gh_wrap_errors.py: make sure all output is processed

The current select-based code to consume subprocess output on the fly
sometimes misses the end of the output. Rework it to consume mixed
stdout+stderr until the end of output instead.

utils/gh_wrap_errors.py

@@ -8,7 +8,6 @@ GitHub workflow commands.
 import argparse
 from os import path as P
 import re
-import select
 import subprocess
 import sys
 
@@ -21,56 +20,22 @@ msg_re = re.compile(r"(.*?):(\d+):(?:(d+)\:)?(.*)?$")
 
 p = subprocess.Popen(
     extra_args, universal_newlines=True,
-    stdout=subprocess.PIPE, stderr=subprocess.PIPE
+    stdin=subprocess.DEVNULL, stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
 )
 
+# If a line is a GNU style error message, emit a corresponding workflow
+# command and trigger an error status.
 exit_status = 0
-
-
-def emit_workflow_command_for_line(line):
-    """
-    If line is a GNU style error message, emit a corresponding workflow
-    command.
-    """
-    global exit_status
-    m = msg_re.match(line)
+for line in p.stdout:
+    m = msg_re.match(line.rstrip())
     if m:
         exit_status = 1
         file, line, _, msg = m.groups()
-        if file.startswith('/'):
+        if file.startswith("/"):
             file = P.relpath(file)
         print(f"::error file={file},line={line}::{msg}")
+    sys.stdout.write(line)
+    sys.stdout.flush()
 
-
-stdout = []
-stderr = []
-
-# Record the subprocess's stdout & stderr while still feeding them to this
-# process' out & err.
-while True:
-    descs, _, _ = select.select((p.stdout.fileno(), p.stderr.fileno()), [], [])
-
-    # Record stdout and stderr, while still forwarding them to this process'
-    # stdout and stderr.
-    for fd in descs:
-        if fd == p.stdout.fileno():
-            read = p.stdout.read()
-            sys.stdout.write(read)
-            stdout.append(read)
-        elif fd == p.stderr.fileno():
-            read = p.stderr.read()
-            sys.stderr.write(read)
-            stderr.append(read)
-
-    returncode = p.poll()
-    if returncode is not None:
-        # Process has terminated, check if the return code is non zero, and if
-        # it is, propagate to exit_status.
-        if returncode != 0:
-            exit_status = returncode
-        break
-
-for line in "".join(stdout + stderr).splitlines():
-    emit_workflow_command_for_line(line)
-
-sys.exit(exit_status)
+p.wait()
+sys.exit(p.returncode if exit_status == 0 else exit_status)