1. 11 Jun, 2020 1 commit
    • Yedidya Feldblum's avatar
      Back out "EventBase::getRecentSteadyTime" · 82dd413a
      Yedidya Feldblum authored
      Summary:
      Original commit changeset: 8eb472ce50bf
      
      This breaks time measurement to include measuring the wait preceding handler/callback invocation.
      
      Reviewed By: w-o-o
      
      Differential Revision: D21986042
      
      fbshipit-source-id: 42701b929878c3f56d736de5ca0417814c64fce1
      82dd413a
  2. 10 Jun, 2020 1 commit
    • Pranav Thulasiram Bhat's avatar
      Raise stackSizeMultiplier for debug builds · aa825c5c
      Pranav Thulasiram Bhat authored
      Summary:
      Debug builds can be prone to stack overflows if they are configurated to use the same stack sizes as prod builds.
      
      This diff effectively doubles the stack size for debug builds to provide more buffer against overflows.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21913754
      
      fbshipit-source-id: bd32cc4dd9c79a50be178e5d6d30804a83e60ee4
      aa825c5c
  3. 09 Jun, 2020 10 commits
    • Mohamad Gebai's avatar
      Add strict versions of prevPowTwo() and nextPowTwo() · cb47018e
      Mohamad Gebai authored
      Summary: Returns the previous/next power of two that are strictly smaller/greater than the parameter.
      
      Reviewed By: yfeldblum, Gownta
      
      Differential Revision: D21920224
      
      fbshipit-source-id: be919063379e5b231cc69d743128fcaeb0ea9f39
      cb47018e
    • Yedidya Feldblum's avatar
      Cut FOR_EACH_KV · c94ebed8
      Yedidya Feldblum authored
      Summary: [Folly] Cut the FOR_EACH_KV macro, which may be replaced by a combination of range-for and structured bindings.
      
      Differential Revision: D21826182
      
      fbshipit-source-id: ce4712afd3d0d7806eb1fca8c97009da117f982e
      c94ebed8
    • Alexey Kuzmenko's avatar
      only allocate trimmedName on win32 · 58171cb5
      Alexey Kuzmenko authored
      Reviewed By: yfeldblum
      
      Differential Revision: D21921631
      
      fbshipit-source-id: e905d6d2a96775a8c3b328fb78c8d32897f634f6
      58171cb5
    • Junqi Wang's avatar
      Allow connect before bind · e31eb32a
      Junqi Wang authored
      Summary: connect will automatically bind the socket if the socket is not bound yet
      
      Reviewed By: yangchi
      
      Differential Revision: D21845740
      
      fbshipit-source-id: 27a5b44476dfc0b2ae5ff2f0a6c1bd4e976eadc9
      e31eb32a
    • Yedidya Feldblum's avatar
      Dead Code: TAsyncSocket · d11cbbc9
      Yedidya Feldblum authored
      Summary: [Thrift] Dead Code: `TAsyncSocket`.
      
      Reviewed By: vitaut
      
      Differential Revision: D20851837
      
      fbshipit-source-id: d4279fb1ab712f338d399220df26d56c4ab7076a
      d11cbbc9
    • Yedidya Feldblum's avatar
      EventBase::getRecentSteadyTime · da13f8bc
      Yedidya Feldblum authored
      Summary:
      [Folly] `EventBase::getRecentSteadyTime`, returning a recent `steady_clock::time_point` which is updated each pass through the event loop, but not for each ready event or function.
      
      Assuming everything in the loop is truly asynchronous, this permits callers to get a reasonably accurate approximation of the current time, but for many callers to blend or amortize the cost of getting that current time.
      
      May only be called from the running IO thread.
      
      Differential Revision: D21866783
      
      fbshipit-source-id: 8eb472ce50bf012616e567b29f6724c67ef8d41a
      da13f8bc
    • Yedidya Feldblum's avatar
      atomic_ref::compare_exchange_strong · 33403f55
      Yedidya Feldblum authored
      Summary: [Folly] `atomic_ref::compare_exchange_strong` and `atomic_ref::compare_exchange_weak`.
      
