[PATCH 2 of 3] debugwireproto: dump server's stderr to temporary file if --noreadstderr

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1520858510 -32400
#      Mon Mar 12 21:41:50 2018 +0900
# Node ID d3dd691a3fce0c501a34ed68d1a08b563a78794c
# Parent  2e2a5376d006ad77ba9a07d341f6bc5418289af1
debugwireproto: dump server's stderr to temporary file if --noreadstderr

Otherwise the server process could stall because of writing too many data
to stderr.

No idea if this is strictly better than the original implementation, but
this allows dropping readstderr=False support from sshpeer.

diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -2716,7 +2716,9 @@ def debugwireproto(ui, repo, **opts):
 
     blocks = list(_parsewirelangblocks(ui.fin))
 
+    logging = (ui.verbose or opts['peer'] == 'raw')
     proc = None
+    stderrfname = None
 
     if opts['localssh']:
         # We start the SSH server in its own process so there is process
@@ -2726,27 +2728,34 @@ def debugwireproto(ui, repo, **opts):
             '-R', repo.root,
             'debugserve', '--sshstdio',
         ]
+        autoreadstderr = not opts['noreadstderr']
+        stderrfd = subprocess.PIPE
+        if not autoreadstderr:
+            # Dump stderr to file so the server process never stalls
+            stderrfd, stderrfname = tempfile.mkstemp(prefix='hg-debugserve-')
         proc = subprocess.Popen(args, stdin=subprocess.PIPE,
-                                stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+                                stdout=subprocess.PIPE, stderr=stderrfd,
                                 bufsize=0)
+        if stderrfd != subprocess.PIPE:
+            os.close(stderrfd)
 
         stdin = proc.stdin
         stdout = proc.stdout
-        stderr = proc.stderr
+        stderr = proc.stderr  # may be None
 
         # We turn the pipes into observers so we can log I/O.
-        if ui.verbose or opts['peer'] == 'raw':
+        if logging:
             stdin = util.makeloggingfileobject(ui, proc.stdin, b'i',
                                                logdata=True)
             stdout = util.makeloggingfileobject(ui, proc.stdout, b'o',
                                                 logdata=True)
+        if logging and stderr:
             stderr = util.makeloggingfileobject(ui, proc.stderr, b'e',
                                                 logdata=True)
 
         # --localssh also implies the peer connection settings.
 
         url = 'ssh://localserver'
-        autoreadstderr = not opts['noreadstderr']
 
         if opts['peer'] == 'ssh1':
             ui.write(_('creating ssh peer for wire protocol version 1\n'))
@@ -2838,7 +2847,8 @@ def debugwireproto(ui, repo, **opts):
         elif action == 'readavailable':
             stdin.close()
             stdout.read()
-            stderr.read()
+            if stderr:
+                stderr.read()
         elif action == 'readline':
             stdout.readline()
         else:
@@ -2852,3 +2862,10 @@ def debugwireproto(ui, repo, **opts):
 
     if proc:
         proc.kill()
+
+    if stderrfname:
+        with open(stderrfname, 'rb') as f:
+            if logging:
+                f = util.makeloggingfileobject(ui, f, b'e', logdata=True)
+            f.read()
+        os.unlink(stderrfname)