[PATCH 1/2] chardev: don't set O_NONBLOCK on SCM_RIGHTS file descriptors.

[Andrey Ryabinin <arbn@yandex-team.com>]

This reverts commit 9b938c7262e4 ("chardev: clear O_NONBLOCK on SCM_RIGHTS file descriptors").
File descriptor passed to QEMU via 'getfd' QMP command always
changed to blocking mode. Instead of that, change blocking mode by QEMU
file descriptors users when necessary, e.g. like migration.

We need to preserve the state of the file descriptor in case it's still
used by an external process and before the QEMU itself started
using it.

E.g. our local migration scenario with TAP networking looks like this:
 1. Create TAP devices and pass file descriptors to source QEMU
 2. Launch destination QEMU (-incoming defer) and pass same descriptors to it.
 3. Start migration

In such scenario setting blocking state at stage (2) will hang source QEMU
since TAP fd suddenly become blocking.

Signed-off-by: Andrey Ryabinin <arbn@yandex-team.com>
---
 chardev/char-socket.c | 3 ---
 io/channel-socket.c   | 3 ---
 migration/fd.c        | 2 ++
 3 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index dc4e218eeb6..c9592fb5836 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -310,9 +310,6 @@ static ssize_t tcp_chr_recv(Chardev *chr, char *buf, size_t len)
             continue;
         }
 
-        /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
-        qemu_socket_set_block(fd);
-
 #ifndef MSG_CMSG_CLOEXEC
         qemu_set_cloexec(fd);
 #endif
diff --git a/io/channel-socket.c b/io/channel-socket.c
index dc9c165de11..8b9679460dc 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -479,9 +479,6 @@ static void qio_channel_socket_copy_fds(struct msghdr *msg,
                 continue;
             }
 
-            /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
-            qemu_socket_set_block(fd);
-
 #ifndef MSG_CMSG_CLOEXEC
             qemu_set_cloexec(fd);
 #endif
diff --git a/migration/fd.c b/migration/fd.c
index 6f2f50475f4..793fffeb169 100644
--- a/migration/fd.c
+++ b/migration/fd.c
@@ -60,6 +60,8 @@ void fd_start_incoming_migration(const char *fdname, Error **errp)
         return;
     }
 
+    qemu_socket_set_block(fd);
+
     trace_migration_fd_incoming(fd);
 
     ioc = qio_channel_new_fd(fd, errp);
-- 
2.35.1