1. 01 Mar, 2016 4 commits
    • Dave Watson's avatar
      Fix narenas size · d2f85c15
      Dave Watson authored
      Summary:Changed from size_t to unsigned in a recent tp2 update
      https://github.com/jemalloc/jemalloc/blame/3c07f803aa282598451eb0664cc94717b769a5e6/test/unit/mallctl.c
      
      Reviewed By: jasone, mhlakhani
      
      Differential Revision: D2991755
      
      fb-gh-sync-id: 46f2be722a30706333eac6428f4183ca9cb85543
      shipit-source-id: 46f2be722a30706333eac6428f4183ca9cb85543
      d2f85c15
    • Michael Lee's avatar
      to_string is not supported everywhere · aa8dce11
      Michael Lee authored
      Summary: Also remove an extra `#include <strstream>` from JsonTest.cpp as it is no longer necessary.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2991381
      
      fb-gh-sync-id: 386b8281726d034933ebdc26733dd35dc5ace949
      shipit-source-id: 386b8281726d034933ebdc26733dd35dc5ace949
      aa8dce11
    • Michael Lee's avatar
      Remove unecessary int main from MemoryTest · b363d9f7
      Michael Lee authored
      Summary: We do not need this main function, so remove it.
      
      Reviewed By: markisaa
      
      Differential Revision: D2992000
      
      fb-gh-sync-id: 30bc0734af0f61e047276dab9639ae29cc999bbb
      shipit-source-id: 30bc0734af0f61e047276dab9639ae29cc999bbb
      b363d9f7
    • Subodh Iyengar's avatar
      Add conversion constructors for Future · 692cc4b1
      Subodh Iyengar authored
      Summary:previously we were not able to do
      
      Future<Base> f = makeFuture<Derived>();
      
      This is because Future did not declare a conversion
      constructor.
      
      Adding a proper conversion ctor for Future is very
      tricky because of the way Core is managed under the hood.
      Core is not movable, and cannot be moved otherwise we
      would have to retain pointers to the Future and Promises which
      pointed to a particular core. This would be inefficient.
      
      Instead we compromise and allow a very small subset of conversions
      from objects whose templated types are convertible and also the
      sizes of their Cores are also the same. As a result, we can
      convert between types like unique_ptrs however not always between
      full objects.
      
      Reviewed By: jsedgwick
      
      Differential Revision: D2943775
      
      fb-gh-sync-id: 7c2388f2fb49d789c80ae2477814e960a099771b
      shipit-source-id: 7c2388f2fb49d789c80ae2477814e960a099771b
      692cc4b1
  2. 29 Feb, 2016 1 commit
    • Pavlo Kushnir's avatar
      Fix SingletonThreadLocal · 8edf04da
      Pavlo Kushnir authored
      Summary: it's really thread local at this point :)
      
      Reviewed By: jmswen
      
      Differential Revision: D2989668
      
      fb-gh-sync-id: 423b9214922c92318181a62c583f58cd204b52e3
      shipit-source-id: 423b9214922c92318181a62c583f58cd204b52e3
      8edf04da
  3. 27 Feb, 2016 1 commit
  4. 26 Feb, 2016 2 commits
    • Andrii Grynenko's avatar
      Fix folly::ThreadLocal to have unique singleton in dev builds · a7d7c07c
      Andrii Grynenko authored
      Summary:This re-uses StaticSingletonManager which was previously used to fix the same issue in folly::Singleton.
      
      Because of the same issue we can no longer use static thread_local for the ThreadEntry. We now rely on pthread_getspecific/pthread_setspecific instead and use static thread_local ThreadEntry* only as a cache (to improve perf).
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2978526
      
      fb-gh-sync-id: cf1d9044afc27b62bd50a1ed931c0c420ae7107e
      shipit-source-id: cf1d9044afc27b62bd50a1ed931c0c420ae7107e
      a7d7c07c
    • Christopher Dykes's avatar
      Create the sys/time.h portability header · 398424e8
      Christopher Dykes authored
      Summary: Windows doesn't have it.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2979215
      
