Summary:
On MacOS compiling unittests with "make chek" shows the following compile
error:
```
../futures/test/RetryingTest.cpp:168:26: error: no matching function for call to 'min'
 newMemLimit.rlim_cur = std::min(1UL << 30, oldMemLimit.rlim_max);
```
Closes https://github.com/facebook/folly/pull/680

Reviewed By: elsteveogrande

Differential Revision: D5921433

Pulled By: yfeldblum

fbshipit-source-id: 236d8336f852750a983c2e268db8811d1a4ed9ee

Summary: Objective-C blocks are stack allocated, and unless there is a proper assignment it isn't retained and the memory is freed. Because folly::Function used to move, it would hold a reference, but after switch to a constructor by-value, it no longer does this and we see a use-after-free.

Reviewed By: yfeldblum, ericniebler

Differential Revision: D5888606

fbshipit-source-id: fe4cabb2f2ae289cce0e7429e0af3935ba314720

Summary:
[Folly] Let `ProcessReturnCode` be publicly constructible.

Via provided constructor functions, which limit how it may be constructed so that it is only constructible into a valid state.

Differential Revision: D5898739

fbshipit-source-id: 7490018adfc39408b4290248ef1220e8fd0238cb

Summary:
[Folly] Avoid tautological compare in `folly/experimental/symbolizer/`.

`x < 0` when `x` is unsigned is tautological and there are warnings against such comparisons. The underlying type of an unscoped enumerations without a fixed underlying type is not specified, and whether it is signed is also not specified; it could be unsigned, and is unsigned in practice in common cases.

Reviewed By: Orvid, eduardo-elizondo

Differential Revision: D5897792

fbshipit-source-id: 24d84f9bf2c61c907585e1b675c2bbf11ef1720b

Summary:
If a lock is stolen from fiber f1, and then fiber f2 is notified before f1 one
wakes up and discovers the crime, then f1 will clear notifiedFiber_ so that f2
thinks the lock was stolen from it as well and hence recurses back into lock().

Reviewed By: andriigrynenko

Differential Revision: D5896323

fbshipit-source-id: 528ec1ed983175d3e08f3dc07b69bbc783a86cfb

Summary:
folly/test/stl_tests/StlVectorTest.cpp was too slow under
mode/dev-asan, resulting in timeouts, and the relinquish test allocated
with malloc and freed with operator delete.  This diff reduces the vector
size for testing under ASAN, and fixes the allocation mismatch.

Reviewed By: Orvid

Differential Revision: D5890969

fbshipit-source-id: 49a9498f6c0c4b3c7165906efd1262e518fea810

Summary: This is more convenient to use (for example with an IOBufEqual matcher).

Reviewed By: yfeldblum

Differential Revision: D5882029

fbshipit-source-id: 6aa12f80479f40bcc2af64dc270fb0a9382983b5

Summary:
This adds a gdb deadlock detector script into a new directory in folly. I
chose to put it under the `experimental` directory and not the top-level
directory as we have only tested these scripts on x86_64 Linux and not other
types of platforms.

This diff includes:
- a README on the contents of this directory and how to use the scripts
- a script to detect deadlocks

gdb directory
---------------

This new directory will contain a collection of gdb scripts that we have
found helpful. These scripts use the [gdb extension Python API](https://sourceware.org/gdb/current/onlinedocs/gdb/Python.html#Python).

To run the scripts, fire up gdb and load a script with `source -v`. Example:

```
$ gdb -p 123456
(gdb) source -v ./folly/experimental/gdb/deadlock.py
Type "deadlock" to detect deadlocks.
(gdb) deadlock
Found deadlock!
...
```

deadlock detector script
----------------------------

Consider the following program that always deadlocks:

```
void deadlock3() {
  std::mutex m1, m2, m3;
  folly::Baton<> b1, b2, b3;

  auto t1 = std::thread([&m1, &m2, &b1, &b2] {
    std::lock_guard<std::mutex> g1(m1);
    b1.post();
    b2.wait();
    std::lock_guard<std::mutex> g2(m2);
  });

  auto t2 = std::thread([&m3, &m2, &b3, &b2] {
    std::lock_guard<std::mutex> g2(m2);
    b2.post();
    b3.wait();
    std::lock_guard<std::mutex> g3(m3);
  });

  auto t3 = std::thread([&m3, &m1, &b3, &b1] {
    std::lock_guard<std::mutex> g3(m3);
    b3.post();
    b1.wait();
    std::lock_guard<std::mutex> g1(m1);
  });

  t1.join();
  t2.join();
  t3.join();
}
```

Once the process has deadlocked, we can use gdb to find the threads and mutexes involved in the deadlock:

```
$ gdb -p 2174496
(gdb) source -v ./folly/experimental/gdb/deadlock.py
Type "deadlock" to detect deadlocks.
(gdb) deadlock
Found deadlock!
Thread 2 (LWP 2174497) is waiting on mutex (0x00007ffcff42a4c0) held by Thread 3 (LWP 2174498)
Thread 3 (LWP 2174498) is waiting on mutex (0x00007ffcff42a4f0) held by Thread 4 (LWP 2174499)
Thread 4 (LWP 2174499) is waiting on mutex (0x00007ffcff42a490) held by Thread 2 (LWP 2174497)
```

Reviewed By: yfeldblum

Differential Revision: D5860868

fbshipit-source-id: 020a32327a79bb066269fe08113695803ce06c7d

Summary:
We didn't previously benchmark the performance of `get()`, which was a strange
omission.

