Merge remote-tracking branch 'remotes/berrange/tags/pull-qio-2017-10-16-1' into staging

Merge QIO 2017/10/16 v1

# gpg: Signature made Mon 16 Oct 2017 17:10:54 BST
# gpg:                using RSA key 0xBE86EBB415104FDF
# gpg: Good signature from "Daniel P. Berrange <dan@berrange.com>"
# gpg:                 aka "Daniel P. Berrange <berrange@redhat.com>"
# Primary key fingerprint: DAF3 A6FD B26B 6291 2D0E  8E3F BE86 EBB4 1510 4FDF

* remotes/berrange/tags/pull-qio-2017-10-16-1:
  io: fix mem leak in websock error path
  io: add trace points for websocket HTTP protocol headers
  io: cope with websock 'Connection' header having multiple values
  io: get rid of bounce buffering in websock write path
  io: pass a struct iovec into qio_channel_websock_encode
  io: get rid of qio_channel_websock_encode helper method
  io: simplify websocket ping reply handling
  io: monitor encoutput buffer size from websocket GSource
  sockets: Handle race condition between binds to the same port
  sockets: factor out create_fast_reuse_socket
  sockets: factor out a new try_bind() function

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

4 files changed, 187 insertions, 124 deletions

diff --git a/include/io/channel-websock.h b/include/io/channel-websock.h
index ff32d86..a7e5e92 100644
--- a/include/io/channel-websock.h
+++ b/include/io/channel-websock.h
@@ -59,9 +59,8 @@ struct QIOChannelWebsock {
     Buffer encinput;
     Buffer encoutput;
     Buffer rawinput;
-    Buffer rawoutput;
-    Buffer ping_reply;
     size_t payload_remain;
+    size_t pong_remain;
     QIOChannelWebsockMask mask;
     guint io_tag;
     Error *io_err;
diff --git a/io/channel-websock.c b/io/channel-websock.c
index d1d471f..df2c3a9 100644
--- a/io/channel-websock.c
+++ b/io/channel-websock.c
@@ -24,11 +24,12 @@
 #include "io/channel-websock.h"
 #include "crypto/hash.h"
 #include "trace.h"
+#include "qemu/iov.h"
 
 #include <time.h>
 
 
-/* Max amount to allow in rawinput/rawoutput buffers */
+/* Max amount to allow in rawinput/encoutput buffers */
 #define QIO_CHANNEL_WEBSOCK_MAX_BUFFER 8192
 
 #define QIO_CHANNEL_WEBSOCK_CLIENT_KEY_LEN 24
@@ -223,6 +224,7 @@ qio_channel_websock_extract_headers(QIOChannelWebsock *ioc,
         goto bad_request;
     }
     *nl = '\0';
+    trace_qio_channel_websock_http_greeting(ioc, buffer);
 
     tmp = strchr(buffer, ' ');
     if (!tmp) {
@@ -339,7 +341,7 @@ static void qio_channel_websock_handshake_send_res_ok(QIOChannelWebsock *ioc,
     char combined_key[QIO_CHANNEL_WEBSOCK_CLIENT_KEY_LEN +
                       QIO_CHANNEL_WEBSOCK_GUID_LEN + 1];
     char *accept = NULL;
-    char *date = qio_channel_websock_date_str();
+    char *date = NULL;
 
     g_strlcpy(combined_key, key, QIO_CHANNEL_WEBSOCK_CLIENT_KEY_LEN + 1);
     g_strlcat(combined_key, QIO_CHANNEL_WEBSOCK_GUID,
@@ -358,6 +360,7 @@ static void qio_channel_websock_handshake_send_res_ok(QIOChannelWebsock *ioc,
         return;
     }
 
+    date = qio_channel_websock_date_str();
     qio_channel_websock_handshake_send_res(
         ioc, QIO_CHANNEL_WEBSOCK_HANDSHAKE_RES_OK, date, accept);
 