      fb-gh-sync-id: 030bdcaa44deb18834903705a26369ce0b13abc6
      shipit-source-id: 030bdcaa44deb18834903705a26369ce0b13abc6
      398424e8
  5. 25 Feb, 2016 7 commits
    • Michael Lee's avatar
      Split tests into test and benchmarks. · 705e7518
      Michael Lee authored
      Summary:Split up
      
        io/test/IOBufCursorTest
        test/BitsTest
        test/CacheLocalityTest
        test/CallOnceTest
        test/ConvTest
        test/LoggingTest
        test/MemoryIdlerTest
        test/RandomTest
        test/ThreadLocalTest
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2973132
      
      fb-gh-sync-id: 9dadbdf49a31e82c3a2e34c2fdb6a2b47aa0928d
      shipit-source-id: 9dadbdf49a31e82c3a2e34c2fdb6a2b47aa0928d
      705e7518
    • Michael Lee's avatar
      Split FileTest to a smaller test and an extended test · 68b1410c
      Michael Lee authored
      Summary: FileTest pulls in subprocess and does a lot more than just directly test File.h
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2971641
      
      fb-gh-sync-id: 16e1096ab3b0f6434a26f84c889ebb082ee3f210
      shipit-source-id: 16e1096ab3b0f6434a26f84c889ebb082ee3f210
      68b1410c
    • Christopher Dykes's avatar
      Create a portability header for working with environment variables · 7bc876aa
      Christopher Dykes authored
      Summary:Because `extern char** environ` is not the correct way to access environ on Windows.
      This also implements setenv and unsetenv for Windows, which means that TestUtils no longer needs to be disabled for the Windows build.
      
      Reviewed By: mzlee
      
      Differential Revision: D2973704
      
      fb-gh-sync-id: 84db7db3494cf183fcbcc25063cb0482ef84ebf4
      shipit-source-id: 84db7db3494cf183fcbcc25063cb0482ef84ebf4
      7bc876aa
    • Nathan Bronson's avatar
      shorten SharedMutex tests that timeout under ASAN · ddb7850c
      Nathan Bronson authored
      Summary:New ASAN options are more thorough but slower, resulting
      in timeouts for some tests.  Decreasing their internal loop count to
      compensate.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2973982
      
      fb-gh-sync-id: 67391936d0b4930f99d00358ebb93a1b3ee87140
      shipit-source-id: 67391936d0b4930f99d00358ebb93a1b3ee87140
      ddb7850c
    • Jonathan Kron's avatar
      Fixing AtomicHashArrayTest ASan Test Failure · 2203c927
      Jonathan Kron authored
      Summary: Both of these tests will unavoidably fail on ASan because of shared library loads, and indestructable objects.  Disabling ASAN testing for these test files.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2968169
      
      fb-gh-sync-id: d2acc71b40541af5e10f15d5087a7c07da05453d
      shipit-source-id: d2acc71b40541af5e10f15d5087a7c07da05453d
      2203c927
    • Christopher Dykes's avatar
      Minor MSVC issues · fd108f7f
      Christopher Dykes authored
      Summary:MSVC doesn't let you have unicode characters in a narrow string without using a utf-8 literal string.
      AsyncTransport needs the definition of IOBuf because of how the standard library implements a few things.
      And longs are only 32-bit under Windows.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2973973
      
      fb-gh-sync-id: c2f3e848e740a65c575598991d43dd79360ec1e3
      shipit-source-id: c2f3e848e740a65c575598991d43dd79360ec1e3
      fd108f7f
    • Orvid King's avatar
      Explicit vtordisp for MSVC · a2962ba8
      Orvid King authored
      Summary:Because sometimes things are just different.
      
      MSVC warns about the `dynamic_cast` done against this class because the class's declaration doesn't show that it would need a vtordisp. While I don't believe the `dynamic_cast` in question is actually called in either the constructor or destructor paths for the class, better to be safe and force the disp to be created.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2887490
      
      Pulled By: Orvid
      
      fb-gh-sync-id: 821a6fc820087ac7120fffd9f8b95a26c2b4e71b
      shipit-source-id: 821a6fc820087ac7120fffd9f8b95a26c2b4e71b
      a2962ba8
  6. 24 Feb, 2016 3 commits
    • Sven Over's avatar
      folly/futures: keep Core alive until callback has returned · 914e877b
      Sven Over authored
      Summary:Callbacks passed to e.g. folly::Future::then (or
      folly::Future::ensure) may delete the promise that keeps the
      Core of the same future alive. The Core must protect itself from
      destruction during callback execution.
      
