A program that can get exactly the log of a supervised process?

["Casper Ti. Vector" <caspervector@gmail.com>]

Many daemons output some lines in their logs (here assumed to have been
redirected to logging pipes in some supervision framework) that can be
used as a kind of markers for readiness.  So if we can implement the
following program, we will able to trivially add readiness support to
these daemons without patching their source code, while reducing the
daemon-specific code (the pattern recogniser) to a minimum:

* The program is double-forked from the ./run script of a service, and
  also watches for the death of the daemon: in case the daemon dies
  before logging the readiness marker (and optionally, if a specified
  timeout is reached), this program dies as well, killing its subprocess
  (the pattern recogniser, discussed below).  To maximise portability
  across supervision frameworks, the death notification can be
  implemented with a pipe that can be written to by an ancillary
  program ran from the ./finish script.

* The program somehow (directly or indirectly) gets a copy of the
  log from the reading side of the logging pipe, and feeds it into a
  specified pattern recogniser, which should exit 0 upon finding the
  readiness marker.  A special exit value of the subprocess can be
  considered a notification for fatal error erroneously ignored by the
  daemon, leading to the program exiting some special value which can
  be handled by a `s6-svc -t' invocation.  In order to avoid potential
  feedback loops, the subprocess should be killed (and waited for)
  before the program outputs any warning; the subprocess should be
  responsible for keeping itself quiet when running normally.

Any idea on how the log "teeing" may be done cleanly (and portably
if possible; something akin to `tail -f' seems unsuitable because of
potential log rotation), and perhaps any flaw or redundancy in the
design above?

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