@@ -373,6 +376,9 @@ static void qio_channel_websock_handshake_process(QIOChannelWebsock *ioc,
     size_t nhdrs = G_N_ELEMENTS(hdrs);
     const char *protocols = NULL, *version = NULL, *key = NULL,
         *host = NULL, *connection = NULL, *upgrade = NULL;
+    char **connectionv;
+    bool upgraded = false;
+    size_t i;
 
     nhdrs = qio_channel_websock_extract_headers(ioc, buffer, hdrs, nhdrs, errp);
     if (!nhdrs) {
@@ -421,6 +427,9 @@ static void qio_channel_websock_handshake_process(QIOChannelWebsock *ioc,
         goto bad_request;
     }
 
+    trace_qio_channel_websock_http_request(ioc, protocols, version,
+                                           host, connection, upgrade, key);
+
     if (!g_strrstr(protocols, QIO_CHANNEL_WEBSOCK_PROTOCOL_BINARY)) {
         error_setg(errp, "No '%s' protocol is supported by client '%s'",
                    QIO_CHANNEL_WEBSOCK_PROTOCOL_BINARY, protocols);
@@ -439,7 +448,16 @@ static void qio_channel_websock_handshake_process(QIOChannelWebsock *ioc,
         goto bad_request;
     }
 
-    if (strcasecmp(connection, QIO_CHANNEL_WEBSOCK_CONNECTION_UPGRADE) != 0) {
+    connectionv = g_strsplit(connection, ",", 0);
+    for (i = 0; connectionv != NULL && connectionv[i] != NULL; i++) {
+        g_strstrip(connectionv[i]);
+        if (strcasecmp(connectionv[i],
+                       QIO_CHANNEL_WEBSOCK_CONNECTION_UPGRADE) == 0) {
+            upgraded = true;
+        }
+    }
+    g_strfreev(connectionv);
+    if (!upgraded) {
         error_setg(errp, "No connection upgrade requested '%s'", connection);
         goto bad_request;
     }
@@ -582,49 +600,48 @@ static gboolean qio_channel_websock_handshake_io(QIOChannel *ioc,
 }
 
 
-static void qio_channel_websock_encode_buffer(QIOChannelWebsock *ioc,
-                                              Buffer *output,
-                                              uint8_t opcode, Buffer *buffer)
+static void qio_channel_websock_encode(QIOChannelWebsock *ioc,
+                                       uint8_t opcode,
+                                       const struct iovec *iov,
+                                       size_t niov,
+                                       size_t size)
 {
     size_t header_size;
+    size_t i;
     union {
         char buf[QIO_CHANNEL_WEBSOCK_HEADER_LEN_64_BIT];
         QIOChannelWebsockHeader ws;
     } header;
 
+    assert(size <= iov_size(iov, niov));
+
     header.ws.b0 = QIO_CHANNEL_WEBSOCK_HEADER_FIELD_FIN |
         (opcode & QIO_CHANNEL_WEBSOCK_HEADER_FIELD_OPCODE);
-    if (buffer->offset < QIO_CHANNEL_WEBSOCK_PAYLOAD_LEN_THRESHOLD_7_BIT) {
-        header.ws.b1 = (uint8_t)buffer->offset;
+    if (size < QIO_CHANNEL_WEBSOCK_PAYLOAD_LEN_THRESHOLD_7_BIT) {
+        header.ws.b1 = (uint8_t)size;
         header_size = QIO_CHANNEL_WEBSOCK_HEADER_LEN_7_BIT;
-    } else if (buffer->offset <
-               QIO_CHANNEL_WEBSOCK_PAYLOAD_LEN_THRESHOLD_16_BIT) {
+    } else if (size < QIO_CHANNEL_WEBSOCK_PAYLOAD_LEN_THRESHOLD_16_BIT) {
         header.ws.b1 = QIO_CHANNEL_WEBSOCK_PAYLOAD_LEN_MAGIC_16_BIT;
-        header.ws.u.s16.l16 = cpu_to_be16((uint16_t)buffer->offset);
+        header.ws.u.s16.l16 = cpu_to_be16((uint16_t)size);
         header_size = QIO_CHANNEL_WEBSOCK_HEADER_LEN_16_BIT;
     } else {
         header.ws.b1 = QIO_CHANNEL_WEBSOCK_PAYLOAD_LEN_MAGIC_64_BIT;
-        header.ws.u.s64.l64 = cpu_to_be64(buffer->offset);
+        header.ws.u.s64.l64 = cpu_to_be64(size);
         header_size = QIO_CHANNEL_WEBSOCK_HEADER_LEN_64_BIT;
     }
     header_size -= QIO_CHANNEL_WEBSOCK_HEADER_LEN_MASK;
 
