Hand the pre-push hook's terminal to edg-check-policy [GH #164]

git supplies the refs being pushed on a pre-push hook's standard input,
so the checker cannot inherit that stream and still read the user's
answers to the interactive prompts.  Reopening sys.stdin inside the
checker rebound only the Python level object and left file descriptor 0
pointing at git's pipe, and it also required a general purpose tool to
know how git invokes hooks.  Open the terminal in the hook instead and
pass it as the child's standard input, which fixes the descriptor for the
whole process tree and keeps that knowledge in the one place that needs
it.

Pushing with no terminal attached previously raised an unhandled OSError
from open('/dev/tty').  The hook now falls back to an empty standard
input, and an unanswerable prompt leaves the finding standing, so such a
push reports its findings instead of failing with a traceback.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: sdarwin

.git-hooks/pre-push.py

@@ -66,6 +66,25 @@
 
 TOOL_PATH_CHECK_POLICY = shutil.which('edg-check-policy')
 
+def terminal_input():
+  '''Open the terminal to hand to the checker as its standard input.
+
+  git supplies the refs being pushed on this hook's stdin, so the checker
+  cannot simply inherit it and still read the user's answers.  On Windows
+  the relaunch above has already arranged a standalone console, whose stdin
+  is usable as it is, so that is inherited instead.
+  '''
+  if platform.system() == 'Windows':
+    return None
+
+  try:
+    return open('/dev/tty')
+  except OSError:
+    # Pushing with no terminal attached, so there is nobody to prompt.  The
+    # checker reports its findings without asking rather than reading the
+    # refs above as if they were answers.
+    return subprocess.DEVNULL
+
 def main():
   if TOOL_PATH_CHECK_POLICY is None:
     print(
@@ -75,11 +94,14 @@ def main():
     )
     sys.exit(1)
 
-  completed_process = subprocess.run([
-    TOOL_PATH_CHECK_POLICY,
-    '--unpushed',
-    '--interactive'
-  ])
+  completed_process = subprocess.run(
+    [
+      TOOL_PATH_CHECK_POLICY,
+      '--unpushed',
+      '--interactive'
+    ],
+    stdin = terminal_input()
+  )
 
   sys.exit(completed_process.returncode)
 

dev_tools/bin/edg-check-policy

@@ -17,7 +17,6 @@ interactive review.
 import argparse
 import json
 import os
-import platform
 import re
 import shutil
 import subprocess
@@ -274,15 +273,24 @@ class PolicyReporter:
     '''Ask the user whether a policy warning is acceptable.
 
     When stdout is a terminal, prompt on stdin for y/n/q.  Otherwise record
-    an error without prompting (e.g. non-interactive pushes).
+    an error without prompting (e.g. non-interactive pushes).  Supplying a
+    stdin that can be read is the caller's job: git hands a hook the refs
+    being pushed on stdin, so the pre-push hook opens the terminal itself
+    and passes that to this tool instead.
     '''
     if not sys.stdout.isatty():
       self.errors += 1
       return
 
     tries = 0
     while True:
-      response = input('    Okay? [ynq] ').lower()
+      try:
+        response = input('    Okay? [ynq] ').lower()
+      except EOFError:
+        # There is nothing to answer with, so let the finding stand.
+        print()
+        self.errors += 1
+        return
 
       if response == 'y':
         return
@@ -719,23 +727,9 @@ def parse_args(argv: Optional[List[str]] = None) -> argparse.Namespace:
   )
   return parser.parse_args(argv)
 
-def _reopen_terminal_input() -> None:
-  '''Point stdin at the terminal so findings can be reviewed.
-
-  A hook is handed the list of refs being pushed on stdin, so interactive
-  review has to read the user's answers from the terminal instead.  On
-  Windows the wrapper has already arranged for a console, whose stdin is
-  usable as it is.
-  '''
-  if platform.system() != 'Windows':
-    sys.stdin = open('/dev/tty')
-
 def main() -> None:
   args = parse_args()
 
-  if args.interactive:
-    _reopen_terminal_input()
-
   repo_dir = edgutil.find_mono_repo_or_exit()
   repository = GitRepository(repo_dir)
   reporter = PolicyReporter(