      This commit also adds a unit test to check for correct behaviour
      in the "self destruction" scenario. The test should usually pass
      even without the fix in Core.h. However, when you run the test
      in Valgrind or ASAN, it will report problems unless the fix in
      Core.h is applied.
      
      Reviewed By: mhx
      
      Differential Revision: D2938094
      
      fb-gh-sync-id: 22796e168e1876ef2e3c7d7619da020be6a22073
      shipit-source-id: 22796e168e1876ef2e3c7d7619da020be6a22073
      914e877b
    • Andrii Grynenko's avatar
      Fix double definition for templated folly::Singletons · 9dd08403
      Andrii Grynenko authored
      Summary: Allow createGlobal() to create dependent objects.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2963111
      
      fb-gh-sync-id: 8e4da48a7a1000934963396b423e8eff98a8aade
      shipit-source-id: 8e4da48a7a1000934963396b423e8eff98a8aade
      9dd08403
    • Yedidya Feldblum's avatar
      Fix Build: folly/test/IndestructibleTest.cpp under LSAN · 2739d937
      Yedidya Feldblum authored
      Summary:[Folly] Fix Build: `folly/test/IndestructibleTest.cpp` under LSAN.
      
      Leaks of Meyers singletons are ignored automatically, while normal locals are not. So we make our instances be Meyers singletons.
      
      Differential Revision: D2968368
      
      fb-gh-sync-id: 385ac4491d9a105885af82d85354af929d69cc80
      shipit-source-id: 385ac4491d9a105885af82d85354af929d69cc80
      2739d937
  7. 23 Feb, 2016 4 commits
    • Andrii Grynenko's avatar
      Fix SIOF in folly/io/async/Request.cpp · 9c3e3be0
      Andrii Grynenko authored
      Summary: Use Meyers singleton instead.
      
      Reviewed By: afrind
      
      Differential Revision: D2963282
      
      fb-gh-sync-id: 7aec36e7cc3b0ab09b2b093202f40d5b0d399f4d
      shipit-source-id: 7aec36e7cc3b0ab09b2b093202f40d5b0d399f4d
      9c3e3be0
    • Beny Luo's avatar
      AsyncSocketException: add detailed information in error message · 787e185c
      Beny Luo authored
      Summary: AsyncSocketException: add detailed information in error message
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2953030
      
      fb-gh-sync-id: a5e26036657ecc6c74ad0369650730a2af13bead
      shipit-source-id: a5e26036657ecc6c74ad0369650730a2af13bead
      787e185c
    • Michael Lee's avatar
      Split benchmarks and tests · d1d243a6
      Michael Lee authored
      Summary:Benchmarks need their own main function, which means they
      aren't really useful in cases we can't use a custom main function.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2962104
      
      fb-gh-sync-id: 25bdc6e5a8bdf8c3aa94d393207a74797b2e1234
      shipit-source-id: 25bdc6e5a8bdf8c3aa94d393207a74797b2e1234
      d1d243a6
    • Lucian Grijincu's avatar
      folly: symbolizer: dwarf: don't fallback to linear scan if address missing from .debug_aranges · efe4f93c
      Lucian Grijincu authored
      Summary:Presence of .debug_aranges implies user expects fast address lookup.
      Some addresses might not be avaialble in .debug_aranges.
      
