worker: flush ui buffers before running the worker

a91c6275 introduces flushing ui buffers after a worker finished. If the ui was
not flushed before the worker was started, fork will copy the existing buffers
to the worker. This causes messages issued before the worker started to be
written to the terminal for each worker.

We are now flushing the ui before we start a worker and add an appropriate test
which will fail before this patch.

tests/test-worker.t

@@ -0,0 +1,54 b''
+Test UI worker interaction
+
+  $ cat > t.py <<EOF
+  > from __future__ import absolute_import, print_function
+  > from mercurial import (
+  >     cmdutil,
+  >     ui as uimod,
+  >     worker,
+  > )
+  > def runme(ui, args):
+  >     for arg in args:
+  >         ui.status('run\n')
+  >         yield 1, arg
+  > cmdtable = {}
+  > command = cmdutil.command(cmdtable)
+  > @command('test', [], 'hg test [COST]')
+  > def t(ui, repo, cost=1.0):
+  >     cost = float(cost)
+  >     ui.status('start\n')
+  >     runs = worker.worker(ui, cost, runme, (ui,), range(8))
+  >     for n, i in runs:
+  >         pass
+  >     ui.status('done\n')
+  > EOF
+  $ abspath=`pwd`/t.py
+  $ hg init
+
+Run tests with worker enable by forcing a heigh cost
+
+  $ hg --config "extensions.t=$abspath" test 100000.0
+  start
+  run
+  run
+  run
+  run
+  run
+  run
+  run
+  run
+  done
+
+Run tests without worker by forcing a low cost
+
+  $ hg --config "extensions.t=$abspath" test 0.0000001
+  start
+  run
+  run
+  run
+  run
+  run
+  run
+  run
+  run
+  done

mercurial/worker.py

@@ -133,6 +133,7 b' def _posixworker(ui, func, staticargs, a'
         if problem[0]:
             killworkers()
     oldchldhandler = signal.signal(signal.SIGCHLD, sigchldhandler)
+    ui.flush()
     for pargs in partition(args, workers):
         pid = os.fork()
         if pid == 0: