qemu-char: append opt to stop truncation of serial file

Our QA team wants to preserve serial output of the guest in between QEMU
runs to perform post-analysis.

By default this behavior is off (file is truncated each time QEMU is
started or device is plugged).
Signed-off-by: NOlga Krishtal <okrishtal@virtuozzo.com>
Signed-off-by: NDenis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1449211324-17856-1-git-send-email-den@openvz.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qapi-schema.json

@@ -3102,11 +3102,14 @@
 #
 # @in:  #optional The name of the input file
 # @out: The name of the output file
+# @append: #optional Open the file in append mode (default false to
+#          truncate) (Since 2.6)
 #
 # Since: 1.4
 ##
 { 'struct': 'ChardevFile', 'data': { '*in' : 'str',
-                                   'out' : 'str' } }
+                                   'out' : 'str',
+                                   '*append': 'bool' } }
 
 ##
 # @ChardevHostdev:

qemu-char.c

@@ -3484,6 +3484,9 @@ static void qemu_chr_parse_file_out(QemuOpts *opts, ChardevBackend *backend,
     }
     backend->u.file = g_new0(ChardevFile, 1);
     backend->u.file->out = g_strdup(path);
+
+    backend->u.file->has_append = true;
+    backend->u.file->append = qemu_opt_get_bool(opts, "append", false);
 }
 
 static void qemu_chr_parse_stdio(QemuOpts *opts, ChardevBackend *backend,
@@ -4041,6 +4044,9 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "chardev",
             .type = QEMU_OPT_STRING,
+        },{
+            .name = "append",
+            .type = QEMU_OPT_BOOL,
         },
         { /* end of list */ }
     },
@@ -4101,7 +4107,13 @@ static CharDriverState *qmp_chardev_open_file(const char *id,
     ChardevFile *file = backend->u.file;
     int flags, in = -1, out;
 
-    flags = O_WRONLY | O_TRUNC | O_CREAT | O_BINARY;
+    flags = O_WRONLY | O_CREAT | O_BINARY;
+    if (file->has_append && file->append) {
+        flags |= O_APPEND;
+    } else {
+        flags |= O_TRUNC;
+    }
+
     out = qmp_chardev_open_file_source(file->out, flags, errp);
     if (out < 0) {
         return NULL;