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.

mercurial/server.py

@@ -30,6 +30,27 b' def runservice(opts, parentfn=None, init'
                runargs=None, appendpid=False):
     '''Run a command as a service.'''
 
+    # When daemonized on Windows, 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.  See 594dd384803c.  On
+    # other platforms, the child can write to the parent's stdio directly, until
+    # it is redirected prior to runfn().
+    if pycompat.iswindows and opts['daemon_postexec']:
+        for inst in opts['daemon_postexec']:
+            if inst.startswith('unlink:'):
+                lockpath = inst[7:]
+                if os.path.exists(lockpath):
+                    procutil.stdout.flush()
+                    procutil.stderr.flush()
+
+                    fd = os.open(lockpath,
+                                 os.O_WRONLY | os.O_APPEND | os.O_BINARY)
+                    try:
+                        os.dup2(fd, 1)
+                        os.dup2(fd, 2)
+                    finally:
+                        os.close(fd)
+
     def writepid(pid):
         if opts['pid_file']:
             if appendpid:
@@ -61,6 +82,12 b' 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 pycompat.iswindows and os.path.exists(lockpath):
+                    with open(lockpath) as log:
+                        for line in log:
+                            procutil.stderr.write(line)
                 raise error.Abort(_('child process failed to start'))
             writepid(pid)
         finally:
@@ -81,10 +108,11 b' 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':
@@ -107,6 +135,11 b' def runservice(opts, parentfn=None, init'
         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.
+        if lockpath:
+            os.unlink(lockpath)
+
     if runfn:
         return runfn()