[Qemu-devel] [PATCH RFC] qemu-char: add logfile facility to socket backend

["Daniel P. Berrange" <berrange@redhat.com>]

Typically a UNIX guest OS will log boot messages to a serial
port in addition to any graphical console. An admin user
may also wish to use the serial port for an interactive
console. A virtualization management system may wish to
collect system boot messages by logging the serial port,
but also wish to allow admins interactive access.

Currently providing such a feature forces the mgmt app
to either provide 2 separate serial ports, one for
logging boot messages and one for interactive console
login, or to proxy all output via a separate service
that can multiplex the two needs onto one serial port.
While both are valid approaches, they each have their
own downsides. The former causes confusion and extra
setup work for VM admins creating disk images. The latter
places an extra burden to re-implement much of the QEMU
chardev backends logic in libvirt or even higher level
mgmt apps and adds extra hops in the data transfer path.

A simpler approach that is satisfactory for many use
cases is to allow the QEMU chardev backends to have a
"logfile" property associated with them.

 $QEMU -chardev socket,host=localhost,port=9000,\
                server=on,nowait,id-charserial0,\
		logfile=/var/log/libvirt/qemu/test-serial0.log
       -device isa-serial,chardev=charserial0,id=serial0

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---
 qapi-schema.json |  4 +++-
 qemu-char.c      | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 61 insertions(+), 8 deletions(-)

diff --git a/qapi-schema.json b/qapi-schema.json
index 8b1a423..c20fe88 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -3056,6 +3056,7 @@
 #          then attempt a reconnect after the given number of seconds.
 #          Setting this to zero disables this function. (default: 0)
 #          (Since: 2.2)
+# @logfile: #optional file to log all data output to (since 2.6)
 #
 # Since: 1.4
 ##
@@ -3064,7 +3065,8 @@
                                      '*wait'      : 'bool',
                                      '*nodelay'   : 'bool',
                                      '*telnet'    : 'bool',
-                                     '*reconnect' : 'int' } }
+                                     '*reconnect' : 'int',
+                                     '*logfile'   : 'str' } }
 
 ##
 # @ChardevUdp:
diff --git a/qemu-char.c b/qemu-char.c
index 2969c44..a10d32e 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -2537,6 +2537,7 @@ static CharDriverState *qemu_chr_open_udp_fd(int fd)
 typedef struct {
 
     GIOChannel *chan, *listen_chan;
+    int logfd;
     guint listen_tag;
     int fd, listen_fd;
     int connected;
@@ -2630,23 +2631,47 @@ static int unix_send_msgfds(CharDriverState *chr, const uint8_t *buf, int len)
 }
 #endif
 
+static size_t fullwrite(int fd, const uint8_t *buf, size_t len)
+{
+    size_t done = 0;
+    ssize_t ret;
+
+    while (done < len) {
+        ret = write(fd, buf + done, len - done);
+        if (ret < 0)
+            break;
+        done += ret;
+    }
+
+    return ret;
+}
+
 /* Called with chr_write_lock held.  */
 static int tcp_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
 {
     TCPCharDriver *s = chr->opaque;
+    int ret;
     if (s->connected) {
 #ifndef _WIN32
         if (s->is_unix && s->write_msgfds_num) {
-            return unix_send_msgfds(chr, buf, len);
+            ret = unix_send_msgfds(chr, buf, len);
         } else
 #endif
         {
-            return io_channel_send(s->chan, buf, len);
+            ret = io_channel_send(s->chan, buf, len);
         }
     } else {
         /* XXX: indicate an error ? */
-        return len;
+        ret = len;
+    }
+
+    if (ret > 0) {
+        /* Ignoring errors from writing to logfile, as logs are
+         * defined to be "best effort" only */
+        fullwrite(s->logfd, buf, ret);
     }
+
+    return ret;
 }
 
 static int tcp_chr_read_poll(void *opaque)
@@ -3085,6 +3110,9 @@ static void tcp_chr_close(CharDriverState *chr)
     if (s->write_msgfds_num) {
         g_free(s->write_msgfds);
     }
+    if (s->logfd != -1) {
+        close(s->logfd);
+    }
     g_free(s);
     qemu_chr_be_event(chr, CHR_EVENT_CLOSED);
 }
@@ -3575,6 +3603,7 @@ static void qemu_chr_parse_socket(QemuOpts *opts, ChardevBackend *backend,
     const char *path = qemu_opt_get(opts, "path");
     const char *host = qemu_opt_get(opts, "host");
     const char *port = qemu_opt_get(opts, "port");
+    const char *logfile = qemu_opt_get(opts, "logfile");
     SocketAddress *addr;
 
     if (!path) {
@@ -3600,6 +3629,8 @@ static void qemu_chr_parse_socket(QemuOpts *opts, ChardevBackend *backend,
     backend->u.socket->wait = is_waitconnect;
     backend->u.socket->has_reconnect = true;
     backend->u.socket->reconnect = reconnect;
+    backend->u.socket->has_logfile = logfile != NULL;
+    backend->u.socket->logfile = g_strdup(logfile);
 
     addr = g_new0(SocketAddress, 1);
     if (path) {
@@ -4041,6 +4072,9 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "chardev",
             .type = QEMU_OPT_STRING,
+        },{
+            .name = "logfile",
+            .type = QEMU_OPT_STRING,
         },
         { /* end of list */ }
     },
@@ -4201,6 +4235,7 @@ static CharDriverState *qmp_chardev_open_socket(const char *id,
 
     s->fd = -1;
     s->listen_fd = -1;
+    s->logfd = -1;
     s->is_unix = addr->type == SOCKET_ADDRESS_KIND_UNIX;
     s->is_listen = is_listen;
     s->is_telnet = is_telnet;
@@ -4219,6 +4254,16 @@ static CharDriverState *qmp_chardev_open_socket(const char *id,
     /* be isn't opened until we get a connection */
     chr->explicit_be_open = true;
 
+    if (sock->has_logfile) {
+        s->logfd = qemu_open(sock->logfile, O_WRONLY | O_CREAT | O_TRUNC, 0666);
+        if (s->logfd < 0) {
+            error_setg_errno(errp, errno,
+                             "Unable to open logfile %s",
+                             sock->logfile);
+            goto error;
+        }
+    }
+
     chr->filename = g_malloc(CHR_MAX_FILENAME_SIZE);
     SocketAddress_to_str(chr->filename, CHR_MAX_FILENAME_SIZE, "disconnected:",
                          addr, is_listen, is_telnet);
@@ -4234,10 +4279,7 @@ static CharDriverState *qmp_chardev_open_socket(const char *id,
     if (s->reconnect_time) {
         socket_try_connect(chr);
     } else if (!qemu_chr_open_socket_fd(chr, errp)) {
-        g_free(s);
-        g_free(chr->filename);
-        g_free(chr);
-        return NULL;
+        goto error;
     }
 
     if (is_listen && is_waitconnect) {
@@ -4248,6 +4290,15 @@ static CharDriverState *qmp_chardev_open_socket(const char *id,
     }
 
     return chr;
+
+ error:
+    if (s->logfd != -1) {
+        close(s->logfd);
+    }
+    g_free(s);
+    g_free(chr->filename);
+    g_free(chr);
+    return NULL;
 }
 
 static CharDriverState *qmp_chardev_open_udp(const char *id,
-- 
2.5.0