-    trace_qio_channel_websock_encode(ioc, opcode, header_size, buffer->offset);
-    buffer_reserve(output, header_size + buffer->offset);
-    buffer_append(output, header.buf, header_size);
-    buffer_append(output, buffer->buffer, buffer->offset);
-}
-
-
-static void qio_channel_websock_encode(QIOChannelWebsock *ioc)
-{
-    if (!ioc->rawoutput.offset) {
-        return;
+    trace_qio_channel_websock_encode(ioc, opcode, header_size, size);
+    buffer_reserve(&ioc->encoutput, header_size + size);
+    buffer_append(&ioc->encoutput, header.buf, header_size);
+    for (i = 0; i < niov && size != 0; i++) {
+        size_t want = iov[i].iov_len;
+        if (want > size) {
+            want = size;
+        }
+        buffer_append(&ioc->encoutput, iov[i].iov_base, want);
+        size -= want;
     }
-    qio_channel_websock_encode_buffer(
-        ioc, &ioc->encoutput, QIO_CHANNEL_WEBSOCK_OPCODE_BINARY_FRAME,
-        &ioc->rawoutput);
-    buffer_reset(&ioc->rawoutput);
 }
 
 
@@ -634,17 +651,22 @@ 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)
 {
-    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++;
     }
-    qio_channel_websock_encode_buffer(
-        ioc, &ioc->encoutput, QIO_CHANNEL_WEBSOCK_OPCODE_CLOSE,
-        &ioc->rawoutput);
-    buffer_reset(&ioc->rawoutput);
+    qio_channel_websock_encode(ioc, QIO_CHANNEL_WEBSOCK_OPCODE_CLOSE,
+                               iov, niov, size);
     qio_channel_websock_write_wire(ioc, NULL);
     qio_channel_shutdown(ioc->master, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
 }
@@ -813,9 +835,10 @@ static int qio_channel_websock_decode_payload(QIOChannelWebsock *ioc,
         error_setg(errp, "websocket closed by peer");
         if (payload_len) {
             /* echo client status */
-            qio_channel_websock_encode_buffer(
-                ioc, &ioc->encoutput, QIO_CHANNEL_WEBSOCK_OPCODE_CLOSE,
-                &ioc->encinput);
+            struct iovec iov = { .iov_base = ioc->encinput.buffer,
+                                 .iov_len = ioc->encinput.offset };
+            qio_channel_websock_encode(ioc, QIO_CHANNEL_WEBSOCK_OPCODE_CLOSE,
+                                       &iov, 1, iov.iov_len);
             qio_channel_websock_write_wire(ioc, NULL);
             qio_channel_shutdown(ioc->master, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
         } else {
@@ -825,11 +848,15 @@ static int qio_channel_websock_decode_payload(QIOChannelWebsock *ioc,
         }
         return -1;
     } else if (ioc->opcode == QIO_CHANNEL_WEBSOCK_OPCODE_PING) {
-        /* ping frames produce an immediate reply */
-        buffer_reset(&ioc->ping_reply);
-        qio_channel_websock_encode_buffer(
-            ioc, &ioc->ping_reply, QIO_CHANNEL_WEBSOCK_OPCODE_PONG,
-            &ioc->encinput);
+        /* ping frames produce an immediate reply, as long as we've not still
+         * got a previous pong queued, in which case we drop the new pong */
+        if (ioc->pong_remain == 0) {
+            struct iovec iov = { .iov_base = ioc->encinput.buffer,
+                                 .iov_len = ioc->encinput.offset };
+            qio_channel_websock_encode(ioc, QIO_CHANNEL_WEBSOCK_OPCODE_PONG,
+                                       &iov, 1, iov.iov_len);
+            ioc->pong_remain = ioc->encoutput.offset;
+        }
     }   /* pong frames are ignored */
 
