[Libguestfs] [PATCH nbdkit] nbd: Hide some state machine debugging behind a debug flag

[Richard W.M. Jones]

When running virt-p2v which uses this plugin, the log file is consumed
by messages about state machine transitions and so on.  In a log file
that was shared with me, out of the 135,023 lines in total,
94,653 (70%) were:

  nbdkit: debug: polling, dir=1

and 18,676 (14%) were:

  nbdkit: debug: cookie X completed state machine, status 0

This commit changes the logging so these state machine transitions are
only printed when you use the debug flag “-D nbd.verbose=1”.  I didn't
document this flag because it's likely only of use to developers who
are reading the code already.

There are some debug messages along error paths which are
(a) generally useful and (b) did not appear in the log file, so I left
those alone.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2083498
Reported-by: Ming Xie
---
 plugins/nbd/nbd.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
index 45bf05e9..d0d6544b 100644
--- a/plugins/nbd/nbd.c
+++ b/plugins/nbd/nbd.c
@@ -66,6 +66,9 @@
 #define USE_VSOCK 1
 #endif
 
+/* Use '-D nbd.verbose=1' for verbose messages about the state machine. */
+NBDKIT_DLL_PUBLIC int nbd_debug_verbose = 0;
+
 /* The per-transaction details */
 struct transaction {
   int64_t cookie;
@@ -421,7 +424,8 @@ nbdplug_reader (void *handle)
 {
   struct handle *h = handle;
 
-  nbdkit_debug ("nbd: started reader thread");
+  if (nbd_debug_verbose)
+    nbdkit_debug ("nbd: started reader thread");
 
   while (!nbd_aio_is_dead (h->nbd) && !nbd_aio_is_closed (h->nbd)) {
     int r;
@@ -433,7 +437,8 @@ nbdplug_reader (void *handle)
     unsigned dir;
 
     dir = nbd_aio_get_direction (h->nbd);
-    nbdkit_debug ("polling, dir=%d", dir);
+    if (nbd_debug_verbose)
+      nbdkit_debug ("polling, dir=%d", dir);
     if (dir & LIBNBD_AIO_DIRECTION_READ)
       fds[0].events |= POLLIN;
     if (dir & LIBNBD_AIO_DIRECTION_WRITE)
@@ -466,8 +471,11 @@ nbdplug_reader (void *handle)
     }
   }
 
-  nbdkit_debug ("state machine changed to %s", nbd_connection_state (h->nbd));
-  nbdkit_debug ("exiting reader thread");
+  if (nbd_debug_verbose) {
+    nbdkit_debug ("state machine changed to %s",
+                  nbd_connection_state (h->nbd));
+    nbdkit_debug ("exiting reader thread");
+  }
   return NULL;
 }
 
@@ -481,8 +489,9 @@ nbdplug_notify (void *opaque, int *error)
    * updated by nbdplug_register, but it's only an informational
    * message.
    */
-  nbdkit_debug ("cookie %" PRId64 " completed state machine, status %d",
-                trans->cookie, *error);
+  if (nbd_debug_verbose)
+    nbdkit_debug ("cookie %" PRId64 " completed state machine, status %d",
+                  trans->cookie, *error);
   trans->err = *error;
   if (sem_post (&trans->sem)) {
     nbdkit_error ("failed to post semaphore: %m");
@@ -514,7 +523,8 @@ nbdplug_register (struct handle *h, struct transaction *trans, int64_t cookie)
     return;
   }
 
-  nbdkit_debug ("cookie %" PRId64 " started by state machine", cookie);
+  if (nbd_debug_verbose)
+    nbdkit_debug ("cookie %" PRId64 " started by state machine", cookie);
   trans->cookie = cookie;
 
   if (write (h->fds[1], &c, 1) == -1 && errno != EAGAIN)
-- 
2.35.1