[PATCH 2/3] chardev: report blocked write to chardev backend

[Nicholas Piggin <npiggin@gmail.com>]

If a chardev socket is not read, it will eventually fill and QEMU
can block attempting to write to it. A difficult bug in avocado
tests where the console socket was not being read from caused this
hang.

warn if a chardev write is blocked for 100ms.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
This is not necessary for the fix but it does trigger in the
failing avocado test without the previous patch applied. Maybe
it would be helpful?

Thanks,
Nick

 chardev/char.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/chardev/char.c b/chardev/char.c
index 996a024c7a..7c375e3cc4 100644
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -114,6 +114,8 @@ static int qemu_chr_write_buffer(Chardev *s,
 {
     ChardevClass *cc = CHARDEV_GET_CLASS(s);
     int res = 0;
+    int nr_retries = 0;
+
     *offset = 0;
 
     qemu_mutex_lock(&s->chr_write_lock);
@@ -126,6 +128,10 @@ static int qemu_chr_write_buffer(Chardev *s,
             } else {
                 g_usleep(100);
             }
+            if (++nr_retries == 1000) { /* 100ms */
+                warn_report("Chardev '%s' write blocked for > 100ms, "
+                            "socket buffer full?", s->label);
+            }
             goto retry;
         }
 
-- 
2.42.0