      Don't do slow lookup in .debug_info, as it can lead to unexpected slowdowns.
      
      override-unit-failures
      
      Reviewed By: philippv
      
      Differential Revision: D2965323
      
      fb-gh-sync-id: 405daefd57cdff4344fd231c5f5b7ff4dcd9df8c
      shipit-source-id: 405daefd57cdff4344fd231c5f5b7ff4dcd9df8c
      efe4f93c
  8. 22 Feb, 2016 3 commits
    • Neel Goyal's avatar
      Fix OSX build · d45626d0
      Neel Goyal authored
      Summary: Fixed the build using the homebrew instructions
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2960409
      
      fb-gh-sync-id: 218b879f26b3baab09676cac51787bf36d0b4bed
      shipit-source-id: 218b879f26b3baab09676cac51787bf36d0b4bed
      d45626d0
    • Nathan Bronson's avatar
      fix SIOF in CacheLocality.h's AccessSpreader · bf385e23
      Nathan Bronson authored
      Summary:This diff moves all data accessed during
      AccessSpreader<>::current(x) into the .data segment, avoiding SIOF
      without adding indirection or dynamic gating as would be the case for
      normal singleton-like constructs.  The diff also trims the AccessSpreader
      API to include only those methods that people actually seem to use.
      
      Reviewed By: djwatson
      
      Differential Revision: D2945205
      
      fb-gh-sync-id: 847e31adc4450217f4ed0575686be261fb504d7c
      shipit-source-id: 847e31adc4450217f4ed0575686be261fb504d7c
      bf385e23
    • Michael Lee's avatar
      Remove extra `int main`s from unit tests. · d31eb712
      Michael Lee authored
      Summary: test/common/TestMain.cpp should provide the necessary initialization.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2947245
      
      fb-gh-sync-id: 75b9d241673667badf4dc8fb81285c60cbb8166a
      shipit-source-id: 75b9d241673667badf4dc8fb81285c60cbb8166a
      d31eb712
  9. 20 Feb, 2016 2 commits
    • Andrii Grynenko's avatar
      Move RequestContext::getStaticContext to .cpp · bb5ed807
      Andrii Grynenko authored
      Summary: All singleton registration has to happen in .cpp. Otherwise singleton may be double registered if inlined.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2956951
      
      fb-gh-sync-id: bea425c31270d614f4b8a780204694168602fe86
      shipit-source-id: bea425c31270d614f4b8a780204694168602fe86
      bb5ed807
    • Andrii Grynenko's avatar
      Support nested FiberManagers · 743a0b48
      Andrii Grynenko authored
      Summary:It's possible to have nested loopUntilNoReady (with different FiberManagers). See unit test for a simple example.
      This diff fixes currentFiberManager_ to be a stack.
      
      Reviewed By: meyering
      
      Differential Revision: D2953468
      
      fb-gh-sync-id: 0abdcb7f43c94e7bb0adef8440699dc8e138d21a
      shipit-source-id: 0abdcb7f43c94e7bb0adef8440699dc8e138d21a
      743a0b48
  10. 19 Feb, 2016 6 commits
    • Andrii Grynenko's avatar
      Fix EventBaseOnDestructionCallback to loop FiberManager · 95ade4c9
      Andrii Grynenko authored
      Summary: EventBase loop should do it as well, but it's possible for LoopCallback to be removed before EventBaseOnDestructionCallback.
      
      Reviewed By: pavlo-fb
      
      Differential Revision: D2951303
      
      fb-gh-sync-id: 43f5a91e7ecffa7ab7feec32fa45e01b28db66a3
      shipit-source-id: 43f5a91e7ecffa7ab7feec32fa45e01b28db66a3
      95ade4c9
    • Andrii Grynenko's avatar
      Fix folly::Singleton to work in dynamically linked binaries · f390a90d
      Andrii Grynenko authored
      Summary:This implements StaticSingletonManager which is then used to create all leaked Meyers singletons.
      
      StaticSingletonManager is a singleton itself, which is created in a separate compilation unit (Singleton.cpp) and so we can be sure that other compilation units will always see a single instance of StaticSingletonManager, even if linked dynamically.
      
