Summary:
Fixed typo in doc comment related to `void toLowerAscii(char* str, size_t length)`.
This fixes the annoying xCode's warning `Parameter 'len' not found in the function declaration`.
Closes https://github.com/facebook/folly/pull/670

Reviewed By: yfeldblum

Differential Revision: D5815034

Pulled By: Orvid

fbshipit-source-id: 9580cd0bbd4924d4ed461a5435f525316e5c285a

Summary: I was wrong, MSVC's implementation has the same bug as everything else.

Reviewed By: yfeldblum

Differential Revision: D5810646

fbshipit-source-id: 9caabbbc2115f57b7e836bb85c9b108588c94ad9

Summary: const -> constexpr in folly::small_vector

Reviewed By: yfeldblum

Differential Revision: D5814264

fbshipit-source-id: 4631bb7f3f04906636e5a188d4aa0d33ad796a3c

Summary: Remove an unneeded autoconf flag, as it causes the build to fail on missing boost-context

Reviewed By: yfeldblum

Differential Revision: D5815646

fbshipit-source-id: cb6544593de9fbf8248506b09c56412b4635b30c

Summary: clang-format folly::small_vector.  No functional change.

Reviewed By: Orvid

Differential Revision: D5808357

fbshipit-source-id: d2ee831e25c60778c699214b875635c22854832d

Make RequestContext provider overridable in order to save cost of setContext() on each fiber context switch

Summary:
Each fiber context switch currently involves the cost of saving/restoring `RequestContext`.  Certain
fibers-based applications such as mcrouter don't use `RequestContext` at all, so updating the current
thread-global `RequestContext` on each fiber context switch is unnecessary overhead.  Avoid this cost
by allowing `FiberManager` to override the `RequestContext` provider at the start and end of each fiber drain
loop.

Reviewed By: andriigrynenko

Differential Revision: D5787837

fbshipit-source-id: ea9041ce228063c8701165366fd1e34132868d22

Summary: Spookyhash got moved a long time ago, and a duplicate test name was added with the migration of executors from wangle to folly.

Reviewed By: yfeldblum

Differential Revision: D5811321

fbshipit-source-id: 789b4d3187a2e1e21c6cf8297f733bb4dffdbd60

Summary: Original problem detected by compiling with `-Werror,-Wunused-value`. On further inspection the only place which uses this detail class ensures that the `max_size` parameter is a power of two already, so we can discard the logic to manipulate `max_size` and put a `CHECK` clause in its place to guard against future changes to the caller that break this assumption.

Reviewed By: yfeldblum

Differential Revision: D5799110

fbshipit-source-id: d21ed9ff196d54ef91e38254df8b1b88bbf29275

Summary: This field was only previously set if setupSessionCache was called on the wangle::ServerSSLContext

Reviewed By: yfeldblum

Differential Revision: D5776057

fbshipit-source-id: ecdbe83816fee2ecbe7ea1b26a67b682a571309a

Summary:
`toLowerAscii` is surprisingly hard to use with `std::string` because (until C++17) `data()` returns a `const char*`. In addition, it is not widely known that `s[0]` is legal even if the string is empty. This can lead to a variety of suboptimal code, from explicitly checking emptiness, to casting away the `const` (which causes UB).

Let's just have a simple overload for it.

Reviewed By: pixelb, philippv, yfeldblum

Differential Revision: D5801970

fbshipit-source-id: 407e2e9e66147a0662a5e09c75921255adff0702

Reviewed By: yfeldblum

Differential Revision: D5802013

fbshipit-source-id: 472b09da2be7234157a7886018004e2f4ed4f279

Summary: This more closely matches std optional and allows folly Optional to be a drop in replacement.

Reviewed By: aary, yfeldblum

Differential Revision: D5775541

fbshipit-source-id: f754c006429fa3c5a866b6b5ffdfd8883ec2dd4f

Summary:
Coroutines can be used to simulate the monadic "do" notion of Haskell. Use coroutines to enable monadic composition with folly::Optional.

```
Optional<int> f() {
  return 7;
}
Optional<int> g(int) {
  return folly::none;
}

void MonadicOptional() {
  Optional<int> r = []() -> Optional<int> {
    int x = co_await f();
    assert(x == 7);
    int y = co_await g(x);
    assert(false); // not executed
    co_return y;
  }();
  assert(!r.hasValue());
}
```

Reviewed By: yfeldblum

Differential Revision: D5763371

fbshipit-source-id: 9babc682244f38da7006d0b3a8444fd4efec1747

Summary:
I noticed this while reading the docs.

(Note: this ignores all push blocking failures!)

