As its name suggests, ChardevClass.chr_sync_read is supposed to do a
blocking read. The only implementation of it, tcp_chr_sync_read, does
set the underlying io channel to the blocking mode indeed.
Therefore a failure return with EAGAIN is not expected from this call.
So do not retry it in qemu_chr_fe_read_all; instead place an assertion
that it doesn't fail with EAGAIN.
The code was introduced in :
commit 7b0bfdf52d694c9a3a96505aa42ce3f8d63acd35
Author: Nikolay Nikolaev <
n.nikolaev@virtualopensystems.com>
Date: Tue May 27 15:03:48 2014 +0300
Add chardev API qemu_chr_fe_read_all
Also touched later by Daniel in:
commit 53628efbc8aa7a7ab5354d24b971f4d69452151d
Author: Daniel P. Berrangé <
berrange@redhat.com>
Date: Thu Mar 31 16:29:27 2016 +0100
char: fix broken EAGAIN retry on OS-X due to errno clobbering
Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
---
chardev/char-fe.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/chardev/char-fe.c b/chardev/char-fe.c
index 7789f7be9c..f94efe928e 100644
--- a/chardev/char-fe.c
+++ b/chardev/char-fe.c
@@ -68,13 +68,10 @@ int qemu_chr_fe_read_all(CharBackend *be, uint8_t *buf, int len)
}
while (offset < len) {
- retry:
res = CHARDEV_GET_CLASS(s)->chr_sync_read(s, buf + offset,
len - offset);
- if (res == -1 && errno == EAGAIN) {
- g_usleep(100);
- goto retry;
- }
+ /* ->chr_sync_read should block */
+ assert(!(res < 0 && errno == EAGAIN));
While I agree with the rationale to clean this code a bit, I am not so sure about replacing it with an assert(). In the past, when we did such things we had unexpected regressions :)
A slightly better approach perhaps is g_warn_if_fail(), although it's not very popular in qemu.
if (res == 0) {
break;
--
2.33.1