     if (payload_len) {
@@ -887,8 +914,6 @@ static void qio_channel_websock_finalize(Object *obj)
     buffer_free(&ioc->encinput);
     buffer_free(&ioc->encoutput);
     buffer_free(&ioc->rawinput);
-    buffer_free(&ioc->rawoutput);
-    buffer_free(&ioc->ping_reply);
     object_unref(OBJECT(ioc->master));
     if (ioc->io_tag) {
         g_source_remove(ioc->io_tag);
@@ -946,13 +971,6 @@ static ssize_t qio_channel_websock_write_wire(QIOChannelWebsock *ioc,
     ssize_t ret;
     ssize_t done = 0;
 
-    /* ping replies take priority over binary data */
-    if (!ioc->ping_reply.offset) {
-        qio_channel_websock_encode(ioc);
-    } else if (!ioc->encoutput.offset) {
-        buffer_move_empty(&ioc->encoutput, &ioc->ping_reply);
-    }
-
     while (ioc->encoutput.offset > 0) {
         ret = qio_channel_write(ioc->master,
                                 (char *)ioc->encoutput.buffer,
@@ -968,6 +986,11 @@ static ssize_t qio_channel_websock_write_wire(QIOChannelWebsock *ioc,
         }
         buffer_advance(&ioc->encoutput, ret);
         done += ret;
+        if (ioc->pong_remain < ret) {
+            ioc->pong_remain = 0;
+        } else {
+            ioc->pong_remain -= ret;
+        }
     }
     return done;
 }
@@ -1026,7 +1049,7 @@ static void qio_channel_websock_set_watch(QIOChannelWebsock *ioc)
         return;
     }
 
-    if (ioc->encoutput.offset || ioc->ping_reply.offset) {
+    if (ioc->encoutput.offset) {
         cond |= G_IO_OUT;
     }
     if (ioc->encinput.offset < QIO_CHANNEL_WEBSOCK_MAX_BUFFER &&
@@ -1100,8 +1123,8 @@ 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) {
@@ -1114,24 +1137,21 @@ 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;
-        }
+    avail = wioc->encoutput.offset >= QIO_CHANNEL_WEBSOCK_MAX_BUFFER ?
+        0 : (QIO_CHANNEL_WEBSOCK_MAX_BUFFER - wioc->encoutput.offset);
+    if (want > avail) {
+        want = avail;
+    }
 
-        buffer_reserve(&wioc->rawoutput, want);
-        buffer_append(&wioc->rawoutput, iov[i].iov_base, want);
-        done += want;
-        if (want < iov[i].iov_len) {
-            break;
-        }
+    if (want) {
+        qio_channel_websock_encode(wioc,
+                                   QIO_CHANNEL_WEBSOCK_OPCODE_BINARY_FRAME,
+                                   iov, niov, want);
     }
 
- done:
+    /* 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) {
@@ -1141,11 +1161,11 @@ 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,
@@ -1208,7 +1228,7 @@ qio_channel_websock_source_check(GSource *source)
     if (wsource->wioc->rawinput.offset || wsource->wioc->io_eof) {
         cond |= G_IO_IN;
     }
-    if (wsource->wioc->rawoutput.offset < QIO_CHANNEL_WEBSOCK_MAX_BUFFER) {
+    if (wsource->wioc->encoutput.offset < QIO_CHANNEL_WEBSOCK_MAX_BUFFER) {
         cond |= G_IO_OUT;
     }
 
