Summary:
Create a new class, ConstructorCallback, that when included as a member in
another class (e.g., Foo) allows other functions to register and receive
callbacks every time any of Foo's constructors are called.

Main implementation concern is to be extremely lightweight (e.g., avoid locks)
assuming that most code calling this will be performance optimized (both
for CPU and mem).

Implement as a static array of functions:

1) Static array instead of dynamic (e.g., vector):
  vector occasionally resizes so avoid locking during resize
2) Don't allow removing callback functions:
  simplify the calling of funcitons, again to avoid locks and races
3) Use function callbacks instead of objects with an observer pattern
  (e.g., like AsyncSocket::LifecycleObserver) to ensure that there
  won't be races on shutdown.  Functions are static where classes can
  de-allocate at shutdown and since we don't allow un-registering the
  call back, it's not easy to remove the race for a class.

Reviewed By: simpkins

Differential Revision: D27056139

fbshipit-source-id: 0846e0d55124cfde2e15acbe18e01ca5e327e7df

Summary:
`DigestBuilder` caches the cpu id and invalidates it when it detects contention, but the critical section is small enough that it is possible for two thread to end up on a conflicting slot but not detect it for some time, causing cache-line pingponging in the meantime.

Switch to `cachedCurrent()` instead, which uses the number of accesses to invalidate the cache, and is successfully adopted by other high-concurrency primitives.

Reviewed By: yfeldblum

Differential Revision: D27726518

fbshipit-source-id: 05d70ad9f101df4a8797e44cc3cbe94cbd1cb7b8

Summary:
GmockHelpersTest.cpp is the only one that fails to build with GCC after this change.
Also only 5 tests are failing at this point.

Reviewed By: pixelb

Differential Revision: D27728248

fbshipit-source-id: ded994f466347a8826ba90429a13237c21d3af5e

Summary:
F14 map `iterator` doesn't define `operator==` or `operator!=` to itself.
Those member functions take `const_iterator` as the right-hand side,
relying on the ability to convert implicitly from `iterator` to
`const_iterator`. This interacts poorly with c++20's default equality
operators, triggering clang's `-Wambiguous-reversed-operator`.

This diff replaces the equality member functions with friends that use
the same left-hand and right-hand types.  iterator != const_iterator
will now compile by implicitly converting the left-hand-side and then
calling `operator!=(const_iterator const&, const_iterator const&)`,
rather than calling `iterator::operator!=(const_iterator const&)`.
This resolves the ambiguity in c++20, and should work fine on earlier
versions, although I have not tested it on anything except c++17.

Pull Request resolved: https://github.com/facebook/folly/pull/1544

Test Plan: does it build?

Reviewed By: yfeldblum

Differential Revision: D27479086

Pulled By: shixiao

fbshipit-source-id: 0d42dbdc056cec58a36de3a816883736a761d0f6

Summary: Consistently use `Observer` and `Snapshot` like everywhere else in these files instead of `folly::Observer::{Observer,Snapshot}`.

Reviewed By: yfeldblum

Differential Revision: D27685699

fbshipit-source-id: 7eca35b9d113d8d9cd8ca631be2ad54c827431ea

Summary:
Nothing uses the skipFraction in LockFreeRingBuffer::currentTail, and
floating point math for discrete fractions can be hard to reason
about, so remove this dead code.

Differential Revision: D27717906

fbshipit-source-id: b25e2bea25ab66c59e8c76a80d75140db474c46d

Summary: To facilitate debugging of T88337980

Reviewed By: rdavies

Differential Revision: D27610329

fbshipit-source-id: 6b01af24e2cfdda9afcbef10d2951ac071d2fd10

Summary:
- Observer implementation backed by hazptrs
- We were using TLObserver in a library our team maintains but were seeing global lock contention on process startup due to the TL https://fburl.com/scuba/offcpu/aud1qe8p (more details here https://fb.workplace.com/groups/wfi/permalink/5462384263810144/)
- Also might be a perf win when compared with TLObserver when using getLocalSnapshot (per micro benchmarks / strobelights in D27535141)

Reviewed By: yfeldblum

Differential Revision: D27565756

fbshipit-source-id: f345a34c5fd2c69fde6b98f698f79640016f6eee

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1556

Reviewed By: Orvid

Differential Revision: D27672765

Pulled By: yfeldblum

fbshipit-source-id: edcf9b300c737799399096a8dbc401b3ab49b88c

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1557

Differential Revision: D27701865

Pulled By: yfeldblum

fbshipit-source-id: f9ed96a8e9283a30f1f96d15b4b8166ade5d3321

Summary: To get this to link across platforms, requires moving the thread-locals into functions. To avoid curious performance hits, requires removing the field initializers.

