construct/ircd/fs_aio.cc

// Matrix Construct
//
// Copyright (C) Matrix Construct Developers, Authors & Contributors
// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

#include <sys/syscall.h>
#include <sys/eventfd.h>
#include "fs_aio.h"

///////////////////////////////////////////////////////////////////////////////
//
// ircd/fs/aio.h

/// True if IOCB_CMD_FSYNC is supported by AIO. If this is false then
/// fs::fsync_opts::async=true flag is ignored.
decltype(ircd::fs::support::aio_fsync)
ircd::fs::support::aio_fsync
{
	#if defined(RWF_SYNC)
		info::kernel_version[0] > 4 ||
		(info::kernel_version[0] >= 4 && info::kernel_version[1] >= 18)
	#else
		false
	#endif
};

/// True if IOCB_CMD_FDSYNC is supported by AIO. If this is false then
/// fs::fsync_opts::async=true flag is ignored.
decltype(ircd::fs::support::aio_fdsync)
ircd::fs::support::aio_fdsync
{
	#if defined(RWF_DSYNC)
		info::kernel_version[0] > 4 ||
		(info::kernel_version[0] >= 4 && info::kernel_version[1] >= 18)
	#else
		false
	#endif
};

decltype(ircd::fs::aio::max_events)
ircd::fs::aio::max_events
{
	{ "name",     "ircd.fs.aio.max_events"  },
	{ "default",  0L                        },
	{ "persist",  false                     },
};

decltype(ircd::fs::aio::max_submit)
ircd::fs::aio::max_submit
{
	{ "name",     "ircd.fs.aio.max_submit"  },
	{ "default",  0L                        },
	{ "persist",  false                     },
};

decltype(ircd::fs::aio::submit_coalesce)
ircd::fs::aio::submit_coalesce
{
	{ "name",     "ircd.fs.aio.submit.coalesce"  },
	{ "default",  true                           },
	{ "description",

	R"(
	Enable coalescing to briefly delay the submission of a request, under
	certain conditions, allowing other contexts to submit additional requests.
	All requests are submitted to the kernel at once, allowing the disk
	controller to plot the most efficient route of the head to satisfy all
	requests with the lowest overall latency. Users with SSD's do not require
	this and latency may be improved by setting it to false, though beware
	of increasing system call overhead.
	)"}
};

//
// init
//

ircd::fs::aio::init::init()
{
	assert(!system);
	if(!aio::enable)
		return;

	// Don't init AIO if the io_uring is established. If it is, that means it
	// was supported by the build, this kernel, and didn't encounter an error
	// to construct. In all other cases AIO can serve as a fallback.
	#if defined(IRCD_USE_IOU)
	if(iou::system)
		return;
	#endif

	// We don't know which storage device (if any one) will be used by this
	// application, and we only have one aio instance shared by everything.
	// To deal with this for now, we look for the most favorable device and
	// tune to it. The caveat here is that if the application makes heavy use
	// of an inferior device on the same system, it wont be optimally utilized.
	if(max_events == 0UL)
		fs::dev::for_each("disk", []
		(const ulong &id, const fs::dev::blk &device)
		{
			max_events._value = std::clamp
			(
				device.queue_depth, size_t(max_events), MAX_EVENTS
			);

			return true;
		});

	// If max_events is still not determined here set a sane default.
	if(max_events == 0UL)
	{
		static const auto MAX_EVENTS_DEFAULT {32UL};
		max_events._value = std::min(MAX_EVENTS, MAX_EVENTS_DEFAULT);
	}

	assert(max_events);
	system = new struct aio::system
	(
		size_t(max_events),
		size_t(max_submit)
	);
}

ircd::fs::aio::init::~init()
noexcept
{
	delete system;
	system = nullptr;
}

///////////////////////////////////////////////////////////////////////////////
//
// ircd/fs/op.h
//
// The contents of this section override weak symbols in ircd/fs.cc when this
// unit is conditionally compiled and linked on AIO-supporting platforms.