Reviewed By: lbrandy

Differential Revision: D5795548

fbshipit-source-id: 9ec5aa13a8913f217b3118e72714d02dc4a9407f

Summary:
I found a data race with TSAN while attempting to run a sanitizer version of my service:

```
WARNING: ThreadSanitizer: data race (pid=266)
  Read of size 8 at 0x7b58000c0c08 by thread T370:
    @ folly::Codel::overloaded(std::chrono::duration<long, std::ratio<1l, 1000000000l> >) at ./folly/executors/Codel.cpp:44
    @ apache::thrift::concurrency::ThreadManager::ImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::Worker<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::run() at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:119
    @ apache::thrift::concurrency::PthreadThread::threadMain(void*) at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:200
    @ __tsan_thread_start_func at crtstuff.c:?

  Previous write of size 8 at 0x7b58000c0c08 by thread T371:
    @ folly::Codel::overloaded(std::chrono::duration<long, std::ratio<1l, 1000000000l> >) at ./folly/executors/Codel.cpp:62
    @ apache::thrift::concurrency::ThreadManager::ImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::Worker<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::run() at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:119
    @ apache::thrift::concurrency::PthreadThread::threadMain(void*) at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:200
    @ __tsan_thread_start_func at crtstuff.c:?

  Location is heap block of size 744 at 0x7b58000c0c00 allocated by thread T314:
    @ operator new(unsigned long) at ??:?
    @ PriorityImplT at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:826
    @ void __gnu_cxx::new_allocator<apache::thrift::concurrency::PriorityThreadManager::PriorityImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > > >::construct<apache::thrift::concurrency::PriorityThreadManager::PriorityImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >, std::array<std::pair<std::shared_ptr<apache::thrift::concurrency::ThreadFactory>, unsigned long>, 5ul>&, bool&, unsigned long&>(apache::thrift::concurrency::PriorityThreadManager::PriorityImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >*, std::array<std::pair<std::shared_ptr<apache::thrift::concurrency::ThreadFactory>, unsigned long>, 5ul>&, bool&, unsigned long&)
    @ std::shared_ptr<apache::thrift::concurrency::PriorityThreadManager> apache::thrift::concurrency::PriorityThreadManager::newPriorityThreadManager<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >(std::array<unsigned long, 5ul> const&, bool, unsigned long) at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:1090
    @ std::shared_ptr<apache::thrift::concurrency::PriorityThreadManager> apache::thrift::concurrency::PriorityThreadManager::newPriorityThreadManager<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >(unsigned long, bool, unsigned long) at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:1100
    @ apache::thrift::ThriftServer::serve() at ./thrift/lib/cpp2/server/ThriftServer.cpp:475
    @ apache::thrift::server::TServer::run() at ./thrift/lib/cpp/server/TServer.h:186
    @ apache::thrift::concurrency::PthreadThread::threadMain(void*) at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:200
    @ __tsan_thread_start_func at crtstuff.c:?

  Thread T370 (tid=638, running) created by thread T314 at:
    @ pthread_create at ??:?
    @ apache::thrift::concurrency::PthreadThread::start() at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:108
    @ apache::thrift::concurrency::ThreadManager::ImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::addWorker(unsigned long) at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:185
    @ apache::thrift::concurrency::PriorityThreadManager::PriorityImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::start() at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:840
    @ apache::thrift::ThriftServer::setup() at ./thrift/lib/cpp2/server/ThriftServer.cpp:347
    @ apache::thrift::ThriftServer::serve() at ./thrift/lib/cpp2/server/ThriftServer.cpp:475
    @ apache::thrift::server::TServer::run() at ./thrift/lib/cpp/server/TServer.h:186
    @ apache::thrift::concurrency::PthreadThread::threadMain(void*) at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:200
    @ __tsan_thread_start_func at crtstuff.c:?

  Thread T371 (tid=639, running) created by thread T314 at:
    @ pthread_create at ??:?
    @ apache::thrift::concurrency::PthreadThread::start() at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:108
    @ apache::thrift::concurrency::ThreadManager::ImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::addWorker(unsigned long) at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:185
    @ apache::thrift::concurrency::PriorityThreadManager::PriorityImplT<folly::LifoSemImpl<std::atomic, folly::Baton<std::atomic, true, true> > >::start() at ./thrift/lib/cpp/concurrency/ThreadManager.tcc:840
    @ apache::thrift::ThriftServer::setup() at ./thrift/lib/cpp2/server/ThriftServer.cpp:347
    @ apache::thrift::ThriftServer::serve() at ./thrift/lib/cpp2/server/ThriftServer.cpp:475
    @ apache::thrift::server::TServer::run() at ./thrift/lib/cpp/server/TServer.h:186
    @ apache::thrift::concurrency::PthreadThread::threadMain(void*) at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:200
    @ __tsan_thread_start_func at crtstuff.c:?

  Thread T314 (tid=582, running) created by main thread at:
    @ pthread_create at ??:?
    @ apache::thrift::concurrency::PthreadThread::start() at ./thrift/lib/cpp/concurrency/PosixThreadFactory.cpp:108
    ...
```

Looks like there is a data race in how `codelMinDelay_` is used. I couldn't get `std::atomic` to compile with `std::chrono::nanoseconds`,
so I used `std::atomic<uint64_t>` and converted between `uint64_t` and time types appropriately.

