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bb38541d9e
We've done this a number of times already where we're getting exclusive access to either the out direction of a connection, or we try to lock out the read side while we are responding to a previous request. They usually are really cumbersome because we reach around to the other direction to stop it from proceeding, or we flag our exclusive access somewhere, and we always need to know whom to notify. PR ElementsProject/lightning#1970 adds two new instances of this: - Streaming a JSON response requires that nothing else should write while the stream is active. - We also want to stop reading new requests while we are responding to one. To remove the complexity of having to know whom to stop and notify when we're done, this adds a simple `io_lock` primitive that can be used to get exclusive access to a connection. This inverts the requirement for notifications, since everybody registers interest in the lock and they get notified if the lock holder releases it.
83 lines
2.1 KiB
C
83 lines
2.1 KiB
C
#include "io_lock.h"
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#include <assert.h>
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#include <ccan/io/io_plan.h>
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struct io_lock {
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bool locked;
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};
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/* Struct to hold information while we wait for the lock to be freed */
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struct io_lock_waiter {
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struct io_plan *(*next)(struct io_conn *conn, void *next_arg);
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void *arg;
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struct io_lock *lock;
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enum io_direction dir;
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};
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struct io_lock *io_lock_new(const tal_t *ctx)
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{
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struct io_lock *lock = tal(ctx, struct io_lock);
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lock->locked = false;
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return lock;
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}
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static struct io_plan *io_lock_try_acquire(struct io_conn *conn,
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struct io_lock_waiter *waiter)
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{
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/* Destructure waiter, since we might be freeing it below */
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struct io_plan *(*next)(struct io_conn *, void *) = waiter->next;
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void *next_arg = waiter->arg;
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if (!waiter->lock->locked) {
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waiter->lock->locked = true;
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tal_free(waiter);
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return next(conn, next_arg);
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} else {
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switch (waiter->dir) {
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case IO_IN:
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return io_wait(conn, waiter->lock, io_lock_try_acquire,
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waiter);
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case IO_OUT:
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return io_out_wait(conn, waiter->lock,
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io_lock_try_acquire, waiter);
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}
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/* Should not happen if waiter->dir is a valid enum
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* value */
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abort();
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}
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}
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static struct io_plan *io_lock_acquire_dir(
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struct io_conn *conn, struct io_lock *lock, enum io_direction dir,
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struct io_plan *(*next)(struct io_conn *, void *), void *arg)
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{
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/* FIXME: We can avoid one allocation if we lock and call next here directly */
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struct io_lock_waiter *waiter = tal(lock, struct io_lock_waiter);
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waiter->next = next;
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waiter->arg = arg;
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waiter->lock = lock;
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waiter->dir = dir;
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return io_lock_try_acquire(conn, waiter);
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}
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struct io_plan *
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io_lock_acquire_out_(struct io_conn *conn, struct io_lock *lock,
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struct io_plan *(*next)(struct io_conn *, void *), void *arg)
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{
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return io_lock_acquire_dir(conn, lock, IO_OUT, next, arg);
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}
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struct io_plan *
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io_lock_acquire_in_(struct io_conn *conn, struct io_lock *lock,
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struct io_plan *(*next)(struct io_conn *, void *), void *arg)
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{
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return io_lock_acquire_dir(conn, lock, IO_IN, next, arg);
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}
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void io_lock_release(struct io_lock *lock)
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{
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assert(lock->locked);
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lock->locked = false;
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io_wake(lock);
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}
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