Fix --pip-log re-use.

Previously, when only 1 resolve target was specified, a `--pip-log`
would be re-used which would append the current Pip log outout with the
prior output. For a normal PEX build, this was potentially confusing,
but for a `pex3 lock` command it could lead to errors since the log is
used to generate the lock.

Fixes pex-tool#2568

pex/resolver.py

@@ -9,6 +9,7 @@
 import hashlib
 import itertools
 import os
+import shutil
 import zipfile
 from abc import abstractmethod
 from collections import OrderedDict, defaultdict
@@ -85,9 +86,7 @@ def create(
     ):
         # type: (...) -> PipLogManager
         log_by_target = {}  # type: Dict[Target, str]
-        if log and len(targets) == 1:
-            log_by_target[targets[0]] = log
-        elif log:
+        if log:
             log_dir = safe_mkdtemp(prefix="pex-pip-log.")
             log_by_target.update(
                 (target, os.path.join(log_dir, "pip.{target}.log".format(target=target.id)))
@@ -113,7 +112,12 @@ def _target_id(target):
     def finalize_log(self):
         # type: () -> None
         target_count = len(self._log_by_target)
-        if target_count <= 1:
+        if target_count == 0:
+            return
+
+        if self.log and target_count == 1:
+            log = next(iter(self._log_by_target.values()))
+            shutil.move(log, self.log)
             return
 
         with safe_open(self.log, "a") as out_fp:

tests/integration/resolve/test_issue_2568.py

@@ -0,0 +1,43 @@
+# Copyright 2024 Pex project contributors.
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import
+
+import re
+from textwrap import dedent
+
+from pex import targets
+from testing.cli import run_pex3
+from testing.pytest.tmp import Tempdir
+
+
+def test_independent_logs_for_independent_runs(tmpdir):
+    # type: (Tempdir) -> None
+
+    lock = tmpdir.join("lock.json")
+    log = tmpdir.join("pip.log")
+    run_pex3(
+        "lock", "sync", "--lock", lock, "--pip-log", log, "ansicolors==1.1.8", "cowsay==6.0"
+    ).assert_success()
+    target = str(targets.current().platform.tag)
+    run_pex3("lock", "sync", "--lock", lock, "--pip-log", log, "ansicolors==1.1.8").assert_success(
+        expected_error_re=r".*{footer}$".format(
+            footer=re.escape(
+                dedent(
+                    """\
+                    Updates for lock generated by {target}:
+                      Deleted cowsay 6
+                    Updates to lock input requirements:
+                      Deleted 'cowsay==6.0'
+                    """
+                ).format(target=target)
+            )
+        ),
+        re_flags=re.DOTALL,
+    )
+    run_pex3("lock", "sync", "--lock", lock, "--pip-log", log, "ansicolors==1.1.8").assert_success(
+        expected_error_re=r".*No updates for lock generated by {target}\.$".format(
+            target=re.escape(target)
+        ),
+        re_flags=re.DOTALL,
+    )