      Differential Revision: D21923944
      
      fbshipit-source-id: 55620473ea253782cc1dbb95baef38c96a9fbc70
      33403f55
    • Andrii Grynenko's avatar
      Crash process if folly::Singleton destruction is taking too long · e223d1a4
      Andrii Grynenko authored
      Summary: This provides a single mechanism to mitigate deadlocks in singleton destructors.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21827615
      
      fbshipit-source-id: 090bad1a91238310e7814663193711df0f4d622a
      e223d1a4
    • Yedidya Feldblum's avatar
      Deduction guide for atomic_ref · 2cf42c9f
      Yedidya Feldblum authored
      Summary: [Folly] Deduction guide for `atomic_ref`.
      
      Reviewed By: markisaa
      
      Differential Revision: D21923854
      
      fbshipit-source-id: 8ad10692b95c653fc65302e6ad0118cbd615a62f
      2cf42c9f
    • Mingtao Yang's avatar
      Deprecate AsyncTransportWrapper · 2586be5b
      Mingtao Yang authored
      Summary:
      All implementations of transports in practice were all AsyncTransportWrappers
      (AsyncSocket, AsyncSSLSocket, AsyncFizzBase, etc.).
      
      AsyncTransportWrapper was confusing. The only additional functionality that
      it provided was the ability to query underlying transports, a piece of
      functionality which makes just as much sense on AsyncTransport.
      
      The simplified model:
      
      "AsyncTransport deals with bidirectional I/O. It is both an AsyncReader, and
      an AsyncWriter. It may be nested."
      
      This diff changes AsyncTransportWrapper usages in folly to refer to
      AsyncTransport, and aliases AsyncTransportWrapper for compatibility.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21915048
      
      fbshipit-source-id: 741fad91b8f7c8080f942168f5513b12602cfe9a
      2586be5b
  4. 08 Jun, 2020 1 commit
  5. 07 Jun, 2020 1 commit
  6. 06 Jun, 2020 2 commits
    • Yedidya Feldblum's avatar
      Mark _r_debug imports as weak · db54c6bf
      Yedidya Feldblum authored
      Summary:
      [Folly] Mark `_r_debug` imports as weak imports. `_r_debug` is available in `<link.h>` but is not a portable API, so detect whether it is available at runtime and use it only if it is available.
      
      Fixes: https://github.com/facebook/folly/issues/1373.
      
      Reviewed By: Orvid
      
      Differential Revision: D21911715
      
      fbshipit-source-id: 660c0bc90d405d9a43826e7bdfc3b04480728b66
      db54c6bf
    • Yedidya Feldblum's avatar
      Dead code: countLoadedElfFiles · 7d367a68
      Yedidya Feldblum authored
      Summary: [Folly] Cut dead code: `countLoadedElfFiles()`.
      
      Differential Revision: D21910945
      
      fbshipit-source-id: 43ff7d118b4876e2cbd4527713c8552963810537
      7d367a68
  7. 05 Jun, 2020 1 commit
    • Yedidya Feldblum's avatar
      Let ElfCache be unbounded · 35ff8b26
      Yedidya Feldblum authored
      Summary:
      [Folly] Let `ElfCache` be unbounded to permit binaries with many dynamically-linked libraries from continuously missing the cache when symbolizing.
      
      Some applications symbolize stacktraces relatively frequently. When the build is using dynamic linking, such as might be done when developing the application or running the unit-tests, the overhead from re-loading `ElfFile` instances can appear. Address this by making the cache be unboundedly large.
      
      This is unlikely to affect applications built using mostly-static linking, such as might be done for production builds, even when they are large applications built from many libraries, since the capacity parameter is usually much larger than the total number of dynamically loaded libraries.
      
      Currently, no substantial work is done when initializing `ElfFile` instances. If substantial work were to be done, especially in an unoptimized build such as might be used when developing the application or running the unit-tests, and instances do not stay in cache, then that substantial work would be repeated whenever symbolizing stacktraces rather than being amortized.
      
