repo: export GIT_TRACE2_PARENT_SID

This helps with people tracing repo/git execution.  We use a similar
format to git, but a little simpler since we always initialize the
env var setting, and we want to avoid too much overhead.

Bug: https://crbug.com/gerrit/12314
Change-Id: I75675b6cc4c6f7c4f5e09f54128eba9456364d04
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254331
Reviewed-by: Josh Steadmon <steadmon@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

repo

@@ -10,6 +10,7 @@ copy of repo in the checkout.
 
 from __future__ import print_function
 
+import datetime
 import os
 import platform
 import subprocess
@@ -478,6 +479,39 @@ def _CheckGitVersion():
     raise CloneFailure()
 
 
+def SetGitTrace2ParentSid(env=None):
+  """Set up GIT_TRACE2_PARENT_SID for git tracing."""
+  # We roughly follow the format git itself uses in trace2/tr2_sid.c.
+  # (1) Be unique (2) be valid filename (3) be fixed length.
+  #
+  # Since we always export this variable, we try to avoid more expensive calls.
+  # e.g. We don't attempt hostname lookups or hashing the results.
+  if env is None:
+    env = os.environ
+
+  KEY = 'GIT_TRACE2_PARENT_SID'
+
+  now = datetime.datetime.utcnow()
+  value = 'repo-%s-P%08x' % (now.strftime('%Y%m%dT%H%M%SZ'), os.getpid())
+
+  # If it's already set, then append ourselves.
+  if KEY in env:
+    value = env[KEY] + '/' + value
+
+  _setenv(KEY, value, env=env)
+
+
+def _setenv(key, value, env=None):
+  """Set |key| in the OS environment |env| to |value|."""
+  if env is None:
+    env = os.environ
+  # Environment handling across systems is messy.
+  try:
+    env[key] = value
+  except UnicodeEncodeError:
+    env[key] = value.encode()
+
+
 def NeedSetupGnuPG():
   if not os.path.isdir(home_dot_repo):
     return True
@@ -514,10 +548,7 @@ def SetupGnuPG(quiet):
       sys.exit(1)
 
   env = os.environ.copy()
-  try:
-    env['GNUPGHOME'] = gpg_dir
-  except UnicodeEncodeError:
-    env['GNUPGHOME'] = gpg_dir.encode()
+  _setenv('GNUPGHOME', gpg_dir, env)
 
   cmd = ['gpg', '--import']
   try:
@@ -723,10 +754,7 @@ def _Verify(cwd, branch, quiet):
       print(file=sys.stderr)
 
   env = os.environ.copy()
-  try:
-    env['GNUPGHOME'] = gpg_dir
-  except UnicodeEncodeError:
-    env['GNUPGHOME'] = gpg_dir.encode()
+  _setenv('GNUPGHOME', gpg_dir, env)
 
   cmd = [GIT, 'tag', '-v', cur]
   proc = subprocess.Popen(cmd,
@@ -901,6 +929,9 @@ def _SetDefaultsTo(gitdir):
 def main(orig_args):
   cmd, opt, args = _ParseArguments(orig_args)
 
+  # We run this early as we run some git commands ourselves.
+  SetGitTrace2ParentSid()
+
   repo_main, rel_repo_dir = None, None
   # Don't use the local repo copy, make sure to switch to the gitc client first.
   if cmd != 'gitc-init':