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在取消 Buffer drain 任务时可能造成的 crash

asyncio/src/ev/buffer.cpp

Lines 272 to 282 in e650d90

    
           co_return co_await zero::async::coroutine::Cancellable{ 
        
                   zero::async::promise::chain<void, std::error_code>([this](const auto &promise) { 
        
                       mPromises[DRAIN_INDEX] = promise; 
        
                   }), 
        
                   [this]() -> tl::expected<void, std::error_code> { 
        
                       std::exchange(mPromises[DRAIN_INDEX], std::nullopt)->reject( 
        
                               make_error_code(std::errc::operation_canceled) 
        
                       ); 
        
                       return {}; 
        
                   } 
        
           };

有一种场景是使用 asyncio::copy 进行转发：

co_await zero::async::coroutine::race(asyncio::copy(local, remote), asyncio::copy(remote, local));

当任务 asyncio::copy(local, remote) 在 local->read 挂起，asyncio::copy(remote, local) 在 local->drain 挂起时，local 触发了关闭事件，由于 promise 现在是线性处理的：

asyncio/src/ev/buffer.cpp

Lines 428 to 449 in e650d90

    
           void asyncio::ev::Buffer::onClose(const std::error_code &ec) { 
        
               mClosed = true; 
        
               mLastError = ec; 
        
               auto [read, drain, waitClosed] = std::exchange(mPromises, {}); 
        
               if (read) 
        
                   read->reject(ec); 
        
               if (drain) 
        
                   drain->reject(ec); 
        
               if (!waitClosed) 
        
                   return; 
        
               if (ec != Error::IO_EOF) { 
        
                   waitClosed->reject(ec); 
        
                   return; 
        
               } 
        
               waitClosed->resolve(); 
        
           }

所以任务 asyncio::copy(local, remote) 首先由于 read promise 被拒绝而结束，但是此时 drain promise 还是 pending 状态。然后 race 尝试取消 asyncio::copy(remote, local)，但是此时 mPromises[DRAIN_INDEX] 已经为空了。

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	co_return co_await zero::async::coroutine::Cancellable{
	zero::async::promise::chain<void, std::error_code>([this](const auto &promise) {
	mPromises[DRAIN_INDEX] = promise;
	}),
	[this]() -> tl::expected<void, std::error_code> {
	std::exchange(mPromises[DRAIN_INDEX], std::nullopt)->reject(
	make_error_code(std::errc::operation_canceled)
	);
	return {};
	}
	};

	void asyncio::ev::Buffer::onClose(const std::error_code &ec) {
	mClosed = true;
	mLastError = ec;

	auto [read, drain, waitClosed] = std::exchange(mPromises, {});

	if (read)
	read->reject(ec);

	if (drain)
	drain->reject(ec);

	if (!waitClosed)
	return;

	if (ec != Error::IO_EOF) {
	waitClosed->reject(ec);
	return;
	}

	waitClosed->resolve();
	}

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