ircd::fs::op
ircd::fs::aio::translate(const int &val)
{
	switch(val)
	{
		case IOCB_CMD_PREAD:     return op::READ;
		case IOCB_CMD_PWRITE:    return op::WRITE;
		case IOCB_CMD_FSYNC:     return op::SYNC;
		case IOCB_CMD_FDSYNC:    return op::SYNC;
		case IOCB_CMD_NOOP:      return op::NOOP;
		case IOCB_CMD_PREADV:    return op::READ;
		case IOCB_CMD_PWRITEV:   return op::WRITE;
	}

	return op::NOOP;
}

///////////////////////////////////////////////////////////////////////////////
//
// fs_aio.h
//

//
// request::fsync
//

ircd::fs::aio::request::fsync::fsync(ctx::dock &waiter,
                                     const int &fd,
                                     const sync_opts &opts)
:request
{
	fd, &opts, &waiter
}
{
	assert(opts.op == op::SYNC);
	aio_lio_opcode = opts.metadata? IOCB_CMD_FSYNC : IOCB_CMD_FDSYNC;

	aio_buf = 0;
	aio_nbytes = 0;
	aio_offset = 0;
}

size_t
ircd::fs::aio::fsync(const fd &fd,
                     const sync_opts &opts)
{
	ctx::dock waiter;
	aio::request::fsync request
	{
		waiter, fd, opts
	};

	request.submit();
	const size_t bytes
	{
		request.complete()
	};

	return bytes;
}

//
// request::read
//

ircd::fs::aio::request::read::read(ctx::dock &waiter,
                                   const int &fd,
                                   const read_opts &opts,
                                   const const_iovec_view &iov)
:request
{
	fd, &opts, &waiter
}
{
	assert(opts.op == op::READ);
	aio_lio_opcode = IOCB_CMD_PREADV;

	aio_buf = uintptr_t(iov.data());
	aio_nbytes = iov.size();
	aio_offset = opts.offset;
}

size_t
ircd::fs::aio::read(const fd &fd,
                    const const_iovec_view &bufs,
                    const read_opts &opts)
{
	ctx::dock waiter;
	aio::request::read request
	{
		waiter, fd, opts, bufs
	};

	const scope_count cur_reads{stats.cur_reads};
	stats.max_reads = std::max(stats.max_reads, stats.cur_reads);

	request.submit();
	const size_t bytes
	{
		request.complete()
	};

	stats.bytes_read += bytes;
	stats.reads++;
	return bytes;
}

size_t
ircd::fs::aio::read(const vector_view<read_op> &op)
{
	const size_t &num(op.size());
	const size_t numbuf
	{
		std::accumulate(std::begin(op), std::end(op), 0UL, []
		(auto ret, const auto &op)
		{
			return ret += op.bufs.size();
		})
	};

	assert(num <=info::iov_max); // use as sanity limit on op count.
	assert(numbuf <= num * info::iov_max);
	aio::request::read request[num];
	struct ::iovec buf[numbuf];
	ctx::dock waiter;
	for(size_t i(0), b(0); i < num; b += op[i].bufs.size(), ++i)
	{
		assert(op[i].bufs.size() <= info::iov_max);
		assert(b + op[i].bufs.size() <= numbuf);
		assert(b <= numbuf);
		const iovec_view iov
		{
			buf + b, op[i].bufs.size()
		};

		assert(op[i].fd);
		assert(op[i].opts);
		new (request + i) request::read
		{
			waiter,
			*op[i].fd,
			*op[i].opts,
			make_iov(iov, op[i].bufs),
		};
	}

	// Update stats
	const scope_count cur_reads{stats.cur_reads, ushort(num)};
	stats.max_reads = std::max(stats.max_reads, stats.cur_reads);

	// Send requests
	for(size_t i(0); i < num; ++i)
		request[i].submit();

