Extend integration test to test sequence of FUSE operation

Original FUSE integration test has limited capabilities. To test more
situations, the new integration test framework introduces a protocol
to communicate between testing thread and the FUSE server. In summary,
this change includes:

1. Remove CompareResult() and break SetExpected() into
   SetServerResponse() and GetServerActualRequest(). We no longer set
   up an expected request because we want to retrieve the actual FUSE
   request made to the FUSE server and check in the testing thread.

2. Declare a serial buffer data structure to save the received requests
   and expected responses sequentially. The data structure contains a
   cursor to indicate the progress of accessing. This change makes
   sequential SetServerResponse() and GetServerActualRequest() possible.

3. Replace 2 single directional pipes with 1 bi-directional socketpair.
   A protocol which starts with FuseTestCmd is used between the testing
   thread and the FUSE server to provide various functionality.

Fixes #3405

1 files changed, 8 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/fuse/dev.go b/pkg/sentry/fsimpl/fuse/dev.go
index e522ff9a0..0efd2d90d 100644
--- a/pkg/sentry/fsimpl/fuse/dev.go
+++ b/pkg/sentry/fsimpl/fuse/dev.go
@@ -307,6 +307,14 @@ func (fd *DeviceFD) writeLocked(ctx context.Context, src usermem.IOSequence, opt
 
 // Readiness implements vfs.FileDescriptionImpl.Readiness.
 func (fd *DeviceFD) Readiness(mask waiter.EventMask) waiter.EventMask {
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+	return fd.readinessLocked(mask)
+}
+
+// readinessLocked implements checking the readiness of the fuse device while
+// locked with DeviceFD.mu.
+func (fd *DeviceFD) readinessLocked(mask waiter.EventMask) waiter.EventMask {
 	var ready waiter.EventMask
 	ready |= waiter.EventOut // FD is always writable
 	if !fd.queue.Empty() {