[committed] malloc: Fix tcache leak on thread destruction [BZ #22111]

[Carlos O'Donell]

Backported to glibc 2.26 to fix the tcache leak.

I also need this for Fedora 27.

commit 1e26d35193efbb29239c710a4c46a64708643320
Author: Carlos O'Donell <carlos@systemhalted.org>
Date:   Thu Sep 28 11:05:18 2017 -0600

    malloc: Fix tcache leak after thread destruction [BZ #22111]
    
    The malloc tcache added in 2.26 will leak all of the elements remaining
    in the cache and the cache structure itself when a thread exits. The
    defect is that we do not set tcache_shutting_down early enough, and the
    thread simply recreates the tcache and places the elements back onto a
    new tcache which is subsequently lost as the thread exits (unfreed
    memory). The fix is relatively simple, move the setting of
    tcache_shutting_down earlier in tcache_thread_freeres. We add a test
    case which uses mallinfo and some heuristics to look for unaccounted for
    memory usage between the start and end of a thread start/join loop. It
    is very reliable at detecting that there is a leak given the number of
    iterations.  Without the fix the test will consume 122MiB of leaked
    memory.


-- 
Cheers,
Carlos.