marker)
+                # create dependency from locked package to retain dependency metadata
+                # if this is not done, we can end-up with incorrect nested dependencies
+                marker = requirement.marker
+                requirement = locked_package.to_dependency()
+                requirement.marker = requirement.marker.intersect(marker)
+
+                key = (requirement.name, requirement.pretty_constraint)
+
+                if pinned_versions:
+                    requirement.set_constraint(
+                        locked_package.to_dependency().constraint
+                    )
+
+                if key not in nested_dependencies:
+                    for require in locked_package.requires:
+                        if require.marker.is_empty():
+                            require.marker = requirement.marker
+                        else:
+                            require.marker = require.marker.intersect(
+                                requirement.marker
+                            )
 
-                    require.marker = require.marker.intersect(locked_package.marker)
-                    next_level_dependencies.append(require)
+                        require.marker = require.marker.intersect(locked_package.marker)
+                        next_level_dependencies.append(require)
 
             if requirement.name in project_level_dependencies and level == 0:
                 # project level dependencies take precedence
                 continue
 
-            if locked_package:
-                # create dependency from locked package to retain dependency metadata
-                # if this is not done, we can end-up with incorrect nested dependencies
-                marker = requirement.marker
-                requirement = locked_package.to_dependency()
-                requirement.marker = requirement.marker.intersect(marker)
-            else:
+            if not locked_package:
                 # we make a copy to avoid any side-effects
                 requirement = deepcopy(requirement)
 
-            if pinned_versions:
-                requirement.set_constraint(
-                    cls.__get_locked_package(requirement, packages_by_name)
-                    .to_dependency()
-                    .constraint
-                )
-
-            # dependencies use extra to indicate that it was activated via parent
-            # package's extras, this is not required for nested exports as we assume
-            # the resolver already selected this dependency
-            requirement.marker = requirement.marker.without_extras()
-
-            key = (requirement.name, requirement.pretty_constraint)
             if key not in nested_dependencies:
                 nested_dependencies[key] = requirement
             else:

tests/utils/test_exporter.py

@@ -696,6 +696,62 @@ foo @ git+https://github.com/foo/foo.git@123456
     assert expected == content
 
 
+def test_exporter_can_export_requirements_txt_with_nested_packages_cyclic(
+    tmp_dir, poetry
+):
+    poetry.locker.mock_lock_data(
+        {
+            "package": [
+                {
+                    "name": "foo",
+                    "version": "1.2.3",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "dependencies": {"bar": {"version": "4.5.6"}},
+                },
+                {
+                    "name": "bar",
+                    "version": "4.5.6",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "dependencies": {"baz": {"version": "7.8.9"}},
+                },
+                {
+                    "name": "baz",
+                    "version": "7.8.9",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "dependencies": {"foo": {"version": "1.2.3"}},
+                },
+            ],
+            "metadata": {
+                "python-versions": "*",
+                "content-hash": "123456789",
+                "hashes": {"foo": [], "bar": [], "baz": []},
+            },
+        }
+    )
+    set_package_requires(poetry, skip={"bar", "baz"})
+
+    exporter = Exporter(poetry)
+
+    exporter.export("requirements.txt", Path(tmp_dir), "requirements.txt")
+
+    with (Path(tmp_dir) / "requirements.txt").open(encoding="utf-8") as f:
+        content = f.read()
+
+    expected = """\
+bar==4.5.6
+baz==7.8.9
+foo==1.2.3
+"""
+
+    assert expected == content
+
+
 def test_exporter_can_export_requirements_txt_with_git_packages_and_markers(
     tmp_dir, poetry
 ):