[Qemu-devel] [PATCH v8 5/7] char: Update send_all() to handle nonblocking chardev write requests

[Amit Shah]

The send_all function is modified to return to the caller in case the
driver cannot handle any more data.  It returns -EAGAIN or
WSAEWOULDBLOCK on non-Windows and Windows platforms respectively.  This
is only done when the caller sets a callback function handler indicating
it's not interested in blocking till the driver has written out all the
data.

Currently there's no driver or caller that supports this.  Future
commits will add such capability.
---
 net/socket.c  |    4 +-
 qemu-char.c   |   67 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
 qemu_socket.h |    2 +-
 3 files changed, 64 insertions(+), 9 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index 1c4e153..902529e 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -56,8 +56,8 @@ static ssize_t net_socket_receive(VLANClientState *nc, const 
uint8_t *buf, size_
     uint32_t len;
     len = htonl(size);
 
-    send_all(s->fd, (const uint8_t *)&len, sizeof(len));
-    return send_all(s->fd, buf, size);
+    send_all(NULL, s->fd, (const uint8_t *)&len, sizeof(len));
+    return send_all(NULL, s->fd, buf, size);
 }
 
 static ssize_t net_socket_receive_dgram(VLANClientState *nc, const uint8_t 
*buf, size_t size)
diff --git a/qemu-char.c b/qemu-char.c
index c16c2d7..a24c4c2 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -514,7 +514,7 @@ static CharDriverState *qemu_chr_open_mux(CharDriverState 
*drv)
 
 
 #ifdef _WIN32
-int send_all(int fd, const void *buf, int len1)
+static int do_send(int fd, const void *buf, int len1, bool nonblock)
 {
     int ret, len;
 
@@ -525,6 +525,8 @@ int send_all(int fd, const void *buf, int len1)
             errno = WSAGetLastError();
             if (errno != WSAEWOULDBLOCK) {
                 return -1;
+            } else if (errno == WSAEWOULDBLOCK && nonblock) {
+                return WSAEWOULDBLOCK;
             }
         } else if (ret == 0) {
             break;
@@ -538,7 +540,7 @@ int send_all(int fd, const void *buf, int len1)
 
 #else
 
-int send_all(int fd, const void *_buf, int len1)
+static int do_send(int fd, const void *_buf, int len1, bool nonblock)
 {
     int ret, len;
     const uint8_t *buf = _buf;
@@ -547,8 +549,12 @@ int send_all(int fd, const void *_buf, int len1)
     while (len > 0) {
         ret = write(fd, buf, len);
         if (ret < 0) {
-            if (errno != EINTR && errno != EAGAIN)
+            if (errno == EAGAIN && nonblock) {
+                return -EAGAIN;
+            }
+            if (errno != EINTR && errno != EAGAIN) {
                 return -1;
+            }
         } else if (ret == 0) {
             break;
         } else {
@@ -560,6 +566,55 @@ int send_all(int fd, const void *_buf, int len1)
 }
 #endif /* !_WIN32 */
 
+int send_all(CharDriverState *chr, int fd, const void *_buf, int len1)
+{
+    int ret, eagain_errno;
+    bool nonblock;
+
+    if (chr->write_blocked) {
+        /*
+         * We don't handle this situation: the caller should not send
+         * us data while we're blocked.
+         *
+         * We could buffer this data here but that'll only encourage
+         * bad behaviour on part of the callers.
+         *
+         * Also, the data already in fd's buffers isn't easily
+         * migratable.  If we want full migration support, all the
+         * data landing here needs to be buffered and on migration,
+         * anything that's unsent needs to be transferred to the
+         * dest. machine (which again isn't a very good way of solving
+         * the problem, as the src may become writable just during
+         * migration and the reader could receive some data twice,
+         * essentially corrupting the data).
+         */
+        return -1;
+    }
+
+    nonblock = false;
+    /*
+     * Ensure the char backend is able to receive and handle the
+     * 'write unblocked' event before we turn on nonblock support.
+     */
+    if (chr && chr->update_fd_handlers && chr->chr_write_unblocked) {
+        nonblock = true;
+    }
+    ret = do_send(fd, _buf, len1, nonblock);
+
+#ifdef _WIN32
+    eagain_errno = WSAEWOULDBLOCK;
+#else
+    eagain_errno = -EAGAIN;
+#endif
+
+    if (nonblock && ret == eagain_errno) {
+        /* Update fd handler to wake up when chr becomes writable */
+        chr->update_fd_handlers(chr, true);
+        chr->write_blocked = true;
+    }
+    return ret;
+}
+
 #ifndef _WIN32
 
 typedef struct {
@@ -573,7 +628,7 @@ static int stdio_nb_clients = 0;
 static int fd_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
 {
     FDCharDriver *s = chr->opaque;
-    return send_all(s->fd_out, buf, len);
+    return send_all(chr, s->fd_out, buf, len);
 }
 
 static int fd_chr_read_poll(void *opaque)
@@ -885,7 +940,7 @@ static int pty_chr_write(CharDriverState *chr, const 
uint8_t *buf, int len)
         pty_chr_update_read_handler(chr);
         return 0;
     }
-    return send_all(s->fd, buf, len);
+    return send_all(chr, s->fd, buf, len);
 }
 
 static int pty_chr_read_poll(void *opaque)
@@ -1950,7 +2005,7 @@ static int tcp_chr_write(CharDriverState *chr, const 
uint8_t *buf, int len)
 {
     TCPCharDriver *s = chr->opaque;
     if (s->connected) {
-        return send_all(s->fd, buf, len);
+        return send_all(chr, s->fd, buf, len);
     } else {
         /* XXX: indicate an error ? */
         return len;
diff --git a/qemu_socket.h b/qemu_socket.h
index 897a8ae..97dd24a 100644
--- a/qemu_socket.h
+++ b/qemu_socket.h
@@ -36,7 +36,7 @@ int inet_aton(const char *cp, struct in_addr *ia);
 int qemu_socket(int domain, int type, int protocol);
 int qemu_accept(int s, struct sockaddr *addr, socklen_t *addrlen);
 void socket_set_nonblock(int fd);
-int send_all(int fd, const void *buf, int len1);
+int send_all(CharDriverState *chr, int fd, const void *buf, int len1);
 
 /* New, ipv6-ready socket helper functions, see qemu-sockets.c */
 int inet_listen_opts(QemuOpts *opts, int port_offset);
-- 
1.7.3.2