Ignore malformed lockfiles in generate-lockfiles

docs/notes/2.34.x.md

@@ -111,6 +111,8 @@ Support for creating multiplatform/foreign platform pexes when using the `uv` re
 
 Fixed a bug that caused `generate-lockfiles --sync` not to have its intended effect when using `uv` lockfiles.
 
+`generate-lockfiles` now overwrites an existing lockfile that fails to parse, rather than failing with a `TOMLDecodeError`.
+
 Fixed a bug that could cause tool execution to fail when resolving the tool from a `uv` lockfile.
 
 Required named indexes to be referenced explicitly in requirement sources when using the uv resolver, to match the pex resolver's behavior.

src/python/pants/backend/python/goals/lockfile.py

@@ -5,6 +5,7 @@
 
 import itertools
 import os.path
+import tomllib
 from collections import defaultdict
 from dataclasses import dataclass
 from operator import itemgetter
@@ -403,9 +404,9 @@ async def generate_uv_lockfile(
                     raise ValueError(
                         f"Expected lockfile entry to be a FileEntry but was a {type(repo_lock_entry)}"
                     )
-        except IntrinsicError:
-            # May fail if the file doesn't exist, which is expected the first time a new lockfile
-            # is generated.
+        except (IntrinsicError, tomllib.TOMLDecodeError):
+            # May fail if the file doesn't exist or is malformed, which is expected the first time
+            # a new lockfile is generated or when the existing one needs to be overwritten.
             pass
 
     uv_config = resolve_config.uv_config()

src/python/pants/backend/python/goals/lockfile_test.py

@@ -912,3 +912,35 @@ def generate_and_get_versions(reqs: FrozenOrderedSet[str], sync: bool) -> dict[s
     # Regenerate with --sync.
     versions = generate_and_get_versions(updated_requirements, sync=True)
     assert {"ansicolors": "1.1.8", "cowsay": "6.0"} == versions
+
+
+def test_uv_lockfile_generation_ignores_malformed_existing_lockfile(
+    rule_runner: PythonRuleRunner,
+) -> None:
+    # Regression test for https://github.com/pantsbuild/pants/issues/23706.
+    rule_runner.set_options(
+        [
+            "--python-resolves={'test': 'test.lock'}",
+            "--python-separate-lockfile-metadata-file",
+            "--generate-lockfiles-sync",
+        ],
+        env_inherit=PYTHON_BOOTSTRAP_ENV,
+    )
+    req = GenerateUvLockfile(
+        requirements=FrozenOrderedSet(["ansicolors==1.1.8"]),
+        find_links=FrozenOrderedSet([]),
+        interpreter_constraints=InterpreterConstraints(["CPython==3.14.*"]),
+        resolve_name="test",
+        lockfile_dest="test.lock",
+        diff=True,
+    )
+
+    rule_runner.write_digest(rule_runner.request(GenerateLockfileResult, [req]).digest)
+    rule_runner.write_files({"test.lock": "not valid toml [[["})
+
+    result = rule_runner.request(GenerateLockfileResult, [req])
+    digest_contents = rule_runner.request(DigestContents, [result.digest])
+    by_path = {fc.path: fc for fc in digest_contents}
+    lock_data = tomllib.loads(by_path["test.lock"].content.decode())
+    packages = {pkg["name"]: pkg["version"] for pkg in lock_data.get("package", [])}
+    assert packages["ansicolors"] == "1.1.8"

src/python/pants/backend/python/util_rules/lockfile_diff.py

@@ -143,9 +143,9 @@ async def _generate_lockfile_diff(
         old_content_entries = await get_digest_contents(loaded.lockfile_digest)
         old_content = next(iter(old_content_entries)).content
         old_packages = _parse_lockfile_packages(old_content, loaded.lockfile_format, path)
-    except EngineError:
-        # May fail if the file doesn't exist, which is expected the first time a new lockfile
-        # is generated.
+    except (EngineError, tomllib.TOMLDecodeError):
+        # May fail if the file doesn't exist or is malformed, which is expected the first time a
+        # new lockfile is generated or when the existing one needs to be overwritten.
         pass
 
     return LockfileDiff.create(