diff --git a/io/trace-events b/io/trace-events
index 801b5dc..f70bad7 100644
--- a/io/trace-events
+++ b/io/trace-events
@@ -48,6 +48,8 @@ qio_channel_websock_handshake_pending(void *ioc, int status) "Websock handshake
 qio_channel_websock_handshake_reply(void *ioc) "Websock handshake reply ioc=%p"
 qio_channel_websock_handshake_fail(void *ioc, const char *msg) "Websock handshake fail ioc=%p err=%s"
 qio_channel_websock_handshake_complete(void *ioc) "Websock handshake complete ioc=%p"
+qio_channel_websock_http_greeting(void *ioc, const char *greeting) "Websocket HTTP request ioc=%p greeting='%s'"
+qio_channel_websock_http_request(void *ioc, const char *protocols, const char *version, const char *host, const char *connection, const char *upgrade, const char *key) "Websocket HTTP request ioc=%p protocols='%s' version='%s' host='%s' connection='%s' upgrade='%s' key='%s'"
 qio_channel_websock_header_partial_decode(void *ioc, size_t payloadlen, unsigned char fin, unsigned char opcode, unsigned char has_mask) "Websocket header decoded ioc=%p payload-len=%zu fin=0x%x opcode=0x%x has_mask=0x%x"
 qio_channel_websock_header_full_decode(void *ioc, size_t headerlen, size_t payloadlen, uint32_t mask) "Websocket header decoded ioc=%p header-len=%zu payload-len=%zu mask=0x%x"
 qio_channel_websock_payload_decode(void *ioc, uint8_t opcode, size_t payload_remain) "Websocket header decoded ioc=%p opcode=0x%x payload-remain=%zu"
diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index d149383..b47fb45 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -149,6 +149,54 @@ int inet_ai_family_from_address(InetSocketAddress *addr,
     return PF_UNSPEC;
 }
 
+static int create_fast_reuse_socket(struct addrinfo *e)
+{
+    int slisten = qemu_socket(e->ai_family, e->ai_socktype, e->ai_protocol);
+    if (slisten < 0) {
+        return -1;
+    }
+    socket_set_fast_reuse(slisten);
+    return slisten;
+}
+
+static int try_bind(int socket, InetSocketAddress *saddr, struct addrinfo *e)
+{
+#ifndef IPV6_V6ONLY
+    return bind(socket, e->ai_addr, e->ai_addrlen);
+#else
+    /*
+     * Deals with first & last cases in matrix in comment
+     * for inet_ai_family_from_address().
+     */
+    int v6only =
+        ((!saddr->has_ipv4 && !saddr->has_ipv6) ||
+         (saddr->has_ipv4 && saddr->ipv4 &&
+          saddr->has_ipv6 && saddr->ipv6)) ? 0 : 1;
+    int stat;
+
+ rebind:
+    if (e->ai_family == PF_INET6) {
+        qemu_setsockopt(socket, IPPROTO_IPV6, IPV6_V6ONLY, &v6only,
+                        sizeof(v6only));
+    }
+
+    stat = bind(socket, e->ai_addr, e->ai_addrlen);
+    if (!stat) {
+        return 0;
+    }
+
+    /* If we got EADDRINUSE from an IPv6 bind & v6only is unset,
+     * it could be that the IPv4 port is already claimed, so retry
+     * with v6only set
+     */
+    if (e->ai_family == PF_INET6 && errno == EADDRINUSE && !v6only) {
+        v6only = 1;
+        goto rebind;
+    }
+    return stat;
+#endif
+}
+
 static int inet_listen_saddr(InetSocketAddress *saddr,
                              int port_offset,
                              bool update_addr,
@@ -158,7 +206,10 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
     char port[33];
     char uaddr[INET6_ADDRSTRLEN+1];
     char uport[33];
-    int slisten, rc, port_min, port_max, p;
+    int rc, port_min, port_max, p;
+    int slisten = 0;
+    int saved_errno = 0;
+    bool socket_created = false;
     Error *err = NULL;
 
     memset(&ai,0, sizeof(ai));
@@ -210,75 +261,66 @@ static int inet_listen_saddr(InetSocketAddress *saddr,
         return -1;
     }
 
