[PATCH 1 of 2] server: add an error feedback mechanism for when the daemon fails to launch

Matt Harbison mharbison72 at gmail.com
Thu Mar 29 02:45:30 UTC 2018


# HG changeset patch
# User Matt Harbison <matt_harbison at yahoo.com>
# Date 1522210269 14400
#      Wed Mar 28 00:11:09 2018 -0400
# Node ID 7a6eccc3b43dd8f6090b23aa38b2f71d33f5bbdc
# Parent  8bac14ce57781d2a45b6d036b3614b1a21917f0d
server: add an error feedback mechanism for when the daemon fails to launch

There's a recurring problem on Windows where `hg serve -d` will randomly fail to
spawn a detached process.  The reason for the failure is completely hidden, and
it takes hours to get a single failure on my laptop.  All this does is redirect
stdout/stderr of the child to a file until the lock file is freed, and then the
parent dumps it out if it fails to spawn.

I chose to put the output into the lock file because that is always cleaned up.
There's no way to report errors after that anyway.  On Windows, killdaemons.py
is roughly `kill -9`, so this ensures that junk won't pile up.

This may end up being a case of EADDRINUSE.  At least that's what I saw spit out
a few times (among other odd errors and missing output on Windows).  But I also
managed to get the same thing on Fedora 26 by running test-hgwebdir.t with
--loop -j10 for several hours.  Running `netstat` immediately after killing that
run printed a wall of sockets in the TIME_WAIT state, which were gone a couple
seconds later.  I couldn't match up ports that failed, because --loop doesn't
print out the message about the port that was used.  So maybe the fix is to
rotate the use of HGPORT[12] in the tests.  But, let's collect some more data
first.

diff --git a/mercurial/server.py b/mercurial/server.py
--- a/mercurial/server.py
+++ b/mercurial/server.py
@@ -30,6 +30,20 @@ def runservice(opts, parentfn=None, init
                runargs=None, appendpid=False):
     '''Run a command as a service.'''
 
+    # When daemonized, redirect stdout/stderr to the lockfile (which gets
+    # cleaned up after the child is up and running), so that the parent can read
+    # and print the error if this child dies early.
+    if opts['daemon_postexec']:
+        for inst in opts['daemon_postexec']:
+            if inst.startswith('unlink:'):
+                lockpath = inst[7:]
+                procutil.stdout.flush()
+                procutil.stderr.flush()
+                logfilefp = open(lockpath, 'w+')
+                os.dup2(logfilefp.fileno(), 1)
+                os.dup2(logfilefp.fileno(), 2)
+                logfilefp.close()
+
     def writepid(pid):
         if opts['pid_file']:
             if appendpid:
@@ -47,6 +61,8 @@ def runservice(opts, parentfn=None, init
         try:
             if not runargs:
                 runargs = procutil.hgcmd() + pycompat.sysargv[1:]
+                # Only hg processes accept --traceback (e.g., not dumbhttp.py)
+                runargs.append('--traceback')
             runargs.append('--daemon-postexec=unlink:%s' % lockpath)
             # Don't pass --cwd to the child process, because we've already
             # changed directory.
@@ -61,6 +77,12 @@ def runservice(opts, parentfn=None, init
                 return not os.path.exists(lockpath)
             pid = procutil.rundetached(runargs, condfn)
             if pid < 0:
+                # If the daemonized process managed to write out an error msg,
+                # report it.
+                if os.path.exists(lockpath):
+                    with open(lockpath) as log:
+                        for line in log:
+                            procutil.stderr.write('server: %s' % line)
                 raise error.Abort(_('child process failed to start'))
             writepid(pid)
         finally:
@@ -81,31 +103,39 @@ def runservice(opts, parentfn=None, init
             os.setsid()
         except AttributeError:
             pass
+
+        lockpath = None
         for inst in opts['daemon_postexec']:
             if inst.startswith('unlink:'):
                 lockpath = inst[7:]
-                os.unlink(lockpath)
             elif inst.startswith('chdir:'):
                 os.chdir(inst[6:])
             elif inst != 'none':
                 raise error.Abort(_('invalid value for --daemon-postexec: %s')
                                   % inst)
-        procutil.hidewindow()
-        procutil.stdout.flush()
-        procutil.stderr.flush()
+
+        if lockpath:
+            procutil.hidewindow()
+            procutil.stdout.flush()
+            procutil.stderr.flush()
 
-        nullfd = os.open(os.devnull, os.O_RDWR)
-        logfilefd = nullfd
-        if logfile:
-            logfilefd = os.open(logfile, os.O_RDWR | os.O_CREAT | os.O_APPEND,
-                                0o666)
-        os.dup2(nullfd, 0)
-        os.dup2(logfilefd, 1)
-        os.dup2(logfilefd, 2)
-        if nullfd not in (0, 1, 2):
-            os.close(nullfd)
-        if logfile and logfilefd not in (0, 1, 2):
-            os.close(logfilefd)
+            nullfd = os.open(os.devnull, os.O_RDWR)
+            logfilefd = nullfd
+            if logfile:
+                logfilefd = os.open(logfile,
+                                    os.O_RDWR | os.O_CREAT | os.O_APPEND,
+                                    0o666)
+            os.dup2(nullfd, 0)
+            os.dup2(logfilefd, 1)
+            os.dup2(logfilefd, 2)
+            if nullfd not in (0, 1, 2):
+                os.close(nullfd)
+            if logfile and logfilefd not in (0, 1, 2):
+                os.close(logfilefd)
+
+            # Only unlink after redirecting stdout/stderr, so Windows doesn't
+            # complain about a sharing violation.
+            os.unlink(lockpath)
 
     if runfn:
         return runfn()
diff --git a/tests/test-serve.t b/tests/test-serve.t
--- a/tests/test-serve.t
+++ b/tests/test-serve.t
@@ -47,8 +47,8 @@ With -v and -p daytime (should fail beca
 
 #if no-root no-windows
   $ KILLQUIETLY=Y
-  $ hgserve -p daytime
-  abort: cannot start server at 'localhost:13': Permission denied
+  $ hgserve -p daytime 2>&1 | egrep -v '^server: (Traceback|Abort|  )'
+  server: abort: cannot start server at 'localhost:13': Permission denied
   abort: child process failed to start
   % errors
   $ KILLQUIETLY=N


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