      Reviewed By: luciang
      
      Differential Revision: D18819555
      
      fbshipit-source-id: 0e75326133e901e7bd49ea3cefe9b318e6d4466f
      35ff8b26
  8. 04 Jun, 2020 1 commit
    • Andrii Grynenko's avatar
      Fix pool resizer to correctly handle VirtualEventBase destruction · 4479aa0c
      Andrii Grynenko authored
      Summary: It's possible that we still have a recurring pool resizer task scheduled after virtual event base is destroyed. This changes makes sure that we don't try to reschedule it if VirtualEventBase is already destroyed.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21875241
      
      fbshipit-source-id: c4622c5d103c6291386055c153ab4aeebe7795a2
      4479aa0c
  9. 03 Jun, 2020 9 commits
    • Martin Thomas Fleischer's avatar
      Fix shell builders (#50) · 3ee93a0f
      Martin Thomas Fleischer authored
      Summary:
      In 0ae204a978c11ddefafd81bd319a078239a44c1c the 'projects_dir' option
      became a required constructor argument since it is called within the
      constructor. However, it has not been adjusted in the subclasses that
      used to set the option after instantiation. This commit fixes the
      'shell_builder' and the 'debian_system_builder'.
      Pull Request resolved: https://github.com/facebook/openr/pull/50
      
      Test Plan:
      1. Go to build directory: `cd build`
      2. Run the `shell_builder` & `debian_system_builder`:
          - `python fbcode_builder/shell_builder.py`
          - `python debian_system_builder/debian_system_builder.py`
      
      `shell_builder` output before:
      ```
      Traceback (most recent call last):
        File "fbcode_builder/shell_builder.py", line 102, in <module>
          builder = ShellFBCodeBuilder()
        File "/home/butjar/tu/ma/openr/build/fbcode_builder/fbcode_builder.py", line 93, in __init__
          self._github_dir = self.option('projects_dir')
        File "/home/butjar/tu/ma/openr/build/fbcode_builder/fbcode_builder.py", line 108, in option
          raise RuntimeError('Option {0} is required'.format(name))
      RuntimeError: Option projects_dir is required
      ```
      
      `shell_builder` output after:
      ```
      set -exo pipefail
      export CCACHE_DIR='/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr/.ccache' CC="ccache ${CC:-gcc}" CXX="ccache ${CXX:-g++}"
      ### Diagnostics ###
      
      # Builder ShellFBCodeBuilder(google/googletest:cmake_defines={u'BUILD_GTEST': u'ON', u'BUILD_SHARED_LIBS': u'OFF'}, google/googletest:git_hash=u'release-1.8.1', facebook/openr:local_repo_dir='/home/butjar/tu/ma/openr', facebook/zstd:git_hash=ShellQuoted(u'$(git describe --abbrev=0 --tags origin/master)'), openr/build:cmake_defines={u'ADD_ROOT_TESTS': u'OFF'}, thom311/libnl:git_hash=u'libnl3_2_25', projects_dir=u'/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr', fmtlib/fmt:git_hash=u'5.3.0', wangle/wangle/build:cmake_defines={u'BUILD_TESTS': u'OFF'}, prefix=u'/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr/installed', fizz/fizz/build:cmake_defines={u'BUILD_TESTS': u'ON'}, ccache_dir=u'/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr/.ccache', zeromq/libzmq:git_hash=u'v4.2.2', make_parallelism=4, jedisct1/libsodium:git_hash=u'stable')
      hostname
      cat /etc/issue || echo no /etc/issue
      g++ --version || echo g++ not installed
      cmake --version || echo cmake not installed
      
      ### Check out fmtlib/fmt, workdir build ###
      
      mkdir -p '/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr' && cd '/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr'
      git clone  https://github.com/'fmtlib/fmt'
      mkdir -p '/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr'/'fmt'/'build' && cd '/home/butjar/.fbcode_builder-sZshomesZsbutjarsZstusZsmasZsopenr'/'fmt'/'build'
      git checkout '5.3.0'
      
      ### Build and install fmtlib/fmt ###
      
      ...
      ```
      
      Reviewed By: steven1327
      
      Differential Revision: D21865881
      
      Pulled By: saifhhasan
      
      fbshipit-source-id: dfd78127d3b2c78721f84a3ecafe0b7198c38f06
      3ee93a0f
    • Martin Thomas Fleischer's avatar
      Fix shebang in shell_builder (#51) · 2e3e2e00
      Martin Thomas Fleischer authored
      Summary:
      Typo in python shebang introduced by 0d19e27, probably by accident.
      