	// Recv results
	size_t ret(0);
	for(size_t i(0); i < num; ++i) try
	{
		op[i].ret = request[i].complete();
		assert(!op[i].ret || op[i].ret == buffers::size(op[i].bufs) || !op[i].opts->blocking);
		ret += op[i].ret;
		stats.bytes_read += op[i].ret;
		stats.reads++;
	}
	catch(const std::system_error &)
	{
		op[i].eptr = std::current_exception();
		op[i].ret = 0;
	}

	return ret;
}

//
// request::write
//

ircd::fs::aio::request::write::write(ctx::dock &waiter,
                                     const int &fd,
                                     const write_opts &opts,
                                     const const_iovec_view &iov)
:request
{
	fd, &opts, &waiter
}
{
	assert(opts.op == op::WRITE);
	aio_lio_opcode = IOCB_CMD_PWRITEV;

	aio_buf = uintptr_t(iov.data());
	aio_nbytes = iov.size();
	aio_offset = opts.offset;

	#if defined(RWF_APPEND)
	if(support::append && opts.offset == -1)
	{
		// AIO departs from pwritev2() behavior and EINVAL's on -1.
		aio_offset = 0;
		aio_rw_flags |= RWF_APPEND;
	}
	#endif

	#if defined(RWF_DSYNC)
	if(support::dsync && opts.sync && !opts.metadata)
		aio_rw_flags |= RWF_DSYNC;
	#endif

	#if defined(RWF_SYNC)
	if(support::sync && opts.sync && opts.metadata)
		aio_rw_flags |= RWF_SYNC;
	#endif

	#ifdef RWF_WRITE_LIFE_SHIFT
	if(support::rwf_write_life && opts.write_life)
		aio_rw_flags |= (opts.write_life << (RWF_WRITE_LIFE_SHIFT));
	#endif
}

size_t
ircd::fs::aio::write(const fd &fd,
                     const const_iovec_view &bufs,
                     const write_opts &opts)
{
	ctx::dock waiter;
	aio::request::write request
	{
		waiter, fd, opts, bufs
	};

	const size_t req_bytes
	{
		fs::bytes(request.iovec())
	};

	// track current write count
	const scope_count cur_writes{stats.cur_writes};
	stats.max_writes = std::max(stats.max_writes, stats.cur_writes);

	// track current write bytes count
	stats.cur_bytes_write += req_bytes;
	const unwind dec{[&req_bytes]
	{
		stats.cur_bytes_write -= req_bytes;
	}};

	// Make the request; ircd::ctx blocks here. Throws on error
	request.submit();
	const size_t bytes
	{
		request.complete()
	};

	// Does linux ever not complete all bytes for an AIO?
	assert(!opts.blocking || bytes == req_bytes);

	stats.bytes_write += bytes;
	stats.writes++;
	return bytes;
}

size_t
ircd::fs::aio::count_queued(const op &type)
{
	assert(system);
	const auto &qcount(system->qcount);
	return std::count_if(begin(system->queue), begin(system->queue)+qcount, [&type]
	(const iocb *const &iocb)
	{
		assert(iocb);
		return aio::translate(iocb->aio_lio_opcode) == type;
	});
}

bool
ircd::fs::aio::for_each_queued(const std::function<bool (const request &)> &closure)
{
	assert(system);
	for(size_t i(0); i < system->qcount; ++i)
		if(!closure(*reinterpret_cast<const request *>(system->queue[i]->aio_data)))
			return false;

	return true;
}

bool
ircd::fs::aio::for_each_completed(const std::function<bool (const request &)> &closure)
{
	assert(system && system->head);
	const auto &max{system->head->nr};
	volatile auto head(system->head->head);
	volatile const auto &tail(system->head->tail);
	for(; head != tail; ++head, head %= max)
		if(!closure(*reinterpret_cast<const request *>(system->ring[head].data)))
			return false;

	return true;
}

//
// request
//

ircd::fs::aio::request::request(const int &fd,
                                const struct opts *const &opts,
                                ctx::dock *const &waiter)
:iocb{0}
,retval{-2L}
,errcode{0L}
,opts{opts}
,waiter{waiter}
{
	assert(system);
	assert(ctx::current);

	aio_flags = IOCB_FLAG_RESFD;
	aio_resfd = system->resfd.native_handle();
	aio_fildes = fd;
	aio_data = uintptr_t(this);
	aio_reqprio = opts? reqprio(opts->priority) : 0;