Differential Revision: D27699178

fbshipit-source-id: 7617b8e4aedeccae4cd95ffe606d1414dc352705

Reviewed By: yfeldblum

Differential Revision: D27695785

fbshipit-source-id: da8666781e03667a0060e8a7daab4206f60fc71d

Summary: There is no sense to return the iterator that points to the outdated node that just was overwritten by `assign_if_equal` call.

Reviewed By: yfeldblum

Differential Revision: D27678357

fbshipit-source-id: f07ce21df6556819f7eb103a623d3f7a46e72072

Summary: DRY the `ShallowCopyRequestContextScopeGuard` ctor taking multiple items.

Differential Revision: D27698685

fbshipit-source-id: 7f23305855918c4248da11c15e5fab839592d372

Summary:
LockFreeRingBuffer is racy for nontrivial types. I could reproduce
ASAN failures with nontrivial types like std::string, and TSAN
failures with trivial types.

Require triviality by default, and sequence the turns, loads, and
stores with atomic_thread_fence.

Reviewed By: yfeldblum

Differential Revision: D27684743

fbshipit-source-id: e0b32855d3ed5c2edbd7ab72092f2c533071a70f

Summary:
In preparation for making LockFreeRingBuffer trivial-only, remove the
ability to convert types during reads and writes.

Reviewed By: yfeldblum

Differential Revision: D27684584

fbshipit-source-id: 092009b2852d000fa3821756d1ec4bfd2bf002c8

Summary:
`SequentialThreadId` is only ever instantiated with `std::atomic` so just make it no longer a template.

This permits moving the implementation into the source file.

Reviewed By: Mizuchi

Differential Revision: D27669406

fbshipit-source-id: ec2cddbff433df31434548b61111a34ca2da5cef

Summary:
Fix a bug in small_vector reported as a warning via gcc.

```
folly/small_vector.h: In instantiation of 'constexpr const size_t folly::small_vector<short int, 10>::MaxInline':
folly/small_vector.h:1183:56:   required from 'class folly::small_vector<short int, 10>'
bits/stl_stack.h:134:47:   required from 'class std::stack<short int, folly::small_vector<short int, 10> >'
folly/small_vector.h:462:36: error: division 'sizeof (short int*) / sizeof (short int)' does not compute the number of array elements [-Werror=sizeof-pointer-div]
  462 |       constexpr_max(sizeof(Value*) / sizeof(Value), RequestedMaxInline)};
      |                     ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
```

Reviewed By: yfeldblum

Differential Revision: D27688663

fbshipit-source-id: 8da5b017c597c542826e26f2c36b0e3b46e87ed9

Summary: Migrate the codebase away from the ad-hoc `folly::uint64ToBufferUnsafe` and to `folly::to_ascii_decimal` which is intended for these cases.

Reviewed By: WillerZ

Differential Revision: D27281875

fbshipit-source-id: 0c98749e4aed9c873853eed2221cf54a89279ff4

Summary:
It's not safe to expect that the executor is alive after a task is added to the queue (this task could be holding the last KeepAlive and when finished - it may unblock the executor shutdown).
The fix is to hold a KeepAlive if some operation has to be done after adding to the queue. However it's safe to avoid this in a case where executor thread count will never drop below 1 (otherwise not having active threads may result in a deadlock) and we already have the maximum number of threads running. This optimization should help avoid grabbing the KeepAlive on the fast path.

Differential Revision: D27584518

fbshipit-source-id: e1242e3f4c40cee4f7e0c6dfca39abe6d17415f1

Summary:
as titled, an ugly hack to support LABEL attribute for 1.7.0 which didn't exist.

getting rid of it now since oss support upgraded to 1.7.1

Differential Revision: D27536730

fbshipit-source-id: 58653e71cbf20a1cda0b7414f66b5f0014d89b84

Summary:
Addresses - at least partly - issue https://github.com/facebook/folly/issues/1550

We'd like to limit our exposure to additional libraries. Fmt is a new dependency. It doesn't look like we need it in actuality; gating the include based on whether the file exists or not makes it possible for clients (specifically React Native for Windows) to not need to fork Folly.

Pull Request resolved: https://github.com/facebook/folly/pull/1551

Reviewed By: yfeldblum

Differential Revision: D27531237

Pulled By: Orvid

fbshipit-source-id: 340a7ff49be81872aaf23044945a1e8ecd157546

Summary:
This can be used to avoid noisy logging if the process is in the middle of a crash: symbolizing the stack trace can take a long time, so we can end up interleaving the crash report with unrelated logs.

In particular, we can use this to guard monitoring code that is more likely to trip while the process is crashing (for example stall detectors).

Reviewed By: yfeldblum, philippv, luciang

Differential Revision: D27663380

fbshipit-source-id: e3bb40292c3c57579b3eb172847ca089b0f9b07a

Summary: They are observed to trigger pathological behavior.

