blockjob: Allow nested pause

This patch changes block_job_pause to increase the pause counter and
block_job_resume to decrease it.

The counter will allow calling block_job_pause/block_job_resume
unconditionally on a job when we need to suspend the IO temporarily.

From now on, each block_job_resume must be paired with a block_job_pause
to keep the counter balanced.

The user pause from QMP or HMP will only trigger block_job_pause once
until it's resumed, this is achieved by adding a user_paused flag in
BlockJob.

One occurrence of block_job_resume in mirror_complete is replaced with
block_job_enter which does what is necessary.

In block_job_cancel, the cancel flag is good enough to instruct
coroutines to quit loop, so use block_job_enter to replace the unpaired
block_job_resume.

Upon block job IO error, user is notified about the entering to the
pause state, so this pause belongs to user pause, set the flag
accordingly and expect a matching QMP resume.

[Extended doc comments as suggested by Paolo Bonzini
<pbonzini@redhat.com>.
--Stefan]

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1428069921-2957-2-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockjob.c

@@ -107,7 +107,7 @@ void block_job_set_speed(BlockJob *job, int64_t speed, Error **errp)
 
 void block_job_complete(BlockJob *job, Error **errp)
 {
-    if (job->paused || job->cancelled || !job->driver->complete) {
+    if (job->pause_count || job->cancelled || !job->driver->complete) {
         error_set(errp, QERR_BLOCK_JOB_NOT_READY,
                   bdrv_get_device_name(job->bs));
         return;
@@ -118,17 +118,26 @@ void block_job_complete(BlockJob *job, Error **errp)
 
 void block_job_pause(BlockJob *job)
 {
-    job->paused = true;
+    job->pause_count++;
 }
 
 bool block_job_is_paused(BlockJob *job)
 {
-    return job->paused;
+    return job->pause_count > 0;
 }
 
 void block_job_resume(BlockJob *job)
 {
-    job->paused = false;
+    assert(job->pause_count > 0);
+    job->pause_count--;
+    if (job->pause_count) {
+        return;
+    }
+    block_job_enter(job);
+}
+
+void block_job_enter(BlockJob *job)
+{
     block_job_iostatus_reset(job);
     if (job->co && !job->busy) {
         qemu_coroutine_enter(job->co, NULL);
@@ -138,7 +147,7 @@ void block_job_resume(BlockJob *job)
 void block_job_cancel(BlockJob *job)
 {
     job->cancelled = true;
-    block_job_resume(job);
+    block_job_enter(job);
 }
 
 bool block_job_is_cancelled(BlockJob *job)
@@ -258,7 +267,7 @@ BlockJobInfo *block_job_query(BlockJob *job)
     info->device    = g_strdup(bdrv_get_device_name(job->bs));
     info->len       = job->len;
     info->busy      = job->busy;
-    info->paused    = job->paused;
+    info->paused    = job->pause_count > 0;
     info->offset    = job->offset;
     info->speed     = job->speed;
     info->io_status = job->iostatus;
@@ -335,6 +344,8 @@ BlockErrorAction block_job_error_action(BlockJob *job, BlockDriverState *bs,
                                     IO_OPERATION_TYPE_WRITE,
                                     action, &error_abort);
     if (action == BLOCK_ERROR_ACTION_STOP) {
+        /* make the pause user visible, which will be resumed from QMP. */
+        job->user_paused = true;
         block_job_pause(job);
         block_job_iostatus_set_err(job, error);
         if (bs != job->bs) {