	#if defined(RWF_HIPRI)
	if(support::hipri && aio_reqprio == reqprio(opts::highest_priority))
		aio_rw_flags |= RWF_HIPRI;
	#endif

	#if defined(RWF_NOWAIT)
	if(support::nowait && opts && !opts->blocking)
		aio_rw_flags |= RWF_NOWAIT;
	#endif
}

ircd::fs::aio::request::~request()
noexcept
{
	assert(aio_data == uintptr_t(this));
}

/// Cancel a request. The handler callstack is invoked directly from here
/// which means any callback will be invoked or ctx will be notified if
/// appropriate.
bool
ircd::fs::aio::request::cancel()
{
	assert(system);
	if(!system->cancel(*this))
		return false;

	stats.bytes_cancel += bytes(iovec());
	stats.cancel++;
	return true;
}

void
ircd::fs::aio::request::submit()
{
	assert(system);
	assert(ctx::current);

	// Update stats for submission phase
	const size_t submitted_bytes(bytes(iovec()));
	stats.bytes_requests += submitted_bytes;
	stats.requests++;

	const uint16_t &curcnt(stats.requests - stats.complete);
	stats.max_requests = std::max(stats.max_requests, curcnt);

	// Wait here until there's room to submit a request
	system->dock.wait([]
	{
		return system->request_avail() > 0;
	});

	// Submit to system
	system->submit(*this);
}

size_t
ircd::fs::aio::request::complete()
{
	// Wait for completion
	while(!wait());
	assert(completed());

	// Update stats for completion phase.
	const size_t submitted_bytes(bytes(iovec()));
	assert(retval <= ssize_t(submitted_bytes));
	stats.bytes_complete += submitted_bytes;
	stats.complete++;

	if(likely(retval != -1))
		return size_t(retval);

	assert(opts);
	const auto blocking
	{
		#if defined(RWF_NOWAIT)
			~aio_rw_flags & RWF_NOWAIT
		#else
			opts->blocking
		#endif
	};

	static_assert(EAGAIN == EWOULDBLOCK);
	if(!blocking && retval == -1 && errcode == EAGAIN)
		return 0UL;

	stats.errors++;
	stats.bytes_errors += submitted_bytes;
	thread_local char errbuf[512]; fmt::sprintf
	{
		errbuf, "fd:%d size:%zu off:%zd op:%u pri:%u #%lu",
		aio_fildes,
		aio_nbytes,
		aio_offset,
		aio_lio_opcode,
		aio_reqprio,
		errcode
	};

	throw std::system_error
	{
		make_error_code(errcode), errbuf
	};
}

/// Block the current context while waiting for results.
///
/// This function returns true when the request completes and it's safe to
/// continue. This function intercepts all exceptions and cancels the request
/// if it's appropriate before rethrowing; after which it is safe to continue.
///
/// If this function returns false it is not safe to continue; it *must* be
/// called again until it no longer returns false.
bool
ircd::fs::aio::request::wait()
try
{
	assert(waiter);
	waiter->wait([this]
	{
		return completed();
	});

	return true;
}
catch(...)
{
	// When the ctx is interrupted we're obliged to cancel the request
	// if it has not reached a completed state.
	if(completed())
		throw;

	// The handler callstack is invoked synchronously on this stack for
	// requests which are still in our userspace queue.
	if(queued())
	{
		cancel();
		throw;
	}

