Moving DestructorCheck from proxygen library to folly.

[folly.git] / folly / io / async / README.md

diff --git a/folly/io/async/README.md b/folly/io/async/README.md
index 3b09dd1761ec38d41596c6300bd7f4d662b72b6d..0918b3966cfb9aba582ff9c19d0c657d45b8928f 100644 (file)
--- a/folly/io/async/README.md
+++ b/folly/io/async/README.md
@@ -77,7 +77,7 @@ Unsupported libevent event types, and why-
 * TIMEOUT - this library has specific timeout support, instead of
   being attached to read/write fds.
 * SIGNAL - similarly, signals are handled separately, see
-  AsyncSignalHandler (TODO:currently in fbthrift)
+  AsyncSignalHandler
 * EV_ET - Currently all the implementations of EventHandler are set up
   for level triggered.  Benchmarking hasn't shown that edge triggered
   provides much improvement.
@@ -173,10 +173,8 @@ a lock on accept()ing from a port, preventing more than ~20k accepts /
 sec.  There are various workarounds (SO_REUSEPORT), but generally
 clients should be using connection pooling instead when possible.
 
-#### AsyncSSLServerSocket
-
-Similar to AsyncServerSocket, but provides callbacks for SSL
-handshaking.
+Since AsyncServerSocket provides an fd, an AsyncSSLSocket or
+AsyncSocket can be made using the same codepath
 
 #### TAsyncUDPServerSocket
 
@@ -251,19 +249,14 @@ per application.   Using HHWheelTimer instead can clean up the code quite
 a bit, because only a single HHWheelTimer is needed per thread, as
 opposed to one AsyncTimeoutSet per timeout time per thread.
 
-### TAsyncSignalHandler
-
-TODO: still in fbthrift
+### AsyncSignalHandler
 
 Used to handle AsyncSignals.  Similar to AsyncTimeout, for code
 clarity, we don't reuse the same fd as a socket to receive signals.
 
 ### AsyncPipe
 
-TODO: not currently open souce
-
 Async reads/writes to a unix pipe, to send data between processes.
-Why don't you just use AsyncSocket for now?
 
 ## Helper Classes
 
@@ -281,7 +274,7 @@ threads.  Major uses for this include:
 
 In this library only runInEventBaseThread save/restores the request
 context, although other Facebook libraries that pass requests between
-threads do also: folly::wangle::future, and fbthrift::ThreadManager, etc
+threads do also: folly::future, and fbthrift::ThreadManager, etc
 
 ### DelayedDestruction
 
@@ -300,6 +293,30 @@ type.  It's also pretty easy to forget to add a DestructorGuard in
 code that calls callbacks.  But it is well worth it to avoid queuing
 callbacks, and the improved P99 times as a result.
 
+### DestructorCheck
+
+Often for an object requesting callbacks from other components (timer,
+socket connect, etc.) there is a chance that the requestor will be
+deallocated before it'll receive the callback.  One of the ways to avoid
+dereferencing the deallocated object from callbacks is to derive the
+object from DelayedDestruction, and add a delayed destruction guard
+to the callback context.  In case if keeping the object around until
+all the requested callbacks fire is too expensive, or if the callback
+requestor can't have private destructor (it's allocated on the stack,
+or as a member of a larger object), DestructorCheck can be used.
+DestructorCheck is not affecting object life time. It helps other
+component to detect safely that the tracked object was deallocated.
+
+The object requesting the callback must be derived from DestructorCheck.
+The callback context should contain an instance of
+DestructorCheck::Safety object initialized with a reference to the
+object requesting the callback.  Safety object can be captured by value
+in the callback lambda, or explicitly added to a predefined callback
+context class. Multiple instances of Safety object can be instantiated
+for the same tracked object.  Once the callback is invoked, before
+dereferencing the requester object, callback code should make sure that
+`destroyed()` method for the corresponding Safety object returns false.
+
 ### EventBaseManager
 
 DANGEROUS.
@@ -357,4 +374,4 @@ Some best practices we've found:
    notify of overload, such as timeouts, or CPU usage.  For sync
    systems, you are almost always limited by the number of threads.
    For more details see [No Time for
-   Asynchrony](https://www.usenix.org/legacy/event/hotos09/tech/full_papers/aguilera/aguilera.pdf)
\ No newline at end of file
+   Asynchrony](https://www.usenix.org/legacy/event/hotos09/tech/full_papers/aguilera/aguilera.pdf)