io: add QIOTask class for async operations

A number of I/O operations need to be performed asynchronously
to avoid blocking the main loop. The caller of such APIs need
to provide a callback to be invoked on completion/error and
need access to the error, if any. The small QIOTask provides
a simple framework for dealing with such probes. The API
docs inline provide an outline of how this is to be used.

Some functions don't have the ability to run asynchronously
(eg getaddrinfo always blocks), so to facilitate their use,
the task class provides a mechanism to run a blocking
function in a thread, while triggering the completion
callback in the main event loop thread. This easily allows
any synchronous function to be made asynchronous, albeit
at the cost of spawning a thread.

In this series, the QIOTask class will be used for things like
the TLS handshake, the websockets handshake and TCP connect()
progress.

The concept of QIOTask is inspired by the GAsyncResult
interface / GTask class in the GIO libraries. The min
version requirements on glib don't allow those to be
used from QEMU, so QIOTask provides a facsimilie which
can be easily switched to GTask in the future if the
min version is increased.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

2 files changed, 160 insertions, 0 deletions

diff --git a/io/Makefile.objs b/io/Makefile.objs
index b02ea908ef..503b95c30b 100644
--- a/io/Makefile.objs
+++ b/io/Makefile.objs
@@ -1,2 +1,3 @@
 io-obj-y = channel.o
 io-obj-y += channel-watch.o
+io-obj-y += task.o
diff --git a/io/task.c b/io/task.c
new file mode 100644
index 0000000000..3127fca771
--- /dev/null
+++ b/io/task.c
@@ -0,0 +1,159 @@
+/*
+ * QEMU I/O task
+ *
+ * Copyright (c) 2015 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "io/task.h"
+#include "qemu/thread.h"
+#include "trace.h"
+
+struct QIOTask {
+    Object *source;
+    QIOTaskFunc func;
+    gpointer opaque;
+    GDestroyNotify destroy;
+};
+
+
+QIOTask *qio_task_new(Object *source,
+                      QIOTaskFunc func,
+                      gpointer opaque,
+                      GDestroyNotify destroy)
+{
+    QIOTask *task;
+
+    task = g_new0(QIOTask, 1);
+
+    task->source = source;
+    object_ref(source);
+    task->func = func;
+    task->opaque = opaque;
+    task->destroy = destroy;
+
+    trace_qio_task_new(task, source, func, opaque);
+
+    return task;
+}
+
+static void qio_task_free(QIOTask *task)
+{
+    if (task->destroy) {
+        task->destroy(task->opaque);
+    }
+    object_unref(task->source);
+
+    g_free(task);
+}
+
+
+struct QIOTaskThreadData {
+    QIOTask *task;
+    QIOTaskWorker worker;
+    gpointer opaque;
+    GDestroyNotify destroy;
+    Error *err;
+    int ret;
+};
+
+
+static gboolean gio_task_thread_result(gpointer opaque)
+{
+    struct QIOTaskThreadData *data = opaque;
+
+    trace_qio_task_thread_result(data->task);
+    if (data->ret == 0) {
+        qio_task_complete(data->task);
+    } else {
+        qio_task_abort(data->task, data->err);
+    }
+
+    error_free(data->err);
+    if (data->destroy) {
+        data->destroy(data->opaque);
+    }
+
+    g_free(data);
+
+    return FALSE;
+}
+
+
+static gpointer qio_task_thread_worker(gpointer opaque)
+{
+    struct QIOTaskThreadData *data = opaque;
+
+    trace_qio_task_thread_run(data->task);
+    data->ret = data->worker(data->task, &data->err, data->opaque);
+    if (data->ret < 0 && data->err == NULL) {
+        error_setg(&data->err, "Task worker failed but did not set an error");
+    }
+
+    /* We're running in the background thread, and must only
+     * ever report the task results in the main event loop
+     * thread. So we schedule an idle callback to report
+     * the worker results
+     */
+    trace_qio_task_thread_exit(data->task);
+    g_idle_add(gio_task_thread_result, data);
+    return NULL;
+}
+
+
+void qio_task_run_in_thread(QIOTask *task,
+                            QIOTaskWorker worker,
+                            gpointer opaque,
+                            GDestroyNotify destroy)
+{
+    struct QIOTaskThreadData *data = g_new0(struct QIOTaskThreadData, 1);
+    QemuThread thread;
+
+    data->task = task;
+    data->worker = worker;
+    data->opaque = opaque;
+    data->destroy = destroy;
+
+    trace_qio_task_thread_start(task, worker, opaque);
+    qemu_thread_create(&thread,
+                       "io-task-worker",
+                       qio_task_thread_worker,
+                       data,
+                       QEMU_THREAD_DETACHED);
+}
+
+
+void qio_task_complete(QIOTask *task)
+{
+    task->func(task->source, NULL, task->opaque);
+    trace_qio_task_complete(task);
+    qio_task_free(task);
+}
+
+void qio_task_abort(QIOTask *task,
+                    Error *err)
+{
+    task->func(task->source, err, task->opaque);
+    trace_qio_task_abort(task);
+    qio_task_free(task);
+}
+
+
+Object *qio_task_get_source(QIOTask *task)
+{
+    object_ref(task->source);
+    return task->source;
+}