	// The handler callstack is invoked asynchronously for requests
	// submitted to the kernel; we *must* wait for that by blocking
	// ctx interrupts and terminations and continue to wait. The caller
	// must loop into this call again until it returns true or throws.
	return false;
}

bool
ircd::fs::aio::request::queued()
const
{
	return !for_each_queued([this]
	(const auto &request)
	{
		return &request != this; // true to continue and return true
	});
}

bool
ircd::fs::aio::request::completed()
const
{
	return retval >= -1L;
}

ircd::fs::const_iovec_view
ircd::fs::aio::request::iovec()
const
{
	return
	{
		reinterpret_cast<const ::iovec *>(aio_buf), aio_nbytes
	};
}

//
// system
//

decltype(ircd::fs::aio::system::eventfd_flags)
ircd::fs::aio::system::eventfd_flags
{
	EFD_CLOEXEC | EFD_NONBLOCK
};

//
// system::system
//

ircd::fs::aio::system::system(const size_t &max_events,
                              const size_t &max_submit)
try
:event
{
	max_events
}
,queue
{
	max_submit?: max_events
}
,resfd
{
	ios::get(), int(syscall(::eventfd, ecount, eventfd_flags))
}
,head
{
	[this]
	{
		aio_context *idp {nullptr};
		syscall<SYS_io_setup>(this->max_events(), &idp);
		return idp;
	}(),
	[](const aio_context *const &head)
	{
		syscall<SYS_io_destroy>(head);
	}
}
,ring
{
	reinterpret_cast<const io_event *>
	(
		reinterpret_cast<const uint8_t *>(head.get()) +
		sizeof(aio_context)
	)
}
{
	assert(head->magic == aio_context::MAGIC);
	if(unlikely(head->magic != aio_context::MAGIC))
		throw panic
		{
			"ircd::fs::aio kernel context structure magic:%u != %u",
			head->magic,
			aio_context::MAGIC,
		};

	assert(sizeof(aio_context) == head->header_length);
	if(unlikely(head->header_length != sizeof(*head)))
		throw panic
		{
			"ircd::fs::aio kernel context structure length:%u != %u",
			head->header_length,
			sizeof(*head),
		};

	log::info
	{
		log, "AIO id:%u fd:%d max_events:%zu max_submit:%zu compat:%x incompat:%x len:%u nr:%u",
		head->id,
		int(resfd.native_handle()),
		this->max_events(),
		this->max_submit(),
		head->compat_features,
		head->incompat_features,
		head->header_length,
		head->nr
	};
}
catch(const std::exception &e)
{
	log::error
	{
		log, "Error starting AIO context %p :%s",
		(const void *)this,
		e.what()
	};
}

ircd::fs::aio::system::~system()
noexcept try
{
	assert(qcount == 0);
	const ctx::uninterruptible::nothrow ui;

	interrupt();
	wait();

	boost::system::error_code ec;
	resfd.close(ec);
}
catch(const std::exception &e)
{
	log::critical
	{
		log, "Error shutting down AIO context %p :%s",
		(const void *)this,
		e.what()
	};
}

bool
ircd::fs::aio::system::interrupt()
{
	if(!resfd.is_open())
		return false;

	if(handle_set)
		resfd.cancel();
	else
		ecount = -1;

	return true;
}

bool
ircd::fs::aio::system::wait()
{
	if(!resfd.is_open())
		return false;

	log::debug
	{
		log, "Waiting for AIO context %p", this
	};

	dock.wait([this]
	{
		return ecount == uint64_t(-1);
	});

	assert(request_count() == 0);
	return true;
}

bool
ircd::fs::aio::system::cancel(request &request)
try
{
	assert(request.aio_data == uintptr_t(&request));
	assert(!request.completed() || request.queued());

	iocb *const cb
	{
		static_cast<iocb *>(&request)
	};

	const auto eit
	{
		std::remove(begin(queue), begin(queue) + qcount, cb)
	};

	const auto qcount
	{
		size_t(std::distance(begin(queue), eit))
	};

