qemu-char: Introduce Memory driver

This driver handles in-memory chardev operations. That's, all writes
to this driver are stored in an internal buffer and it doesn't talk
to the external world in any way.

Right now it's very simple: it supports only writes. But it can be
easily extended to support more operations.

This is going to be used by the monitor's "HMP passthrough via QMP"
feature, which needs to run monitor handlers without a backing
device.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qemu-char.c

@@ -2275,6 +2275,70 @@ static CharDriverState *qemu_chr_open_socket(QemuOpts *opts)
     return NULL;
 }
 
+/***********************************************************/
+/* Memory chardev */
+typedef struct {
+    size_t outbuf_size;
+    size_t outbuf_capacity;
+    uint8_t *outbuf;
+} MemoryDriver;
+
+static int mem_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
+{
+    MemoryDriver *d = chr->opaque;
+
+    /* TODO: the QString implementation has the same code, we should
+     * introduce a generic way to do this in cutils.c */
+    if (d->outbuf_capacity < d->outbuf_size + len) {
+        /* grow outbuf */
+        d->outbuf_capacity += len;
+        d->outbuf_capacity *= 2;
+        d->outbuf = qemu_realloc(d->outbuf, d->outbuf_capacity);
+    }
+
+    memcpy(d->outbuf + d->outbuf_size, buf, len);
+    d->outbuf_size += len;
+
+    return len;
+}
+
+void qemu_chr_init_mem(CharDriverState *chr)
+{
+    MemoryDriver *d;
+
+    d = qemu_malloc(sizeof(*d));
+    d->outbuf_size = 0;
+    d->outbuf_capacity = 4096;
+    d->outbuf = qemu_mallocz(d->outbuf_capacity);
+
+    memset(chr, 0, sizeof(*chr));
+    chr->opaque = d;
+    chr->chr_write = mem_chr_write;
+}
+
+QString *qemu_chr_mem_to_qs(CharDriverState *chr)
+{
+    MemoryDriver *d = chr->opaque;
+    return qstring_from_substr((char *) d->outbuf, 0, d->outbuf_size - 1);
+}
+
+/* NOTE: this driver can not be closed with qemu_chr_close()! */
+void qemu_chr_close_mem(CharDriverState *chr)
+{
+    MemoryDriver *d = chr->opaque;
+
+    qemu_free(d->outbuf);
+    qemu_free(chr->opaque);
+    chr->opaque = NULL;
+    chr->chr_write = NULL;
+}
+
+size_t qemu_chr_mem_osize(const CharDriverState *chr)
+{
+    const MemoryDriver *d = chr->opaque;
+    return d->outbuf_size;
+}
+
 QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename)
 {
     char host[65], port[33], width[8], height[8];

qemu-char.h

@@ -6,6 +6,7 @@
 #include "qemu-option.h"
 #include "qemu-config.h"
 #include "qobject.h"
+#include "qstring.h"
 
 /* character device */
 
@@ -100,6 +101,12 @@ CharDriverState *qemu_chr_open_eventfd(int eventfd);
 
 extern int term_escape_char;
 
+/* memory chardev */
+void qemu_chr_init_mem(CharDriverState *chr);
+void qemu_chr_close_mem(CharDriverState *chr);
+QString *qemu_chr_mem_to_qs(CharDriverState *chr);
+size_t qemu_chr_mem_osize(const CharDriverState *chr);
+
 /* async I/O support */
 
 int qemu_set_fd_handler2(int fd,