aio: task_abort was a mistake (fixes #2208)

The use of task_abort to prematurely fail an aio at scheduling
time was a mistake, because it could have led to duplicate calls
to nng_aio_finish().  We do need to ensure that we leave an indicator
so that nni_aio_schedule can return the abort status to caller,
in the case that abort is called between the nni_aio_begin and
nni_aio_schedule calls.

This is more or less the same changes for main in commit 3ca7bcc0edd0f26c33264d32e7b6f07276e72e3c
but backported to stable. (A hand backport was needed.)

Author: gdamore

src/core/aio.c

@@ -201,8 +201,6 @@ nni_aio_stop(nni_aio *aio)
 
 		if (fn != NULL) {
 			fn(aio, arg, NNG_ECANCELED);
-		} else {
-			nni_task_abort(&aio->a_task);
 		}
 
 		nni_aio_wait(aio);
@@ -228,8 +226,6 @@ nni_aio_close(nni_aio *aio)
 
 		if (fn != NULL) {
 			fn(aio, arg, NNG_ECLOSED);
-		} else {
-			nni_task_abort(&aio->a_task);
 		}
 	}
 }
@@ -347,6 +343,7 @@ nni_aio_begin(nni_aio *aio)
 	aio->a_result    = 0;
 	aio->a_count     = 0;
 	aio->a_cancel_fn = NULL;
+	aio->a_abort     = false;
 
 	// We should not reschedule anything at this point.
 	if (aio->a_stop) {
@@ -373,7 +370,6 @@ nni_aio_schedule(nni_aio *aio, nni_aio_cancel_fn cancel, void *data)
 		// Convert the relative timeout to an absolute timeout.
 		switch (aio->a_timeout) {
 		case NNG_DURATION_ZERO:
-			nni_task_abort(&aio->a_task);
 			return (NNG_ETIMEDOUT);
 		case NNG_DURATION_INFINITE:
 		case NNG_DURATION_DEFAULT:
@@ -386,8 +382,13 @@ nni_aio_schedule(nni_aio *aio, nni_aio_cancel_fn cancel, void *data)
 	}
 
 	nni_mtx_lock(&eq->eq_mtx);
+	if (aio->a_abort) {
+		int rv = aio->a_result;
+		nni_mtx_unlock(&eq->eq_mtx);
+		return (rv);
+	}
+
 	if (aio->a_stop) {
-		nni_task_abort(&aio->a_task);
 		nni_mtx_unlock(&eq->eq_mtx);
 		return (NNG_ECLOSED);
 	}
@@ -419,13 +420,17 @@ nni_aio_abort(nni_aio *aio, int rv)
 	arg               = aio->a_cancel_arg;
 	aio->a_cancel_fn  = NULL;
 	aio->a_cancel_arg = NULL;
+	if (fn == NULL) {
+		// We haven't been scheduled yet,
+		// so make sure that the schedule will abort.
+		aio->a_abort  = true;
+		aio->a_result = rv;
+	}
 	nni_mtx_unlock(&eq->eq_mtx);
 
 	// Stop any I/O at the provider level.
 	if (fn != NULL) {
 		fn(aio, arg, rv);
-	} else {
-		nni_task_abort(&aio->a_task);
 	}
 }
 

src/core/aio.h

@@ -223,6 +223,7 @@ struct nng_aio {
 	bool         a_expire_ok;  // Expire from sleep is ok
 	bool         a_expiring;   // Expiration in progress
 	bool         a_use_expire; // Use expire instead of timeout
+	bool         a_abort;      // Abort the operation.
 	nni_task     a_task;
 
 	// Read/write operations.