	// We know something was erased if the qcount no longer matches
	const bool erased_from_queue
	{
		this->qcount > qcount
	};

	// Make the qcount accurate again after any erasure.
	assert(!erased_from_queue || this->qcount == qcount + 1);
	assert(erased_from_queue || this->qcount == qcount);
	if(erased_from_queue)
	{
		this->qcount--;
		dock.notify_one();
		stats.cur_queued--;
	}

	// Setup an io_event result which we will handle as a normal event
	// immediately on this stack. We create our own cancel result if
	// the request was not yet submitted to the system so the handler
	// remains agnostic to our userspace queues.
	io_event result {0};
	if(erased_from_queue)
	{
		result.data = cb->aio_data;
		result.obj = uintptr_t(cb);
		result.res = -1;
		result.res2 = ECANCELED;
	} else {
		assert(!request.queued());
		syscall_nointr<SYS_io_cancel>(head.get(), cb, &result);
		in_flight--;
		stats.cur_submits--;
		dock.notify_one();
	}

	handle_event(result);
	return true;
}
catch(const std::system_error &e)
{
	assert(request.aio_data == uintptr_t(&request));
	log::critical
	{
		"AIO(%p) cancel(fd:%d size:%zu off:%zd op:%u pri:%u) #%lu :%s",
		this,
		request.aio_fildes,
		request.aio_nbytes,
		request.aio_offset,
		request.aio_lio_opcode,
		request.aio_reqprio,
		e.code().value(),
		e.what()
	};

	return false;
}

bool
ircd::fs::aio::system::submit(request &request)
{
	assert(request.opts);
	assert(qcount < queue.size());
	assert(qcount + in_flight < max_events());
	assert(request.aio_data == uintptr_t(&request));
	assert(!request.completed());
	const ctx::critical_assertion ca;

	queue.at(qcount++) = static_cast<iocb *>(&request);
	stats.cur_queued++;
	stats.max_queued = std::max(stats.max_queued, stats.cur_queued);
	assert(stats.cur_queued == qcount);

	// Determine whether this request will trigger a flush of the queue
	// and be submitted itself as well.
	const bool submit_now
	{
		// By default a request is not submitted to the kernel immediately
		// to benefit from coalescing unless one of the conditions is met.
		false

		// Submission coalescing is disabled by the configuration
		|| !aio::submit_coalesce

		// The nodelay flag is set by the user.
		|| request.opts->nodelay

		// The queue has reached its limits.
		|| qcount >= max_submit()
	};

	const size_t submitted
	{
		submit_now? submit() : 0
	};

	// Only post the chaser when the queue has one item. If it has more
	// items the chaser was already posted after the first item and will
	// flush the whole queue down to 0.
	if(qcount == 1)
	{
		static ios::descriptor descriptor
		{
			"ircd::fs::aio chase"
		};

		auto handler(std::bind(&system::chase, this));
		ircd::post(descriptor, std::move(handler));
	}

	return true;
}

/// The chaser is posted to the IRCd event loop after the first request.
/// Ideally more requests will queue up before the chaser reaches the front
/// of the IRCd event queue and executes.
void
ircd::fs::aio::system::chase()
noexcept try
{
	if(!qcount)
		return;

	const auto submitted
	{
		submit()
	};

	stats.chases++;
	assert(!qcount);
}
catch(const std::exception &e)
{
	terminate
	{
		panic
		{
			"AIO(%p) system::chase() qcount:%zu :%s", this, qcount, e.what()
		}
	};
}

