Don't clear lock link database during analysis.

pex/pip/tool.py

@@ -415,8 +415,6 @@ def analyze(self, line):
 
                     # VCS requirements are satisfied by a singular source; so we need not consult
                     # links collected in this round.
-                    self._links.clear()
-
                     self._resolved_requirements.append(
                         ResolvedRequirement(
                             requirement=requirement,
@@ -467,7 +465,6 @@ def analyze(self, line):
 
                 additional_artifacts = self._links[project_name_and_version]
                 additional_artifacts.discard(partial_artifact)
-                self._links.clear()
 
                 self._resolved_requirements.append(
                     ResolvedRequirement(

tests/integration/cli/commands/test_issue_1711.py

@@ -0,0 +1,114 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+import os
+
+from pex.cli.testing import run_pex3
+from pex.compatibility import PY3
+from pex.pep_440 import Version
+from pex.pep_503 import ProjectName
+from pex.resolve import lockfile
+from pex.resolve.locked_resolve import Artifact, LockedRequirement
+from pex.resolve.resolved_requirement import Fingerprint
+from pex.testing import run_pex_command
+from pex.typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from typing import Any
+
+
+def pypi_artifact(
+    hash,  # type: str
+    path,  # type: str
+):
+    # type: (...) -> Artifact
+    return Artifact.from_url(
+        url="https://files.pythonhosted.org/packages/{}".format(path),
+        fingerprint=Fingerprint(algorithm="sha256", hash=hash),
+    )
+
+
+def test_backtrack_links_preserved(tmpdir):
+    # type: (Any) -> None
+
+    lock = os.path.join(str(tmpdir), "lock")
+    create_lock_args = [
+        "lock",
+        "create",
+        "--resolver-version",
+        "pip-2020-resolver",
+        "--style",
+        "universal",
+        "--interpreter-constraint",
+        ">=3.7,<3.10",
+        "psutil",
+        "psutil<5.5",  # force a back-track
+        "-o",
+        lock,
+        "--indent",
+        "2",
+    ]
+
+    def lock_as_json():
+        with open(lock) as fp:
+            return fp.read()
+
+    def assert_psutil_basics():
+        # type: () -> LockedRequirement
+        lock_file = lockfile.load(lock)
+        assert 1 == len(
+            lock_file.locked_resolves
+        ), "Expected 1 resolve for universal style:\n{json}".format(json=lock_as_json())
+        locked_resolve = lock_file.locked_resolves[0]
+
+        locked_requirements_by_project_name = {
+            locked_requirement.pin.project_name: locked_requirement
+            for locked_requirement in locked_resolve.locked_requirements
+        }
+        psutil = locked_requirements_by_project_name.get(ProjectName("psutil"))
+        assert psutil is not None, "Expected lock to resolve psutil:\n{json}".format(
+            json=lock_as_json()
+        )
+        assert Version("5.4.8") == psutil.pin.version, (
+            "Expected lock to resolve psutil to <5.5 due to the second requirement but otherwise "
+            "as high as possible, which should be 5.4.8 but was: {version}\n{json}".format(
+                version=psutil.pin.version, json=lock_as_json()
+            )
+        )
+        return psutil
+
+    # 1st prove this does the wrong thing on prior broken versions of Pex.
+    # N.B.: For some reason, this works with old Pex under Python 2.7; i.e.: It appears Pip behaves
+    # differently - likely because of some collection implementation difference.
+    if PY3:
+        run_pex_command(
+            args=["pex==2.1.77", "-c", "pex3", "--"] + create_lock_args
+        ).assert_success()
+        psutil_old = assert_psutil_basics()
+        assert 0 == len(psutil_old.additional_artifacts), (
+            "Expected old versions of Pex to incorrectly wipe out the additional artifacts "
+            "database when backtracking needs to retrieve saved links later:\n{json}".format(
+                json=lock_as_json()
+            )
+        )
+
+    # Now show it currently works.
+    run_pex3(*create_lock_args).assert_success()
+    psutil_current = assert_psutil_basics()
+    assert {
+        pypi_artifact(
+            hash="1c71b9716790e202a00ab0931a6d1e25db1aa1198bcacaea2f5329f75d257fff",
+            path="50/00/ae52663b879333aa5c65fc9a87ddc24169f8fdd1831762a1ba9c9be7740d/psutil-5.4.8-cp37-cp37m-win_amd64.whl",
+        ),
+        pypi_artifact(
+            hash="bfcea4f189177b2d2ce4a34b03c4ac32c5b4c22e21f5b093d9d315e6e253cd81",
+            path="21/1e/fe6731e5f03ddf2e57d5b307f25bba294262bc88e27a0fbefdb3515d1727/psutil-5.4.8-cp37-cp37m-win32.whl",
+        ),
+        pypi_artifact(
+            hash="6e265c8f3da00b015d24b842bfeb111f856b13d24f2c57036582568dc650d6c3",
+            path="e3/58/0eae6e4466e5abf779d7e2b71fac7fba5f59e00ea36ddb3ed690419ccb0f/psutil-5.4.8.tar.gz",
+        ),
+    } == set(psutil_current.iter_artifacts()), (
+        "Expected a full set of artifacts even after the lock resolve backtracked from "
+        "psutil latest to psutil<5.5 before settling:\n{json}".format(json=lock_as_json())
+    )