[PATCH] mwrap 2.0.0 mwrap - LD_PRELOAD malloc wrapper for Ruby

[Eric Wong <e@80x24.org>]

mwrap is designed to answer the question:

   Which lines of Ruby are hitting malloc the most?

mwrap wraps all malloc-family calls to trace the Ruby source
location of such calls and bytes allocated at each callsite.
As of mwrap 2.0.0, it can also function as a leak detector
and show live allocations at every call site.  Depending on
your application and workload, the overhead is roughly a 50%
increase memory and runtime.

It works best for allocations under GVL, but tries to track
numeric caller addresses for allocations made without GVL so you
can get an idea of how much memory usage certain extensions and
native libraries use.

It requires the concurrent lock-free hash table from the
Userspace RCU project: https://liburcu.org/

It does not require recompiling or rebuilding Ruby, but only
supports Ruby trunk (2.6.0dev+) on a few platforms:

* GNU/Linux
* FreeBSD (tested 11.1)

It may work on NetBSD, OpenBSD and DragonFly BSD.


Changes in 2.0.0:

This release includes significant changes to track live
allocations and frees.  It can find memory leaks from malloc
with less overhead than valgrind's leakchecker and there is a
new Rack endpoint (MwrapRack) which can display live allocation
stats.

API additions:

* Mwrap#[] - https://80x24.org/mwrap/Mwrap.html#method-c-5B-5D
* Mwrap::SourceLocation - https://80x24.org/mwrap/Mwrap/SourceLocation.html
* MwrapRack - https://80x24.org/mwrap/MwrapRack.html

Incompatible changes:

* Mwrap.clear now an alias to Mwrap.reset; as it's unsafe
  to implement the new Mwrap#[] API otherwise:
  https://80x24.org/mwrap-public/20180716211933.5835-12-e@80x24.org/

26 changes since v1.0.0:

      README: improve usage example
      MANIFEST: add .document
      add benchmark
      use __attribute__((weak)) instead of dlsym
      Mwrap.dump: do not segfault on invalid IO arg
      bin/mwrap: support LISTEN_FDS env from systemd
      support per-allocation headers for per-alloc tracking
      mwrap: use malloc to do our own memalign
      hold RCU read lock to insert each allocation
      realloc: do not copy if allocation failed
      internal_memalign: do not assume real_malloc succeeds
      ensure ENOMEM is preserved in errno when appropriate
      memalign: check alignment on all public functions
      reduce stack usage from file names
      resolve real_malloc earlier for C++ programs
      allow analyzing live allocations via Mwrap[location]
      alias Mwrap.clear to Mwrap.reset
      implement accessors for SourceLocation
      mwrap_aref: quiet -Wshorten-64-to-32 warning
      fixes for FreeBSD 11.1...
      use memrchr to extract address under glibc
      do not track allocations for constructor and Init_
      disable memalign tracking by default
      support Mwrap.quiet to temporarily disable allocation tracking
      mwrap_rack: Rack app to track live allocations
      documentation updates for 2.0.0 release