-    /* create socket + bind */
+    /* create socket + bind/listen */
     for (e = res; e != NULL; e = e->ai_next) {
         getnameinfo((struct sockaddr*)e->ai_addr,e->ai_addrlen,
 		        uaddr,INET6_ADDRSTRLEN,uport,32,
 		        NI_NUMERICHOST | NI_NUMERICSERV);
-        slisten = qemu_socket(e->ai_family, e->ai_socktype, e->ai_protocol);
+
+        slisten = create_fast_reuse_socket(e);
         if (slisten < 0) {
-            if (!e->ai_next) {
-                error_setg_errno(errp, errno, "Failed to create socket");
-            }
             continue;
         }
 
-        socket_set_fast_reuse(slisten);
-
+        socket_created = true;
         port_min = inet_getport(e);
         port_max = saddr->has_to ? saddr->to + port_offset : port_min;
         for (p = port_min; p <= port_max; p++) {
-#ifdef IPV6_V6ONLY
-            /*
-             * Deals with first & last cases in matrix in comment
-             * for inet_ai_family_from_address().
-             */
-            int v6only =
-                ((!saddr->has_ipv4 && !saddr->has_ipv6) ||
-                 (saddr->has_ipv4 && saddr->ipv4 &&
-                  saddr->has_ipv6 && saddr->ipv6)) ? 0 : 1;
-#endif
             inet_setport(e, p);
-#ifdef IPV6_V6ONLY
-        rebind:
-            if (e->ai_family == PF_INET6) {
-                qemu_setsockopt(slisten, IPPROTO_IPV6, IPV6_V6ONLY, &v6only,
-                                sizeof(v6only));
+            rc = try_bind(slisten, saddr, e);
+            if (rc) {
+                if (errno == EADDRINUSE) {
+                    continue;
+                } else {
+                    error_setg_errno(errp, errno, "Failed to bind socket");
+                    goto listen_failed;
+                }
             }
-#endif
-            if (bind(slisten, e->ai_addr, e->ai_addrlen) == 0) {
-                goto listen;
+            if (!listen(slisten, 1)) {
+                goto listen_ok;
             }
-
-#ifdef IPV6_V6ONLY
-            /* If we got EADDRINUSE from an IPv6 bind & V6ONLY is unset,
-             * it could be that the IPv4 port is already claimed, so retry
-             * with V6ONLY set
-             */
-            if (e->ai_family == PF_INET6 && errno == EADDRINUSE && !v6only) {
-                v6only = 1;
-                goto rebind;
+            if (errno != EADDRINUSE) {
+                error_setg_errno(errp, errno, "Failed to listen on socket");
+                goto listen_failed;
             }
-#endif
-
-            if (p == port_max) {
-                if (!e->ai_next) {
-                    error_setg_errno(errp, errno, "Failed to bind socket");
-                }
+            /* Someone else managed to bind to the same port and beat us
+             * to listen on it! Socket semantics does not allow us to
+             * recover from this situation, so we need to recreate the
+             * socket to allow bind attempts for subsequent ports:
+             */
+            closesocket(slisten);
+            slisten = create_fast_reuse_socket(e);
+            if (slisten < 0) {
+                error_setg_errno(errp, errno,
+                                 "Failed to recreate failed listening socket");
+                goto listen_failed;
             }
         }
+    }
+    error_setg_errno(errp, errno,
+                     socket_created ?
+                     "Failed to find an available port" :
+                     "Failed to create a socket");
+listen_failed:
+    saved_errno = errno;
+    if (slisten >= 0) {
         closesocket(slisten);
     }
     freeaddrinfo(res);
+    errno = saved_errno;
     return -1;
 
-listen:
-    if (listen(slisten,1) != 0) {
-        error_setg_errno(errp, errno, "Failed to listen on socket");
-        closesocket(slisten);
-        freeaddrinfo(res);
-        return -1;
-    }
+listen_ok:
     if (update_addr) {
         g_free(saddr->host);
         saddr->host = g_strdup(uaddr);