beast support per-op cancellation

- websocket supports cancellation.
 - Iterating test for ws cancellation.
 - Only terminal cancellation is forwarded by default.
 - basic_stream supports cancellation.
 - supported cancellation is documented.
 - http cancellation additions.
 - Added cancellation_slot tests to http, utils and saved_handler.
 - Added post to write.cpp, to avoid SIGSEV in test.
 - Refresher describes cancellation in more detail.

include/boost/beast/http/impl/write.hpp

@@ -178,8 +178,18 @@ class write_op
     Stream& s_;
     serializer<isRequest, Body, Fields>& sr_;
     std::size_t bytes_transferred_ = 0;
+    net::cancellation_state st_{this->
+        beast::async_base<Handler, beast::executor_type<Stream>>
+            ::get_cancellation_slot()};
 
 public:
+    using cancellation_slot_type = net::cancellation_slot;
+    cancellation_slot_type get_cancellation_slot() const noexcept
+    {
+        return st_.slot();
+    }
+
+
     template<class Handler_>
     write_op(
         Handler_&& h,
@@ -227,6 +237,8 @@ public:
                         s_, sr_, std::move(*this));
                 }
                 bytes_transferred_ += bytes_transferred;
+                if (!ec && st_.cancelled() != net::cancellation_type::none)
+                    ec = net::error::operation_aborted;
                 if(ec)
                     goto upcall;
                 if(Predicate{}(sr_))

include/boost/beast/http/read.hpp

@@ -200,6 +200,19 @@ read_some(
     @note The completion handler will receive as a parameter the total number
     of bytes transferred from the stream. This may be zero for the case where
     there is sufficient pre-existing message data in the dynamic buffer.
+
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
 */
 template<
     class AsyncReadStream,
@@ -397,6 +410,19 @@ read_header(
     there is sufficient pre-existing message data in the dynamic buffer. The
     implementation will call @ref basic_parser::eager with the value `false`
     on the parser passed in.
+
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
 */
 template<
     class AsyncReadStream,
@@ -594,6 +620,19 @@ read(
     there is sufficient pre-existing message data in the dynamic buffer. The
     implementation will call @ref basic_parser::eager with the value `true`
     on the parser passed in.
+
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
 */
 template<
     class AsyncReadStream,
@@ -800,6 +839,19 @@ read(
     there is sufficient pre-existing message data in the dynamic buffer. The
     implementation will call @ref basic_parser::eager with the value `true`
     on the parser passed in.
+
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
 */
 template<
     class AsyncReadStream,

include/boost/beast/http/write.hpp

@@ -161,6 +161,18 @@ write_some(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see serializer
 */
 template<
@@ -291,6 +303,18 @@ write_header(
     @note The implementation will call @ref serializer::split with
     the value `true` on the serializer passed in.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see serializer
 */
 template<
@@ -412,6 +436,18 @@ write(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see serializer
 */
 template<
@@ -637,6 +673,18 @@ write(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see message
 */
 template<
@@ -699,6 +747,18 @@ async_write(
     this function. Invocation of the handler will be performed in a
     manner equivalent to using `net::post`.
 
+    @par Per-Operation Cancellation
+
+    This asynchronous operation supports cancellation for the following
+    net::cancellation_type values:
+
+    @li @c net::cancellation_type::terminal
+
+    if the `stream` also supports terminal cancellation.
+
+    `terminal` cancellation leaves the stream in an undefined state,
+    so that only closing it is guaranteed to succeed.
+
     @see message
 */
 template<