[Qemu-block] [RFC patch 1/3] blockjob: add block_job_start_shim

[John Snow]

The purpose of this shim is to allow us to pause pre-started jobs.
The purpose of *that* is to allow us to buffer a pause request that
will be able to take effect before the job ever does any work, allowing
us to create jobs during a quiescent state (under which they will be
automatically paused), then resuming the jobs after the critical section
in any order, either:

(1) -block_job_start
    -block_job_resume (via e.g. drained_end)

(2) -block_job_resume (via e.g. drained_end)
    -block_job_start

The problem that requires a shim is the idea that a job must start in the
busy=true state only its first time-- all subsequent entries require busy
to be false, and the toggling of this state is otherwise handled during
existing pause and yield points.

The shim simply allows us to mandate that a job can "start," set busy to
true, then immediately pause only if necessary.

Signed-off-by: John Snow <address@hidden>
---
 blockjob.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/blockjob.c b/blockjob.c
index 69126af..6d08ce5 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -250,16 +250,28 @@ static bool block_job_started(BlockJob *job)
     return job->co;
 }
 
+/**
+ * This code must run after the job's coroutine has been entered,
+ * but not before.
+ */
+static void coroutine_fn block_job_start_shim(void *opaque)
+{
+    BlockJob *job = opaque;
+
+    assert(job && job->driver && job->driver->start);
+    block_job_pause_point(job);
+    job->driver->start(job);
+}
+
 void block_job_start(BlockJob *job)
 {
     assert(job && !block_job_started(job) && job->paused &&
-           !job->busy && job->driver->start);
-    job->co = qemu_coroutine_create(job->driver->start, job);
-    if (--job->pause_count == 0) {
-        job->paused = false;
-        job->busy = true;
-        qemu_coroutine_enter(job->co);
-    }
+           job->driver && job->driver->start);
+    job->co = qemu_coroutine_create(block_job_start_shim, job);
+    job->pause_count--;
+    job->busy = true;
+    job->paused = false;
+    qemu_coroutine_enter(job->co);
 }
 
 void block_job_ref(BlockJob *job)
-- 
2.9.3