Append stats to the file

Author: AlexTereshenkov

src/python/pants/goal/stats_aggregator.py

@@ -4,9 +4,11 @@
 from __future__ import annotations
 
 import base64
+import datetime
 import logging
 from collections import Counter
 from dataclasses import dataclass
+from pathlib import Path
 from typing import Optional
 
 from pants.engine.internals.scheduler import Workunit
@@ -69,7 +71,7 @@ def _log_or_write_to_file(output_file: Optional[str], lines: list[str]) -> None:
     if lines:
         text = "\n".join(lines)
         if output_file:
-            with safe_open(output_file, "w") as fh:
+            with safe_open(output_file, "a") as fh:
                 fh.write(text)
             logger.info(f"Wrote Pants stats to {output_file}")
         else:
@@ -103,6 +105,13 @@ def __call__(
             return
 
         output_lines = []
+        if self.output_file:
+            timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
+            # have an empty line between stats of different Pants invocations
+            space = "\n\n" if Path(self.output_file).exists() else ""
+            output_lines.append(
+                f"{space}{timestamp} Executing goals: {','.join(context._run_tracker.goals)}"
+            )
 
         if self.log:
             # Capture global counters.

src/python/pants/goal/stats_aggregator_integration_test.py

@@ -59,6 +59,18 @@ def test_writing_to_output_file() -> None:
             "roots",
         ]
         run_pants(argv).assert_success()
+        argv = [
+            "--backend-packages=['pants.backend.python']",
+            "--stats-log",
+            "--stats-memory-summary",
+            "--stats-output-file=stats.txt",
+            "list",
+            "::",
+        ]
+        run_pants(argv).assert_success()
         output_file_contents = Path("stats.txt").read_text()
         for item in ("Counters:", "Memory summary"):
+            assert output_file_contents.count(item) == 2
+
+        for item in ("roots", "list"):
             assert item in output_file_contents