      StaticSingletonManager then keeps a dictionary of typeid -> object pointer, which is used to de-duplicate same singleton being re-created from different compilation units (linked dynamically), usually because of code inlining.
      
      override-unit-failures
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2913027
      
      fb-gh-sync-id: 1f5015a79a7a8297ebf5f0fe3fd0cc7eb44f706b
      shipit-source-id: 1f5015a79a7a8297ebf5f0fe3fd0cc7eb44f706b
      f390a90d
    • Tianjiao Yin's avatar
      fix dangling pointer bug in IPAddressV6 · ac31e5f5
      Tianjiao Yin authored
      Summary: `mask(numBits)` returns a temporary variable. After its lifetime ends, subbytes becomes a dangling pointer.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2953699
      
      fb-gh-sync-id: 5d5a35716ecaa1b3d96edb5a459615756848b92f
      shipit-source-id: 5d5a35716ecaa1b3d96edb5a459615756848b92f
      ac31e5f5
    • Yedidya Feldblum's avatar
      Indestructible · 7bf18354
      Yedidya Feldblum authored
      Summary:[Folly] `Indestructible`.
      
      For when you need a Meyers singleton that will never be destructed, even at program exit.
      
      Good for large data structures with many heap allocations, as long as they have no behavior. No point in ever destructing them.
      
      HT: Proxygen, and Orvid.
      
      Reviewed By: andriigrynenko
      
      Differential Revision: D2947959
      
      fb-gh-sync-id: 7dd1f725edf137ba81fbf4032a0819fd7c627261
      shipit-source-id: 7dd1f725edf137ba81fbf4032a0819fd7c627261
      7bf18354
    • Lucian Grijincu's avatar
      folly: symbolizer: slow address->{file+line-number} lookup if `.debug_aranges`... · 4d4ca9ec
      Lucian Grijincu authored
      folly: symbolizer: slow address->{file+line-number} lookup if `.debug_aranges` is missing (e.g. --strip-debug-non-line)
      
      Summary:Binaries linked with `gold` and `--strip-debug-non-line` don't have an `.debug_aranges` section
      
      We still want to map `address->{file+line-number}` to get nice stack
      traces even though this might be slower (linear search all compilation unit entries in `.debug_info`).
      
      Before:
      ```
      $ # link with gold + --strip-debug-non-line
      $ folly/experimental/exception_tracer/exception_tracer_test
      E0217 15:02:13.694947 1321814 ExceptionTracer.cpp:179] Exception type: std::runtime_error (9 frames)
          @ 000000000040ad2d __cxa_throw
          @ 0000000000409df3 bar()
          @ 0000000000409eab baz()
          @ 0000000000407c77 main
          @ 00007f00dd9860f5 __libc_start_main
          @ 000000000040991b (unknown)
      ```
      
      After (similar to the output without `--strip-debug-non-line`):
      ```
      E0217 18:37:37.579596 1583124 ExceptionTracer.cpp:179] Exception type: std::runtime_error (9 frames)
          @ 000000000040ad6d __cxa_throw
                             ./folly/experimental/exception_tracer/ExceptionTracerLib.cpp:57
          @ 0000000000409e33 bar()
                             ./folly/experimental/exception_tracer/ExceptionTracerTest.cpp:24
          @ 0000000000409eeb baz()
                             ./folly/experimental/exception_tracer/ExceptionTracerTest.cpp:51
          @ 0000000000407c87 main
                             ./folly/experimental/exception_tracer/ExceptionTracerTest.cpp:96
          @ 00007f1d16ff80f5 __libc_start_main
          @ 000000000040995b (unknown)
      ```
      
      Differential Revision: D2947965
      
      fb-gh-sync-id: e517bab324b1dcb70cadc9a5211ce794e35c83a5
      shipit-source-id: e517bab324b1dcb70cadc9a5211ce794e35c83a5
      4d4ca9ec
    • Christopher Dykes's avatar
      Don't use a VLA for the double->string buffer. · ad925ffa
      Christopher Dykes authored
      Summary: MSVC is actually smarter about this than GCC, as MSVC doesn't support VLAs, it tries to eval the length at compile time. GCC on the other hand doesn't try to eval it at compile time, resulting in compiles via CMake telling us that this is a VLA.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2911929
      
      fb-gh-sync-id: ffaa133bcf4129a3e02f7e875966d3ae6a97be6a
      shipit-source-id: ffaa133bcf4129a3e02f7e875966d3ae6a97be6a
      ad925ffa
  11. 18 Feb, 2016 2 commits
    • Sven Over's avatar
      folly/ApplyTuple.h: fix const-correctness & other issues, simplify · ca09289d
      Sven Over authored
      Summary:The existing implementation of folly::applyTuple does not support
      mutable callables (such as mutable lambdas and other functor objects
      that only implement non-const operator()).
      