Reviewed By: akrieger

Differential Revision: D5863567

fbshipit-source-id: 468b249da9120fcb84f3303ac5e2157761b6369d

Summary:
Only for clang for the moment, as more work is needed for GCC.
MSVC builds have always had unused variable warnings enabled, I just hadn't gotten around to cleaning up all of the tests for it to be able to enable it.

Reviewed By: yfeldblum

Differential Revision: D5827840

fbshipit-source-id: ab503b5791fcc58d685b8327179b810880c5dea7

Summary: It was horribly broken in multiple ways. It didn't support input > 4gb on Windows, was truncating the entire table to chars, and simply couldn't be compiled because it was exceeding MSVC's max macro instantion depth of 255.

Reviewed By: yfeldblum

Differential Revision: D5827332

fbshipit-source-id: b08268c88db10c607b27231689b3ee3ec7553c1f

Summary: This makes it now both work, and compile, on Windows.

Reviewed By: yfeldblum

Differential Revision: D5811100

fbshipit-source-id: 5d6bfc04ed8e60417615da15bd197769e0c79c11

fix strange recursive std::is_constructible instantiation involving the Function converting constructor

Summary: In rare and obscure cases, the fact that `folly::Function`'s perfectly-forwarding, implicit converting constructor was SFINAE-ing on the argument's constructibility was causing a recursive template instantiation of std::is_constructible. Switch to passing the argument by value to avoid the need to test for constructibility.

Reviewed By: yfeldblum

Differential Revision: D5847034

fbshipit-source-id: ce2ad1d2490206c6cae84c17544bd9fcd6ff47bb

Summary: Coroutines are only available on MSVC when the /await compiler switch was passed, in which case the `_RESUMABLE_FUNCTIONS_SUPPORTED` macro is defined.

Reviewed By: Orvid

Differential Revision: D5837303

fbshipit-source-id: af3349646e8875e3eac1bc3bbf47203f41f0bbde

Summary:
await_resume is only called once, so this allows it to move the value out.
At the same time remove a redundant clear (but keep the existing requirement that the promise type is an OptionalPromise), and clean up the tests.
Also add a test to make sure the coroutine is cleaned up

Reviewed By: ericniebler

Differential Revision: D5834861

fbshipit-source-id: 7ad487e818969cdf6fe27c9e82931aa247daf4e4

Summary: getAutoUncompressionCodec() currently only allows unambiguous headers. Allow a single "terminal codec" to be called if all other codecs can't uncompress or throw.

Reviewed By: terrelln

Differential Revision: D5804833

fbshipit-source-id: 057cb6e13a48fea20508d5c028234afddf7435f6

Summary:
LTO has better optimization which results in the malloc() being optimized away in this function.

Note we don't use folly::dontOptimizeAway() because
that introduces a circular dependency between the
malloc and benchmark targets.

Reviewed By: marksantaniello, meyering, interwq

Differential Revision: D5840883

fbshipit-source-id: 5dd9b152f70b7a49f125b6d79b4dfa40f4cdac59

Summary:
NoHeap small_vector's no longer require a move constructor.

Also changes const -> constexpr in a bunch of places.

Reviewed By: nbronson

Differential Revision: D5804361

fbshipit-source-id: 0f00fee6d318fa9296612409805d4ffcfbadb974

Summary: Just a circular buffer in the middle of a pipeline.

Reviewed By: yfeldblum

Differential Revision: D5791551

fbshipit-source-id: 2808a53df9b8cd2a402da0678a6b4ed0e6ca6e00

Summary:
As described in the comment in `folly/stats/Histogram.h`, MSVC 2017.3 has issues with `/permissive-`.
The bug has been fixed upstream, it's just waiting for release.

These explicit template instantiations are never forward-declared anywhere, so, as long as `Histogram-defs.h` has been included, it's perfectly safe to just not do them.

https://developercommunity.visualstudio.com/content/problem/81223/incorrect-error-c5037-with-permissive.html

Reviewed By: yfeldblum

Differential Revision: D5828891

fbshipit-source-id: 4db8407c9d35aa5bb3f7b81aa24f611b5787fb6e

Summary: The namespace alias was trying to alias a namespace that doesn't exist unless Folly is being compiled with zlib support.

Reviewed By: terrelln

Differential Revision: D5810929

fbshipit-source-id: c659d8f775fc9b99e57cc95f5914418507e546fb

Summary:
Because the way the #if was written, the dummy crc32c_hw function accidentally
ended in folly namespace instead of folly::details as expected. Fix this.

Reviewed By: yfeldblum

Differential Revision: D5826187

fbshipit-source-id: ba0f2207a00bc21eda7a727f3f1025e1106d55f9

Summary:
The signed counters used in a couple of places can in theory (and practice)
overflow, yielding UB. Use unsigned counters instead.

Reviewed By: luciang, philippv

Differential Revision: D5818606

fbshipit-source-id: c5928b779e76b309b00b7f6308a220e2bf6cbf79

Summary: Revert D5787837. Breaks `onSet()`/`onUnset()` behavior.

Reviewed By: palmtenor

Differential Revision: D5817063

fbshipit-source-id: c7dea636fa60eb616d4ebe0a9d418bc96b3018ae

Summary: The test uses DCHECK, but does not include glog/logging.h.

Reviewed By: yfeldblum

Differential Revision: D5818647

fbshipit-source-id: fcb2630ac2b12acd1a7b84e228349b2887e976cd

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