/// The submitter submits all queued requests and resets our userspace queue
/// count down to zero.
size_t
ircd::fs::aio::system::submit()
noexcept try
{
	assert(qcount > 0);
	assert(in_flight + qcount <= MAX_EVENTS);
	assert(in_flight + qcount <= max_events());
	const bool idle
	{
		in_flight == 0
	};

	size_t submitted; do
	{
		submitted = io_submit();
	}
	while(qcount > 0 && !submitted);

	in_flight += submitted;
	qcount -= submitted;
	assert(!qcount);

	stats.submits += bool(submitted);
	stats.cur_queued -= submitted;
	stats.cur_submits += submitted;
	stats.max_submits = std::max(stats.max_submits, stats.cur_submits);
	assert(stats.cur_queued == qcount);
	assert(stats.cur_submits == in_flight);

	if(idle && submitted > 0 && !handle_set)
		set_handle();

	return submitted;
}
catch(const std::exception &e)
{
	ircd::terminate{ircd::error
	{
		"AIO(%p) system::submit() qcount:%zu :%s",
		this,
		qcount,
		e.what()
	}};

	__builtin_unreachable();
}

size_t
ircd::fs::aio::system::io_submit()
try
{
	#ifdef RB_DEBUG_FS_AIO_SUBMIT_BLOCKING
	const size_t count[3]
	{
		count_queued(op::READ),
		count_queued(op::WRITE),
		count_queued(op::SYNC),
	};

	prof::syscall_usage_warning warning
	{
		"fs::aio::system::submit(in_flight:%zu qcount:%zu r:%zu w:%zu s:%zu)",
		in_flight,
		qcount,
		count[0],
		count[1],
		count[2],
	};
	#endif

	assert(qcount > 0);
	const auto ret
	{
		syscall<SYS_io_submit>(head.get(), qcount, queue.data())
	};

	#ifdef RB_DEBUG_FS_AIO_SUBMIT_BLOCKING
	stats.stalls += warning.timer.sample() > 0;
	#endif

	assert(!qcount || ret > 0);
	return ret;
}
catch(const std::system_error &e)
{
	log::error
	{
		log, "AIO(%p): io_submit() inflight:%zu qcount:%zu :%s",
		this,
		in_flight,
		qcount,
		e.what()
	};

	switch(e.code().value())
	{
		// Manpages sez that EBADF is thrown if the fd in the FIRST iocb has
		// an issue.
		case int(std::errc::bad_file_descriptor):
			dequeue_one(e.code());
			return 0;

		case int(std::errc::invalid_argument):
		{
			dequeue_all(e.code());
			return 0;
		}
	}

	throw;
}

void
ircd::fs::aio::system::dequeue_all(const std::error_code &ec)
{
	while(qcount > 0)
		dequeue_one(ec);
}

void
ircd::fs::aio::system::dequeue_one(const std::error_code &ec)
{
	assert(qcount > 0);
	iocb *const cb(queue.front());
	std::rotate(begin(queue), begin(queue)+1, end(queue));
	stats.cur_queued--;
	qcount--;

	io_event result {0};
	assert(cb->aio_data == uintptr_t(static_cast<request *>(cb)));
	result.data = cb->aio_data;
	result.obj = uintptr_t(cb);
	result.res = -1;
	result.res2 = ec.value();
	handle_event(result);
}

void
ircd::fs::aio::system::set_handle()
try
{
	assert(!handle_set);
	handle_set = true;
	ecount = 0;

	const asio::mutable_buffers_1 bufs
	{
		&ecount, sizeof(ecount)
	};

	auto handler
	{
		std::bind(&system::handle, this, ph::_1, ph::_2)
	};

	resfd.async_read_some(bufs, ios::handle(handle_descriptor, std::move(handler)));
}
catch(...)
{
	handle_set = false;
	throw;
}

decltype(ircd::fs::aio::system::handle_descriptor)
ircd::fs::aio::system::handle_descriptor
{
	"ircd::fs::aio sigfd",

	// allocator; custom allocation strategy because this handler
	// appears to excessively allocate and deallocate 120 bytes; this
	// is a simple asynchronous operation, we can do better (and perhaps
	// even better than this below).
	[](auto &handler, const size_t &size)
	{
		assert(ircd::fs::aio::system);
		auto &system(*ircd::fs::aio::system);

		if(unlikely(!system.handle_data))
		{
			system.handle_size = size;
			system.handle_data = std::make_unique<uint8_t[]>(size);
		}

		assert(system.handle_size == size);
		return system.handle_data.get();
	},

	// no deallocation; satisfied by class member unique_ptr
	[](auto &handler, void *const &ptr, const auto &size) {},

