locker: enable poetry lock if an incompatible lock file exists

src/poetry/installation/installer.py

@@ -218,7 +218,7 @@ def _do_install(self) -> int:
 
         locked_repository = Repository("poetry-locked")
         if self._update:
-            if self._locker.is_locked() and not self._lock:
+            if not self._lock and self._locker.is_locked():
                 locked_repository = self._locker.locked_repository()
 
                 # If no packages have been whitelisted (The ones we want to update),

src/poetry/packages/locker.py

@@ -73,10 +73,7 @@ def is_locked(self) -> bool:
         """
         Checks whether the locker has been locked (lockfile found).
         """
-        if not self._lock.exists():
-            return False
-
-        return "package" in self.lock_data
+        return self._lock.exists()
 
     def is_fresh(self) -> bool:
         """
@@ -256,12 +253,18 @@ def set_lock_data(self, root: Package, packages: list[Package]) -> bool:
             "content-hash": self._content_hash,
         }
 
-        if not self.is_locked() or lock != self.lock_data:
+        do_write = True
+        if self.is_locked():
+            try:
+                lock_data = self.lock_data
+            except RuntimeError:
+                # incompatible, invalid or no lock file
+                pass
+            else:
+                do_write = lock != lock_data
+        if do_write:
             self._write_lock_data(lock)
-
-            return True
-
-        return False
+        return do_write
 
     def _write_lock_data(self, data: TOMLDocument) -> None:
         self.lock.write(data)

tests/console/commands/test_lock.py

@@ -67,6 +67,13 @@ def poetry_with_old_lockfile(
     return _project_factory("old_lock", project_factory, fixture_dir)
 
 
+@pytest.fixture
+def poetry_with_incompatible_lockfile(
+    project_factory: ProjectFactory, fixture_dir: FixtureDirGetter
+) -> Poetry:
+    return _project_factory("incompatible_lock", project_factory, fixture_dir)
+
+
 def test_lock_check_outdated(
     command_tester_factory: CommandTesterFactory,
     poetry_with_outdated_lockfile: Poetry,
@@ -150,3 +157,34 @@ def test_lock_no_update(
 
     for package in packages:
         assert locked_repository.find_packages(package.to_dependency())
+
+
+@pytest.mark.parametrize("is_no_update", [False, True])
+def test_lock_with_incompatible_lockfile(
+    command_tester_factory: CommandTesterFactory,
+    poetry_with_incompatible_lockfile: Poetry,
+    repo: TestRepository,
+    is_no_update: bool,
+) -> None:
+    repo.add_package(get_package("sampleproject", "1.3.1"))
+
+    locker = Locker(
+        lock=poetry_with_incompatible_lockfile.pyproject.file.path.parent
+        / "poetry.lock",
+        local_config=poetry_with_incompatible_lockfile.locker._local_config,
+    )
+    poetry_with_incompatible_lockfile.set_locker(locker)
+
+    tester = command_tester_factory("lock", poetry=poetry_with_incompatible_lockfile)
+    if is_no_update:
+        # not possible because of incompatible lock file
+        expected = (
+            "(?s)lock file is not compatible .*"
+            " regenerate the lock file with the `poetry lock` command"
+        )
+        with pytest.raises(RuntimeError, match=expected):
+            tester.execute("--no-update")
+    else:
+        # still possible because lock file is not required
+        status_code = tester.execute()
+        assert status_code == 0

tests/fixtures/incompatible_lock/pyproject.toml

@@ -0,0 +1,15 @@
+[tool.poetry]
+name = "foobar"
+version = "0.1.0"
+description = ""
+authors = ["Poetry Developer <developer@python-poetry.org>"]
+
+[tool.poetry.dependencies]
+python = "^3.8"
+sampleproject = ">=1.3.1"
+
+[tool.poetry.dev-dependencies]
+
+[build-system]
+requires = ["poetry-core>=1.0.0"]
+build-backend = "poetry.core.masonry.api"