hw/xen: Add xs_node_read() helper function

This returns the full contents of the node, having created the node path
from the printf-style format string provided in its arguments.

This will save various callers from having to do so for themselves (and
from using xs_node_scanf() with the non-portable %ms format string.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
[remove double newline and constify trace parameters]
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

hw/xen/trace-events

@@ -39,6 +39,7 @@ xs_node_create(const char *node) "%s"
 xs_node_destroy(const char *node) "%s"
 xs_node_vprintf(char *path, char *value) "%s %s"
 xs_node_vscanf(char *path, char *value) "%s %s"
+xs_node_read(const char *path, const char *value) "%s %s"
 xs_node_watch(char *path) "%s"
 xs_node_unwatch(char *path) "%s"
 

hw/xen/xen-bus-helper.c

@@ -142,6 +142,28 @@ int xs_node_scanf(struct qemu_xs_handle *h,  xs_transaction_t tid,
     return rc;
 }
 
+char *xs_node_read(struct qemu_xs_handle *h, xs_transaction_t tid,
+                   unsigned int *len, Error **errp,
+                   const char *path_fmt, ...)
+{
+    char *path, *value;
+    va_list ap;
+
+    va_start(ap, path_fmt);
+    path = g_strdup_vprintf(path_fmt, ap);
+    va_end(ap);
+
+    value = qemu_xen_xs_read(h, tid, path, len);
+    trace_xs_node_read(path, value);
+    if (!value) {
+        error_setg_errno(errp, errno, "failed to read from '%s'", path);
+    }
+
+    g_free(path);
+
+    return value;
+}
+
 struct qemu_xs_watch *xs_node_watch(struct qemu_xs_handle *h, const char *node,
                                     const char *key, xs_watch_fn fn,
                                     void *opaque, Error **errp)

include/hw/xen/xen-bus-helper.h

@@ -38,6 +38,15 @@ int xs_node_scanf(struct qemu_xs_handle *h,  xs_transaction_t tid,
                   const char *fmt, ...)
     G_GNUC_SCANF(6, 7);
 
+/*
+ * Unlike other functions here, the printf-formatted path_fmt is for
+ * the XenStore path, not the contents of the node.
+ */
+char *xs_node_read(struct qemu_xs_handle *h, xs_transaction_t tid,
+                   unsigned int *len, Error **errp,
+                   const char *path_fmt, ...)
+    G_GNUC_PRINTF(5, 6);
+
 /* Watch node/key unless node is empty, in which case watch key */
 struct qemu_xs_watch *xs_node_watch(struct qemu_xs_handle *h, const char *node,
                                     const char *key, xs_watch_fn fn,