Set tracing to always on and save to .repo/TRACE_FILE.

- add `--trace_to_stderr` option so stderr will include trace outputs and any other errors that get sent to stderr
- while TRACE_FILE will only include trace outputs

piggy-backing on: https://gerrit-review.googlesource.com/c/git-repo/+/349154

Change-Id: I3895a84de4b2784f17fac4325521cd5e72e645e2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/350114
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Joanna Wang <jojwang@google.com>

repo_trace.py

@@ -15,26 +15,128 @@
 """Logic for tracing repo interactions.
 
 Activated via `repo --trace ...` or `REPO_TRACE=1 repo ...`.
+
+Temporary: Tracing is always on. Set `REPO_TRACE=0` to turn off.
+To also include trace outputs in stderr do `repo --trace_to_stderr ...`
 """
 
 import sys
 import os
+import tempfile
+import time
+from contextlib import ContextDecorator
 
 # Env var to implicitly turn on tracing.
 REPO_TRACE = 'REPO_TRACE'
 
-_TRACE = os.environ.get(REPO_TRACE) == '1'
+# Temporarily set tracing to always on unless user expicitly sets to 0.
+_TRACE = os.environ.get(REPO_TRACE) != '0'
+
+_TRACE_TO_STDERR = False
+
+_TRACE_FILE = None
+
+_TRACE_FILE_NAME = 'TRACE_FILE'
+
+_MAX_SIZE = 5  # in mb
+
+_NEW_COMMAND_SEP = '+++++++++++++++NEW COMMAND+++++++++++++++++++'
+
+
+def IsStraceToStderr():
+  return _TRACE_TO_STDERR
 
 
 def IsTrace():
   return _TRACE
 
 
+def SetTraceToStderr():
+  global _TRACE_TO_STDERR
+  _TRACE_TO_STDERR = True
+
+
 def SetTrace():
   global _TRACE
   _TRACE = True
 
 
-def Trace(fmt, *args):
-  if IsTrace():
-    print(fmt % args, file=sys.stderr)
+def _SetTraceFile():
+  global _TRACE_FILE
+  _TRACE_FILE = _GetTraceFile()
+
+
+class Trace(ContextDecorator):
+
+    def _time(self):
+      """Generate nanoseconds of time in a py3.6 safe way"""
+      return int(time.time()*1e+9)
+
+    def __init__(self, fmt, *args, first_trace=False):
+      if not IsTrace():
+        return
+      self._trace_msg = fmt % args
+
+      if not _TRACE_FILE:
+        _SetTraceFile()
+
+      if first_trace:
+        _ClearOldTraces()
+        self._trace_msg = '%s %s' % (_NEW_COMMAND_SEP, self._trace_msg)
+
+
+    def __enter__(self):
+      if not IsTrace():
+        return self
+
+      print_msg = f"PID: {os.getpid()} START: {self._time()} :" + self._trace_msg + '\n'
+
+      with open(_TRACE_FILE, 'a') as f:
+        print(print_msg, file=f)
+
+      if _TRACE_TO_STDERR:
+        print(print_msg, file=sys.stderr)
+
+      return self
+
+    def __exit__(self, *exc):
+      if not IsTrace():
+        return False
+
+      print_msg = f"PID: {os.getpid()} END: {self._time()} :" + self._trace_msg + '\n'
+
+      with open(_TRACE_FILE, 'a') as f:
+        print(print_msg, file=f)
+
+      if _TRACE_TO_STDERR:
+        print(print_msg, file=sys.stderr)
+
+      return False
+
+
+def _GetTraceFile():
+  """Get the trace file or create one."""
+  # TODO: refactor to pass repodir to Trace.
+  repo_dir = os.path.dirname(os.path.dirname(__file__))
+  trace_file = os.path.join(repo_dir, _TRACE_FILE_NAME)
+  print('Trace outputs in %s' % trace_file)
+  return trace_file
+
+def _ClearOldTraces():
+  """Clear traces from old commands if trace file is too big.
+
+  Note: If the trace file contains output from two `repo`
+        commands that were running at the same time, this
+        will not work precisely.
+  """
+  if os.path.isfile(_TRACE_FILE):
+    while os.path.getsize(_TRACE_FILE)/(1024*1024) > _MAX_SIZE:
+      temp = tempfile.NamedTemporaryFile(mode='w', delete=False)
+      with open(_TRACE_FILE, 'r', errors='ignore') as fin:
+        trace_lines = fin.readlines()
+        for i , l in enumerate(trace_lines):
+          if 'END:' in l and _NEW_COMMAND_SEP in l:
+            temp.writelines(trace_lines[i+1:])
+            break
+      temp.close()
+      os.replace(temp.name, _TRACE_FILE)