      Found while skimming the code.
      Pull Request resolved: https://github.com/facebook/openr/pull/51
      
      Reviewed By: steven1327
      
      Differential Revision: D21865922
      
      Pulled By: saifhhasan
      
      fbshipit-source-id: 5f2c2c2fac82078070920915812139f5fef1c7fe
      2e3e2e00
    • Yedidya Feldblum's avatar
      Move AlignedSysAllocator computation of max · dcd9f349
      Yedidya Feldblum authored
      Summary: [Folly] Move `AlignedSysAllocator` computation of max value over input alignment values from two places to a single place.
      
      Differential Revision: D19229096
      
      fbshipit-source-id: 4e95c7b7e0afe99c37116150184eb180683c7d13
      dcd9f349
    • Lukas Piatkowski's avatar
      rust-shed: add secure_utils to the shed · 3b29cdba
      Lukas Piatkowski authored
      Reviewed By: StanislavGlebik
      
      Differential Revision: D21549859
      
      fbshipit-source-id: 0e143354a60578732ae1eed8c3c71b9f859e3958
      3b29cdba
    • Zhengxu Chen's avatar
      Specify error handler for text encoding in logging functions. · 19443ec0
      Zhengxu Chen authored
      Summary: Currently output from build command is decoded with "surrogateescape" error handler, but when writing to log files/stdout we don't specify error handlers to be also "surrogateescape" according to https://docs.python.org/3/library/codecs.html#error-handlers, which could cause exception when there's surrogate characters logged in message.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21850411
      
      fbshipit-source-id: 21c51d1ab2132171ae29f2d1fbe42655ebee94c5
      19443ec0
    • Andrew Huang's avatar
      Forward newly negotiated sessions to corresponding SSLSession · 94ed107a
      Andrew Huang authored
      Summary:
      When a session is newly negotiated, pass it to the SSLSession belonging to the corresponding AsyncSSLSocket through SSLSessionManager. That SSLSession then becomes resumable.
      
      This allows SSLSessions obtained from an AsyncSSLSocket to later become resumable once the session is negotiated (asynchronously in TLS 1.3).
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21035731
      
      fbshipit-source-id: acbaa258798e7774e742c5765ef33e24573f1c78
      94ed107a
    • Andrew Huang's avatar
      Refactor AsyncSSLSocket session code into SSLSessionManager · 0ad1b3bb
      Andrew Huang authored
      Summary: Refactor AsyncSSLSocket session code into a new class called SSLSessionManager, which is a member of AsyncSSLSocket. Will allow for SSLContext to interact with SSLSession through SSLSessionManager, without depending on AsyncSSLSocket.
      
      Reviewed By: yfeldblum, mingtaoy
      
      Differential Revision: D21164020
      
      fbshipit-source-id: e6b38336127fb000587bfe6948c2274afe8d85e0
      0ad1b3bb
    • Junqi Wang's avatar
      Move connect next to bind · abde1c0d
      Junqi Wang authored
      Summary: codecleanupraccoon
      
      Reviewed By: mjoras
      
      Differential Revision: D21845742
      
      fbshipit-source-id: 5adf159ed4ec7a48c76cffdb02d039888ab1686d
      abde1c0d
    • Victor Zverovich's avatar
      Update fmt version to 6.2.1 · 7e211332
      Victor Zverovich authored
      Summary:
      Update fmt version to 6.2.1 for better compatibility with the version used in fbcode. Among other things this fixes fbthrift build failure on Travis:
      
      ```
      /home/fbthrift/thrift/lib/cpp2/async/RocketClientChannel.cpp:70:67:   required from here
      /home/install/include/fmt/core.h:492:3: error: static assertion failed: don't know how to format the type, include fmt/ostream.h if it provides an operator<< that should be used
      ```
      which is caused by trying to format an enum class without a formatter - only supported as of 6.0.
      