Reviewed By: yfeldblum

Differential Revision: D5759588

fbshipit-source-id: 8213f3789808265ddfe5ab122f0f86490d0ea6ea

Summary:
For the initial cutover, just pull the copies in folly into the wangle namespace
The main problem with that approach is that forward declarations of wangle components no longer work, so I fixed those manually

ALSO, IMPORTANT: This is a great, once in a lifetime opportunity to rename/restructure these components. I have a few ideas that I'll noodle and share eventually, e.g. changing LifoSemMPMCQueue so it's just descriptive and not an enumeration of implementation details. Please chip in with yr ideas!

Reviewed By: yfeldblum

Differential Revision: D5694213

fbshipit-source-id: 4fc0ea9359d1216191676fc9729fb53a3f06339f

Summary:
- Current behavior compresses empty strings to empty strings. This is undesirable as decompression using underlying decompressor (side-stepping the codec) will fail. This change passes empty strings to the underlying compressor
- Decompressing empty string -> empty string was kept for backwards compatibility
- Fix `getUncompressedLength` for zlib

Reviewed By: terrelln, yfeldblum

Differential Revision: D5740034

fbshipit-source-id: 5a747ea4963dad872103209aa4410197f6c605db

Summary:
[Folly] Fix `constexpr_min` after {D5739715} (facebook/folly@6d7c6d55f0f4b7b75607608ef9037db58083368f).

Add new unit-tests for `constexpr_min` and `constexpr_max` to avoid silly mistakes like this in the future.

It was defined recursively in terms of `constexpr_max` rather than, as intended, in terns of `constexpr_min`. HT Mandar12.

Reviewed By: Orvid

Differential Revision: D5761831

fbshipit-source-id: 2dad5833e05679232b3c529d33325a0205c2e4e4

Reviewed By: meyering

Differential Revision: D5757469

fbshipit-source-id: ad70c47251993c79a502d30f312534e28c9241d4

Summary: Fulfilling starts destroying the evb in another thread which races with the socket closing on the evb.

Reviewed By: siyengar, ngoyal

Differential Revision: D5755271

fbshipit-source-id: ace37eee63e684c97ca0fe503293eee83514e0ac

Summary: title

Reviewed By: yfeldblum

Differential Revision: D5751299

fbshipit-source-id: d3967c2499da9a2c98c2862b9bdc9994c12edc76

Summary:
[Folly] Extract non-portability `constexpr` math functions to new header.

This new header, `folly/ConstexprMath.h`, is specifically for compile-time-computable math functions. We start with `min`, `max`, `abs`, and `log2`.

Included substantive changes:
* Add new tests for `constexpr_strlen`, which remains in the portability header.
* Make `constexpr_min` and `constexpr_max` variadic.
* Make `constexpr_log2` tail-recursive, remove `const_log2` in `FixedString.h`, and move the related comment.

Reviewed By: Orvid

Differential Revision: D5739715

fbshipit-source-id: 29d3cc846ce98bb4bdddcc8b0fa80e4d32075fe0

Summary:
When trying to build with targets files on OSX, I couldn't use the symbolizer because it needs StackTrace, which requires libunwind and elf. This makes it so that we only build on linux for now. This also makes it so that we set FOLLY_USE_SYMBOLIZER in autoconf, since that wasn't set before.

Does a few things surrounding usage of the symbolizer library:
- Introduce FOLLY_USE_SYMBOLIZER in folly-config.h and USE_SYMBOLIZER as an AM definition
-- Filter some code out of init and some other random libs that optionally need the symbolizer
- Fix libdwarf detection. Previously on a fresh ubuntu container, we didn't find libdwarf/dwarf.h, so we stopped trying before looking at dwarf.h

Reviewed By: yfeldblum

Differential Revision: D5644352

fbshipit-source-id: f0a3580c41122e5e8fdfd17a9fdbb0921be21401

Summary: <array> is sometimes necessary for using `std::array`. Fix up

Reviewed By: aary

Differential Revision: D5740866

fbshipit-source-id: 13bafadd26fdd0f2eff3513115b43811682e7cda

Summary:
Code was breaking in versions of C++ where constexpr functions are
required to have one line bodies

Reviewed By: yfeldblum, mzlee

Differential Revision: D5739062

fbshipit-source-id: 6c509f1daf77751d33ce9c173a0d7f1d3bd2a006

Dynamic MPMCQueue: Eliminate cases of enqueue indefinite blocking and failure in the extensible version that impossible under the default pre-allocated version

Summary:
Currently under the extensible version (Dynamic == true), some enqueue operations may block indefinitely or fail (return false) even though such outcomes are impossible under the default (Dynamic == false) pre-allocated version.

