mythtranscode: Fix #244 by detecting write errors

[twitham1]

Fix for #244 which was originally https://code.mythtv.org/trac/ticket/12602

Explanation of the patch:

• multiplex.h:
  the mx multiplex structure gets a new "int error", the error count.
  write_out_packs and finish_mpg now return int, the error count (so 0 = success)
  currently unused: callers test the error count instead
• multiplex.c:
  if write() fails, increment error count
  log only first 10 failures so we don't flood syslog and cause it to drop messages
  if close() fails, increment error count
• mpeg2fix.cpp:
  copy error count to static variable and pass it to pthread_exit
  capture error count in pthread_join and log/fail if non-zero

This all causes mythtv to keep the original recording if disk fills during transcode, instead of truncating the recording. Of course user will need to go free space larger than this recording for a successful transcode.

In mpeg2fix.cpp there is also a commented section where I tried to exit on first write failure. This just deadlocked the other thread. It might be possible to cancel the other thread if you wish to abort at first write error but I was unsure how to do this. As-is, we just continue to fail and return the failure count when the reader is done. I monitored with top and the size doesn't grow so I assume the data is just being dropped rather than filling memory, which is a good thing.

[twitham1]

See also #163 for original pull request against fixes/0.27.

[twitham1]

So we applied all fixes Beirdo recommended here and in #163 . Of course it still works at avoiding data loss at full disk during transcode, thereby resolving bug #12602. So not sure what more should be done here other than merge into master. Unless someone else wants to confirm the fix. How to do that is described in the bug intro at #244 or https://code.mythtv.org/trac/ticket/12602

[twitham1]

I reset my branch to current master and reduced the commit complexity down to the actual changes only. Detailed commit history is over in #163 if needed.

[twitham1]

Since @linuxdude42 most recently twiddled code in mythtranscode, I wonder if he would be willing to take a look at this?

[linuxdude42]

The changes look reasonable to me. I see only one minor flaw, which is that the "|| 0" in "return mx->error || 0;" is redundant.

[twitham1]

Good point, that || 0 is a habit from my Perl coding, for upgrading an undef to zero. But in this C code the error is already initialized to 0 so this is not needed. You may remove my silly but harmless mistake if you like.

So glad to see this bug fixed, thank you sir!