	// continuation
	true,
};

/// Handle notifications that requests are complete.
void
ircd::fs::aio::system::handle(const boost::system::error_code &ec,
                              const size_t bytes)
noexcept try
{
	namespace errc = boost::system::errc;

	assert((bytes == 8 && !ec && ecount >= 1) || (bytes == 0 && ec));
	assert(!ec || ec.category() == asio::error::get_system_category());
	assert(handle_set);
	handle_set = false;

	switch(ec.value())
	{
		case errc::success:
			handle_events();
			break;

		case errc::interrupted:
			break;

		case errc::operation_canceled:
			throw ctx::interrupted();

		default:
			throw_system_error(ec);
	}

	if(in_flight > 0 && !handle_set)
		set_handle();
}
catch(const ctx::interrupted &)
{
	log::debug
	{
		log, "AIO context %p interrupted", this
	};

	ecount = -1;
	dock.notify_all();
}

void
ircd::fs::aio::system::handle_events()
noexcept try
{
	assert(!ctx::current);

	// The number of completed requests available in events[]. This syscall
	// is restarted by us on EINTR. After restart, it may or may not find any ready
	// events but it never blocks to do so.
	const auto count
	{
		syscall_nointr<SYS_io_getevents>(head.get(), 0, event.size(), event.data(), nullptr)
	};

	// The count should be at least 1 event. The only reason to return 0 might
	// be related to an INTR; this assert will find out and may be commented.
	//assert(count > 0);
	assert(count >= 0);

	in_flight -= count;
	stats.cur_submits -= count;
	stats.handles++;
	if(likely(count))
		dock.notify_one();

	for(ssize_t i(0); i < count; ++i)
		handle_event(event[i]);
}
catch(const std::exception &e)
{
	log::error
	{
		log, "AIO(%p) handle_events: %s",
		this,
		e.what()
	};
}

void
ircd::fs::aio::system::handle_event(const io_event &event)
noexcept try
{
	// The kernel always references the iocb in `event.obj`
	auto *const iocb
	{
		reinterpret_cast<struct ::iocb *>(event.obj)
	};

	// We referenced our request (which extends the same iocb anyway)
	// for the kernel to carry through as an opaque in `event.data`.
	auto *const request
	{
		reinterpret_cast<aio::request *>(event.data)
	};

	// Check that everything lines up.
	assert(request && iocb);
	assert(iocb == static_cast<struct ::iocb *>(request));
	assert(request->aio_data);
	assert(request->aio_data == event.data);
	assert(request->aio_data == iocb->aio_data);
	assert(request->aio_data == uintptr_t(request));

	// Assert that we understand the return-value semantics of this interface.
	assert(event.res2 >= 0);
	assert(event.res == -1 || event.res2 == 0);

	// Set result indicators
	request->retval = std::max(event.res, -1LL);
	request->errcode = event.res >= -1? event.res2 : std::abs(event.res);

	// Notify the waiting context. Note that we are on the main async stack
	// but it is safe to notify from here.
	assert(request->waiter);
	request->waiter->notify_one();
	stats.events++;
}
catch(const std::exception &e)
{
	log::critical
	{
		log, "Unhandled request(%lu) event(%p) error: %s",
		event.data,
		&event,
		e.what()
	};
}

size_t
ircd::fs::aio::system::request_avail()
const
{
	assert(request_count() <= max_events());
	return max_events() - request_count();
}

size_t
ircd::fs::aio::system::request_count()
const
{
	return qcount + in_flight;
}

size_t
ircd::fs::aio::system::max_submit()
const
{
	return queue.size();
}

size_t
ircd::fs::aio::system::max_events()
const
{
	return event.size();
}