      Reviewed By: stevegury, avalonalex
      
      Differential Revision: D21860076
      
      fbshipit-source-id: 1857ab65822956b005980b8dfff7a967508f507c
      7e211332
  10. 01 Jun, 2020 3 commits
    • Yedidya Feldblum's avatar
      Cut FOR_EACH_ENUMERATE · 06274a0d
      Yedidya Feldblum authored
      Summary: [Folly] Cut the `FOR_EACH_ENUMERATE` macro, which may be replaced by a combination of range-for, `ranges::view::enumerate`, and structured bindings.
      
      Reviewed By: markisaa
      
      Differential Revision: D21813019
      
      fbshipit-source-id: fc9ac09a4e2f72f1433d0a518f03d5cd69a59c55
      06274a0d
    • Dan Melnic's avatar
      Expose async read/write functionality · f6784e42
      Dan Melnic authored
      Summary: Expose async read/write functionality
      
      Reviewed By: yfeldblum, kevin-vigor
      
      Differential Revision: D21731597
      
      fbshipit-source-id: 95a0af53c7b85e1bf1a2619586091b6633bc7b8a
      f6784e42
    • soyoo's avatar
      Fix typo in stop_watch.h (#1110) · 8f91f848
      soyoo authored
      Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1110
      
      Reviewed By: Orvid
      
      Differential Revision: D21110331
      
      Pulled By: yfeldblum
      
      fbshipit-source-id: 57522232efb7268bc290e4f22057f531a380fb81
      8f91f848
  11. 29 May, 2020 5 commits
    • Lee Howes's avatar
      Have SemiFuture::within mark internal futures as consumed to avoid treating them as discarded. · 15449325
      Lee Howes authored
      Summary:
      SemiFuture::within creates two valid SemiFutures and promptly overwrites them, both discarding valid SemiFutures and wasting the core allocation. It later drops the updated versions without directly consuming them, instead stealing the deferred executors.
      
      This update initializes the two SemiFutures as empty, and safely terminates the chain with trivial callbacks so the SemiFutures can be treated as consumed.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21769500
      
      fbshipit-source-id: eab13a78b17ecfc937b42c8d5e01e1292158d9ed
      15449325
    • Chad Austin's avatar
      fix some warnings on macOS · 8f13bf87
      Chad Austin authored
      Differential Revision: D21757385
      
      fbshipit-source-id: 28af042285d343944452e1c2cf94794a121c0e06
      8f13bf87
    • Sergey Anpilov's avatar
      Fix "unused parameter" in getValueOrUnit in non-debug builds · 6840cc81
      Sergey Anpilov authored
      Summary:
      `value` could trigger "unused parameter" warning in NDEBUG builds. The fix is straightforward, to use the new-ish `[[maybe_unused]]` attribute.
      
      The alternatives are to use `FOLLY_MAYBE_UNUSED` or even manual `(void)value;`. But since this is coroutines library, it should be reasonably safe to assume the compiler supports the cleaner and more expressive native C++17 option.
      
      Reviewed By: lewissbaker
      
      Differential Revision: D21784931
      
      fbshipit-source-id: 895bf95d65df6533d768b65c44a898a73e0a636f
      6840cc81
    • Chad Austin's avatar
      don't use clock_gettime fallback on modern macOS · c85f8af6
      Chad Austin authored
      Summary:
      There were two problems with the clock_gettime fallback logic on
      macOS. One is that it used undefined macros if AvailabilityMacros.h
      wasn't included. Another is that it compiled the fallback when the
      minimum iPhone OS version (as when building on macOS) was not set, and
      vice versa.
      
      Fix that.
      
      Differential Revision: D21757362
      
      fbshipit-source-id: 99fd17260cb25ed07c39ba79466e835f08a1fc7c
      c85f8af6
    • Matt Dordal's avatar
      add a method to access a threadlocal without creating an object · fd295717
      Matt Dordal authored
      Summary:
      It may be useful to see if a threadlocal has been created on a thread before accessing it.
      
      One way this could be useful: the data is only relevant to a few threads but will be accessed by many. This would allow you to save the memory on the non-relevant threads.
      