Differential Revision: D27659117

fbshipit-source-id: 5e4e770e61ffde02075364f2247614cf5bbcf8ce

Reviewed By: Mizuchi

Differential Revision: D27559375

fbshipit-source-id: aad3cc1a5bef017a52d2530068ca18be2eb50cd1

Summary: as title, marking this import as manual will make it so autodeps won't double list it in the TARGETS file.

Reviewed By: yfeldblum, luciang

Differential Revision: D27418310

fbshipit-source-id: 0c32ff1ad0ad7997a83b050f7fe3cdc522f582cc

Summary: Allow callers to `IOBuf::takeOwnership()` to specify valid data in the middle of the underlying buffer.

Reviewed By: yfeldblum

Differential Revision: D27583632

fbshipit-source-id: 4651dec5e53c5eea341b2b4ad3782adfbe7ff779

Summary:
There appear always to be problems with `extern template struct`. In this case, some compilers either not emitting or dropping thread-local wrapper functions.

```
stderr: Undefined symbols for architecture x86_64:
  "thread-local wrapper routine for folly::AccessSpreader<std::__1::atomic>::cpuCache", referenced from:
      folly::AccessSpreader<std::__1::atomic>::cachedCurrent(unsigned long) in CacheLocality.cpp.o
  "thread-local wrapper routine for folly::SequentialThreadId<std::__1::atomic>::currentId", referenced from:
      folly::SequentialThreadId<std::__1::atomic>::get() in CacheLocality.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

Reviewed By: Mizuchi

Differential Revision: D27579816

fbshipit-source-id: b539816fb1f62d9e50e22d6cbc3c91a0d82ac629

Summary: Extract its content to an outline function and pass it the template immediates it needs.

Reviewed By: Mizuchi

Differential Revision: D27579474

fbshipit-source-id: 193c769d83f886b4fefea4a34118c44f7da08bd1

Summary: Adds a portability header for later ease of use when deciding to include external_deps libunwind in TARGETS files

Reviewed By: yfeldblum

Differential Revision: D27416623

fbshipit-source-id: fc42f010302b8caa280fdf5cbdf013ed1b23af04

Summary: Required for msvc 32-bit builds - the result of `to_ascii_port_clzll` is used in an arithmetic expression.

Reviewed By: SarahDesouky, Orvid

Differential Revision: D27585004

fbshipit-source-id: b84b5b6d6fe417f3c6adafcaf826505275aa827c

Reviewed By: yfeldblum

Differential Revision: D27575037

fbshipit-source-id: fa5817ee3fe1eca99e83f26a13e43ec4a4a6e7cf

Reviewed By: yfeldblum

Differential Revision: D27253836

fbshipit-source-id: a72531e0deca9f0f6582339d483ed56f2eb37d87

Summary: Add a CHECK() to the IOBuf constructor to ensure the underlying buffer does not include any bytes that have been poisoned by AddressSanitizer.

Reviewed By: yfeldblum

Differential Revision: D25627483

fbshipit-source-id: 5bca59e545c32ba7b61430d7cd4787712eecb204

Reviewed By: Orvid

Differential Revision: D27559344

fbshipit-source-id: cf90be5a19da87bffe72fb6646296dd46889bc90

Summary:
Close-on-exec is an important feature to avoid leaking file descriptors to spawned processes.

This diff adds `File::dupCloseOnExec()` function which is equivalent to `File::dup` function, but sets close-on-exec flag where supported (i. e. everywhere except Windows).

Practically most users want to have `closeOnExec = true` by default, but we need to preserve backwards compatibility, thus this diff leaves `dup` function as is.

Reviewed By: yfeldblum

Differential Revision: D27495214

fbshipit-source-id: 540deb2bc6c8fec626a0120bc20c345950dc8af7

Reviewed By: Orvid

Differential Revision: D27555992

fbshipit-source-id: d68dc183f7c7b7c64975ede21e6f3018575fb7c9

Reviewed By: Orvid

Differential Revision: D27537202

fbshipit-source-id: d1aa8ad81f3749129dd6f1795e788a0e55d9f06a

Summary: Between gnu and msvc compilers, the builtins and intrinsics `__builtin_clzll` and `__lzcnt64` have different return types. They are wrapped in a portability function in `to_ascii` which currently forces a return type, which return type could trip implicit-conversion warnings. Avoid forcing the return type to avoid tripping implicit-conversion warnings.

Reviewed By: luciang

Differential Revision: D27535348

fbshipit-source-id: 3339d5d04e095f936c18d2b39ac1fb4b32f8a810

Summary: The ambiguity is that the name appears as both `folly::to_ascii_size` and `folly::detail::to_ascii_size`, so that anything which imports both `folly` and `folly::detail` or which is within `folly::detail` may, depending on the particular compiler, find an unqualified call ambiguous.

Differential Revision: D27535255

fbshipit-source-id: 05db44e6b153d75b691d67c878785cab1655051d