      This commit adds a few more unit tests and changes the implementation
      so that new and existing tests pass.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2942622
      
      fb-gh-sync-id: 82478f290e9fd2020358ff79ef0a6bcf8a43738c
      shipit-source-id: 82478f290e9fd2020358ff79ef0a6bcf8a43738c
      ca09289d
    • Yedidya Feldblum's avatar
      Sort the Makefile headers a bit · 72cab06f
      Yedidya Feldblum authored
      Summary:[Folly] Sort the `Makefile` headers a bit.
      
      Just manually moving a few obviously out-of-order entries.
      
      Reviewed By: Orvid
      
      Differential Revision: D2945668
      
      fb-gh-sync-id: 604976d25e5913ef01d591e23737d61dacfbfc86
      shipit-source-id: 604976d25e5913ef01d591e23737d61dacfbfc86
      72cab06f
  12. 17 Feb, 2016 4 commits
    • Nathan Bronson's avatar
      clang-format some code in preparation for real changes · 3e4e50d5
      Nathan Bronson authored
      Reviewed By: djwatson
      
      Differential Revision: D2945770
      
      fb-gh-sync-id: 9e4ee3b052b0bbb33ef796b8070edd24c6942589
      shipit-source-id: 9e4ee3b052b0bbb33ef796b8070edd24c6942589
      3e4e50d5
    • Lovro Puzar's avatar
      Add call_once, wrapper around std::call_once with a fast path · 13de7740
      Lovro Puzar authored
      Summary: std::call_once is a nice API but the current GCC implementation is slower than it needs to be.  Adding a header-only wrapper with an additional atomic bool.
      
      Reviewed By: luciang
      
      Differential Revision: D2938884
      
      fb-gh-sync-id: 5939c94fe62a1523053dcff26c880ecaec9e1150
      shipit-source-id: 5939c94fe62a1523053dcff26c880ecaec9e1150
      13de7740
    • Michael Lee's avatar
      Handle wrapvFull when IOV_MAX is not defined. · 8f7fb4fb
      Michael Lee authored
      Summary:--Perhaps this should move to Portability.h instead.  There is another instance of this in io/async/AsyncSocket.cpp.--
      
      Added `portability/SysUio.h` to handle the c-macro conversion into a usable value.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2940778
      
      fb-gh-sync-id: 897e44b430c02e5a7d826f3e8da9e4979b7b898c
      shipit-source-id: 897e44b430c02e5a7d826f3e8da9e4979b7b898c
      8f7fb4fb
    • Anirudh Ramachandran's avatar
      ECDSA async/offloading support for proxygen · 167e2483
      Anirudh Ramachandran authored
      Summary: Minor changes to AsyncSSLSocket for async crypto
      
      Reviewed By: siyengar
      
      Differential Revision: D2516765
      
      fb-gh-sync-id: 354baeb94e6f63e8d5cdf8455ff5ca49a6aa479c
      shipit-source-id: 354baeb94e6f63e8d5cdf8455ff5ca49a6aa479c
      167e2483
  13. 16 Feb, 2016 1 commit
    • Iaroslav Tverdokhlib's avatar
      Fix wrapvFull for the case when iovec* has size more than 1024 · 7488256f
      Iaroslav Tverdokhlib authored
      Summary: `folly::wrapvFull` fail if the passed in `iovec*` has more than 1024 (`IOV_MAX`) elements. In particular, it returns -1 with errno 22 [Invalid argument]. The fix is to limit maximum size of iovec* to IOV_MAX that is passed in to `readv/writev/...` in a single iteration of outer loop.
      
      Reviewed By: yfeldblum
      
      Differential Revision: D2935540
      
      fb-gh-sync-id: 6c0a073ac0b59db3d53fb4269b13ddfcc479efb1
      shipit-source-id: 6c0a073ac0b59db3d53fb4269b13ddfcc479efb1
      7488256f