      Differential Revision: D21731260
      
      fbshipit-source-id: 0b1835b454f9b0c457f722ab38f84f89aca383fb
      fd295717
  12. 28 May, 2020 5 commits
    • Andrew Huang's avatar
      (Trivial) Add detailed comment for session cache settings in SSLContext · 0674ab06
      Andrew Huang authored
      Summary: Add detailed comment to SSL_CTX_set_session_cache_mode call in SSLContext, since any changes can result in breaking functionality or increased memory usage.
      
      Reviewed By: mingtaoy
      
      Differential Revision: D21769914
      
      fbshipit-source-id: 61a172f24cda870d6c13a24d279509e2e2804538
      0674ab06
    • Lewis Baker's avatar
      Make collectAll, collectAllRange, collectAllWindowed consistently rethrow... · e19b938b
      Lewis Baker authored
      Make collectAll, collectAllRange, collectAllWindowed consistently rethrow exception from first failure
      
      Summary:
      The void-returning variants of collectAllRange() and collectAllWindowed() were
      previously failing with the error from the first task to fail whereas the other
      variants of collectAll() were failing with an arbitrary error (typically first
      by argument order) in the case of multiple errors.
      
      This was problematic because after the first task failed it requests cancellation
      of the other tasks, which then often will complete with an 'OperationCancelled'
      error. This mean that, unless the original error occurred on the first failed
      task in argument order, the error that caused the cancellation would be discarded
      and instead, the collectAll() operation would result in an 'OperationCancelled'
      error.
      
      This change updates those algorithms to now consistently fail with the error
      from the first task to fail (in time) as this error will typically be the most
      useful in determining the root cause of the failure.
      
      Also it now discards any errors that occur in child tasks if they fail after
      cancellation was requested on the parent task. In this case the collectAll()
      will complete with an 'OperationCancelled' error.
      
      Also made a few drive-by changes to avoid rethrowing exceptions where possible
      by using `co_yield co_error(e)`.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21681812
      
      fbshipit-source-id: dfe15d0672dcbdeb4e156962cdffe767ddf91a0b
      e19b938b
    • Lewis Baker's avatar
      Add cancellation support for folly::coro::UnboundedQueue::dequeue() · 70f914ba
      Lewis Baker authored
      Summary:
      Adds the ability to request cancellation of the folly::coro::UnboundedQueue's
      dequeue() operation by injecting a CancellationToken using co_withCancellation().
      
      Most of the work for supporting this was implemented in the change to
      Semaphore::co_wait() to support cancellation.
      
      This diff just adds some tests for cancellation of UnboundedQueue::dequeue()
      and provides a more efficient code-path for the cancellation-case that does
      not involve throwing an exception.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21555325
      
      fbshipit-source-id: 7fbae8aa426b40bbea4d0d6dec85df4bad813733
      70f914ba
    • Lewis Baker's avatar
      Make folly::fibers::Semaphore::co_wait() cancellable · 955b416f
      Lewis Baker authored
      Summary:
      The `Semaphore::co_await()` method now response to requests for cancellation
      communicated through the awaiting context's CancellationToken.
      
      This allows interrupting a wait-operation if the result is no longer required,
      in which case the co_wait() task completes with the OperationCancelled exception.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21655946
      
      fbshipit-source-id: 8c7f862580ddebd2b4d3eb3cc3ab5e01ad5ee289
      955b416f
    • Lewis Baker's avatar
      Change folly::fibers::Semaphore to use an intrusive list of waiters · f8a3d164
      Lewis Baker authored
      Summary:
      Now use an intrusive list structure to store the list of waiters waiting
      for the semaphore to be signalled.
      
      This is primarily motivated by the need to support efficient cancellation,
      which requires the ability to efficiently remove a specific element from
      the waiter list without having to scan the list like we would for the
      existing `std::deque`-based list storage.
      
      Another beneift of this change is that it now eliminates memory allocations
      that were previously required by the std::deque container.
      
      The `Semaphore::future_wait()` implementation should now also have one fewer
      allocations as the allocation of the Baton and the Baton::Waiter are now fused.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D21692257
      
      fbshipit-source-id: 9beff176b250ae3f37df9e2138f93b15d2607353
      f8a3d164