Fix delayed deadlock when transcript thread dies (#1)

Test plan:

- mount a small tmpfs with 'sudo mount -t tmpfs none /mnt -o size=4096'

- create a zdaemon config file
    <runner>
      program yes
      transcript /mnt/transcript.log
    </runner>

- run zdaemon -C conf start

- wait a few milliseconds for /mnt to fill up

- pgrep yes

If the 'yes' program is still running, we have a deadlock (strace and
you'll find it blocked on write()).  This is the situation before this
patch, as described in bug #1.

If the 'yes' program is dead, the deadlock is fixed.

- run zdaemon -C conf status

The daemon manager should be stopped (it's not functional without the
dead transcript thread).

Automating this test is left as an exercise for the reader.  :(

CHANGES.rst

@@ -25,6 +25,9 @@ Change log
   manager was waiting before respawning a crashed program.
   https://github.com/zopefoundation/zdaemon/issues/13.
 
+- Fix delayed deadlock when the transcript file runs into a full disk
+  (https://github.com/zopefoundation/zdaemon/issues/1).
+
 
 4.0.1 (2014-12-26)
 ==================

src/zdaemon/zdrun.py

@@ -625,14 +625,20 @@ def __init__(self, filename):
         thread.start()
 
     def copy(self):
-        lock = self.lock
-        i = [self.read_from]
-        o = e = []
-        while 1:
-            ii, oo, ee = select.select(i, o, e)
-            with lock:
-                for fd in ii:
-                    self.write(os.read(fd, 8192))
+        try:
+            lock = self.lock
+            i = [self.read_from]
+            o = e = []
+            while 1:
+                ii, oo, ee = select.select(i, o, e)
+                with lock:
+                    for fd in ii:
+                        self.write(os.read(fd, 8192))
+        finally:
+            # since there's no reader from this pipe we want the other side to
+            # get a SIGPIPE as soon as it tries to write to it, instead of
+            # deadlocking when the pipe buffer becomes full.
+            os.close(self.read_from)
 
     def reopen(self):
         new_file = open(self.filename, 'ab', 0)