locker: propagate cumulative markers to nested deps

This change ensures that markers are propagated from top level
dependencies to the deepest level by walking top to bottom instead of
iterating over all available packages.

In addition, we also compress any dependencies with the same name and
constraint to provide a more concise representation.

Resolves: #3112

poetry/packages/locker.py

@@ -226,19 +226,40 @@ def __get_locked_package(
             # return only with project level dependencies
             return dependencies
 
-        nested_dependencies = list()
+        nested_dependencies = dict()
+
+        def __walk_level(__dependencies):  # type: (List[Dependency]) -> None
+            if not __dependencies:
+                return
+
+            _next_level = []
+
+            for requirement in __dependencies:
+                __locked_package = __get_locked_package(requirement)
+
+                if __locked_package:
+                    for require in __locked_package.requires:
+                        if not require.marker.is_empty():
+                            require.marker = require.marker.intersect(
+                                requirement.marker
+                            )
+                        else:
+                            require.marker = requirement.marker
+                        require.marker = require.marker.intersect(
+                            __locked_package.marker
+                        )
+                        _next_level.append(require)
 
-        for pkg in packages:  # type: Package
-            for requirement in pkg.requires:  # type: Dependency
                 if requirement.name in project_level_dependencies:
                     # project level dependencies take precedence
                     continue
 
-                locked_package = __get_locked_package(requirement)
-                if locked_package:
+                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
-                    requirement = locked_package.to_dependency()
+                    marker = requirement.marker
+                    requirement = __locked_package.to_dependency()
+                    requirement.marker = requirement.marker.intersect(marker)
                 else:
                     # we make a copy to avoid any side-effects
                     requirement = deepcopy(requirement)
@@ -266,11 +287,20 @@ def __get_locked_package(
                     # this dependency was not from a project requirement
                     requirement.marker = marker.intersect(pkg.marker)
 
-                if requirement not in nested_dependencies:
-                    nested_dependencies.append(requirement)
+                key = (requirement.name, requirement.pretty_constraint)
+                if key not in nested_dependencies:
+                    nested_dependencies[key] = requirement
+                else:
+                    nested_dependencies[key].marker = nested_dependencies[
+                        key
+                    ].marker.intersect(requirement.marker)
+
+            return __walk_level(_next_level)
+
+        __walk_level(dependencies)
 
         return sorted(
-            itertools.chain(dependencies, nested_dependencies),
+            itertools.chain(dependencies, nested_dependencies.values()),
             key=lambda x: x.name.lower(),
         )
 

tests/utils/test_exporter.py

@@ -2,6 +2,7 @@
 
 import pytest
 
+from poetry.core.packages import dependency_from_pep_508
 from poetry.core.toml.file import TOMLFile
 from poetry.factory import Factory
 from poetry.packages import Locker as BaseLocker
@@ -175,6 +176,86 @@ def test_exporter_can_export_requirements_txt_with_standard_packages_and_markers
     assert expected == content
 
 
+def test_exporter_can_export_requirements_txt_with_nested_packages_and_markers(
+    tmp_dir, poetry
+):
+    poetry.locker.mock_lock_data(
+        {
+            "package": [
+                {
+                    "name": "a",
+                    "version": "1.2.3",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "marker": "python_version < '3.7'",
+                    "dependencies": {"b": ">=0.0.0", "c": ">=0.0.0"},
+                },
+                {
+                    "name": "b",
+                    "version": "4.5.6",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "marker": "platform_system == 'Windows'",
+                    "dependencies": {"d": ">=0.0.0"},
+                },
+                {
+                    "name": "c",
+                    "version": "7.8.9",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                    "marker": "sys_platform == 'win32'",
+                    "dependencies": {"d": ">=0.0.0"},
+                },
+                {
+                    "name": "d",
+                    "version": "0.0.1",
+                    "category": "main",
+                    "optional": False,
+                    "python-versions": "*",
+                },
+            ],
+            "metadata": {
+                "python-versions": "*",
+                "content-hash": "123456789",
+                "hashes": {"a": [], "b": [], "c": [], "d": []},
+            },
+        }
+    )
+    set_package_requires(poetry, skip={"b", "c", "d"})
+
+    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 = {
+        "a": dependency_from_pep_508("a==1.2.3; python_version < '3.7'"),
+        "b": dependency_from_pep_508(
+            "b==4.5.6; platform_system == 'Windows' and python_version < '3.7'"
+        ),
+        "c": dependency_from_pep_508(
+            "c==7.8.9; sys_platform == 'win32' and python_version < '3.7'"
+        ),
+        "d": dependency_from_pep_508(
+            "d==0.0.1; python_version < '3.7' and platform_system == 'Windows' and sys_platform == 'win32'"
+        ),
+    }
+
+    for line in content.strip().split("\n"):
+        dependency = dependency_from_pep_508(line)
+        assert dependency.name in expected
+        expected_dependency = expected.pop(dependency.name)
+        assert dependency == expected_dependency
+        assert dependency.marker == expected_dependency.marker
+
+    assert expected == {}
+
+
 def test_exporter_can_export_requirements_txt_with_standard_packages_and_hashes(
     tmp_dir, poetry
 ):