io: get rid of bounce buffering in websock write path

Currently most outbound I/O on the websock channel gets copied into the
rawoutput buffer, and then immediately copied again into the encoutput
buffer, with a header prepended. Now that qio_channel_websock_encode
accepts a struct iovec, we can trivially remove this bounce buffering
and write directly to encoutput.

In doing so, we also now correctly validate the encoutput size against
the QIO_CHANNEL_WEBSOCK_MAX_BUFFER limit.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

include/io/channel-websock.h

@@ -59,7 +59,6 @@ struct QIOChannelWebsock {
     Buffer encinput;
     Buffer encoutput;
     Buffer rawinput;
-    Buffer rawoutput;
     size_t payload_remain;
     size_t pong_remain;
     QIOChannelWebsockMask mask;

io/channel-websock.c

@@ -24,6 +24,7 @@
 #include "io/channel-websock.h"
 #include "crypto/hash.h"
 #include "trace.h"
+#include "qemu/iov.h"
 
 #include <time.h>
 
@@ -633,19 +634,22 @@ static ssize_t qio_channel_websock_write_wire(QIOChannelWebsock *, Error **);
 static void qio_channel_websock_write_close(QIOChannelWebsock *ioc,
                                             uint16_t code, const char *reason)
 {
-    struct iovec iov;
-    buffer_reserve(&ioc->rawoutput, 2 + (reason ? strlen(reason) : 0));
-    *(uint16_t *)(ioc->rawoutput.buffer + ioc->rawoutput.offset) =
-        cpu_to_be16(code);
-    ioc->rawoutput.offset += 2;
+    struct iovec iov[2] = {
+        { .iov_base = &code, .iov_len = sizeof(code) },
+    };
+    size_t niov = 1;
+    size_t size = iov[0].iov_len;
+
+    cpu_to_be16s(&code);
+
     if (reason) {
-        buffer_append(&ioc->rawoutput, reason, strlen(reason));
+        iov[1].iov_base = (void *)reason;
+        iov[1].iov_len = strlen(reason);
+        size += iov[1].iov_len;
+        niov++;
     }
-    iov.iov_base = ioc->rawoutput.buffer;
-    iov.iov_len = ioc->rawoutput.offset;
     qio_channel_websock_encode(ioc, QIO_CHANNEL_WEBSOCK_OPCODE_CLOSE,
-                               &iov, 1, iov.iov_len);
-    buffer_reset(&ioc->rawoutput);
+                               iov, niov, size);
     qio_channel_websock_write_wire(ioc, NULL);
     qio_channel_shutdown(ioc->master, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
 }
@@ -893,7 +897,6 @@ static void qio_channel_websock_finalize(Object *obj)
     buffer_free(&ioc->encinput);
     buffer_free(&ioc->encoutput);
     buffer_free(&ioc->rawinput);
-    buffer_free(&ioc->rawoutput);
     object_unref(OBJECT(ioc->master));
     if (ioc->io_tag) {
         g_source_remove(ioc->io_tag);
@@ -1103,8 +1106,8 @@ static ssize_t qio_channel_websock_writev(QIOChannel *ioc,
                                           Error **errp)
 {
     QIOChannelWebsock *wioc = QIO_CHANNEL_WEBSOCK(ioc);
-    size_t i;
-    ssize_t done = 0;
+    ssize_t want = iov_size(iov, niov);
+    ssize_t avail;
     ssize_t ret;
 
     if (wioc->io_err) {
@@ -1117,32 +1120,21 @@ static ssize_t qio_channel_websock_writev(QIOChannel *ioc,
         return -1;
     }
 
-    for (i = 0; i < niov; i++) {
-        size_t want = iov[i].iov_len;
-        if ((want + wioc->rawoutput.offset) > QIO_CHANNEL_WEBSOCK_MAX_BUFFER) {
-            want = (QIO_CHANNEL_WEBSOCK_MAX_BUFFER - wioc->rawoutput.offset);
-        }
-        if (want == 0) {
-            goto done;
-        }
-
-        buffer_reserve(&wioc->rawoutput, want);
-        buffer_append(&wioc->rawoutput, iov[i].iov_base, want);
-        done += want;
-        if (want < iov[i].iov_len) {
-            break;
-        }
+    avail = wioc->encoutput.offset >= QIO_CHANNEL_WEBSOCK_MAX_BUFFER ?
+        0 : (QIO_CHANNEL_WEBSOCK_MAX_BUFFER - wioc->encoutput.offset);
+    if (want > avail) {
+        want = avail;
     }
 
- done:
-    if (wioc->rawoutput.offset) {
-        struct iovec iov = { .iov_base = wioc->rawoutput.buffer,
-                             .iov_len = wioc->rawoutput.offset };
+    if (want) {
         qio_channel_websock_encode(wioc,
                                    QIO_CHANNEL_WEBSOCK_OPCODE_BINARY_FRAME,
-                                   &iov, 1, iov.iov_len);
-        buffer_reset(&wioc->rawoutput);
+                                   iov, niov, want);
     }
+
+    /* Even if want == 0, we'll try write_wire in case there's
+     * pending data we could usefully flush out
+     */
     ret = qio_channel_websock_write_wire(wioc, errp);
     if (ret < 0 &&
         ret != QIO_CHANNEL_ERR_BLOCK) {
@@ -1152,11 +1144,11 @@ static ssize_t qio_channel_websock_writev(QIOChannel *ioc,
 
     qio_channel_websock_set_watch(wioc);
 
-    if (done == 0) {
+    if (want == 0) {
         return QIO_CHANNEL_ERR_BLOCK;
     }
 
-    return done;
+    return want;
 }
 
 static int qio_channel_websock_set_blocking(QIOChannel *ioc,