This diff eliminates such cases by changing the algorithms for the extensible version. Some of the high-level changes:
- The offset formula for an expansion guarantees that no enqueue operation left behind in a closed array does not have an existing dequeue operation that unblocks it. The old formula was 1 + max(head, tail). The new formula is max(head, current offset) + current capacity.
- Conditional operations validate state after the success of CAS.

Reviewed By: djwatson

Differential Revision: D5701013

fbshipit-source-id: 4917c5b35b7e2a2fddfd2e11fb5aeb478502137c

Summary:
[Folly] `constexpr_log2`.

Useful for anything that needs to compute log2 at compile time.

Reviewed By: eduardo-elizondo

Differential Revision: D5734727

fbshipit-source-id: 8eab7991eea2104570eecd8e84ede6160bb0b549

Summary: [Folly] Shrink `MicroSpinLock.h` transitive includes and inline methods.

Reviewed By: meyering

Differential Revision: D5732693

fbshipit-source-id: 386816f0d97c145ff8a4180d41c8a682694aa6cb

Summary: This comment is outdated.

Reviewed By: yfeldblum

Differential Revision: D5730049

fbshipit-source-id: f6088c4354210fafb019656cdf8246abf90fbc06

Summary:
Before this change, future context will not be cleaned up until timekeeper times out. These objects has been occupying memory when more shorter future tasks are registered.
Switch to use weak ptr to hold context, so that context objects are deallocated as soon as the future completes (or times out)

Reviewed By: yfeldblum

Differential Revision: D5692040

fbshipit-source-id: b3b74a29b2ccafef6c4a06011699b069feb3a847

Summary:
Let clients choose whether to use fibers or not, but default
to the same autodetected setting as prior logic.

Reviewed By: yfeldblum

Differential Revision: D5585605

fbshipit-source-id: 9a4bf9f9bc23b4e15601f30c40602f21798c9685

Summary:
MSVC warns this might be used but uninitialized. Force
the struct to be zero initialized.

Reviewed By: yfeldblum

Differential Revision: D5585320

fbshipit-source-id: 9454a2a4a66c7689f42b1eb211dc57f5d3b88fea

Summary:
Off-by-one error in DIR::open() would result in paths not
ending in a trailing separator to fail to open. Fix the arithmetic.

Reviewed By: Orvid

Differential Revision: D5579657

fbshipit-source-id: 79507bc398549033eb26b2ffa788d66241deb623

Summary:
This implementation should suffice for MSVC. _ReadWriteBarrier()
generates no instructions but instructs the compiler not to reorder accesses
around it, and we have to rely on MSVC not currently having any other
optimizations for const members to worry about.

Reviewed By: yfeldblum

Differential Revision: D5723782

fbshipit-source-id: a68caa4673a5c410b326c1d2e3c0752bd945f4a6

Summary: [Folly] Apply `clang-format` to `folly/experimental/exception_tracer/`.

Reviewed By: Orvid

Differential Revision: D5722994

fbshipit-source-id: 2e52c920119ba58d123aaffd59491d7561c37c17

Summary:
This reverts commit 1744685ff9fa8d3620aef2545c8fe3ebc481df06

bypass-lint

Differential Revision: D5714883

fbshipit-source-id: 0cdb5f2cac095a9202cb1310ff8e75e75b4afd74

Summary: This enables the last of the magic internally.

Reviewed By: yfeldblum

Differential Revision: D5719795

fbshipit-source-id: fb59a0d7873e9a9f3b73f556b42b647091ca7e67

Summary: [Folly] Shrink `MicroSpinLock.h` transitive includes and inline methods.

Reviewed By: Orvid

Differential Revision: D5714883

fbshipit-source-id: 1744685ff9fa8d3620aef2545c8fe3ebc481df06

Summary: Currently FunctionScheduler calls that cancel/restart timer for a function of particular id are O(n). By introducing hashmap that translate id to pointer of particular RepeatFunc, we make it O(1).

Reviewed By: simpkins

Differential Revision: D5668557

fbshipit-source-id: e5e8bf9bd75b6d5d42f0bfa398d476703e5801fa

Summary:
The current definition of void_t was leading to errors because unused
template parameters are ignored and SFINAE SFIAEs

Reviewed By: yfeldblum

Differential Revision: D5700825

fbshipit-source-id: d23336070c217e8594980d6db710cb417b014236