qga: add qga_open_cloexec() helper

QGA calls qemu_open_old() in various places. Calling qemu_open() instead
isn't a great alternative, as it has special "/dev/fdset" handling and
depends on QEMU internal monitor data structures.

Instead, provide a simple helper for QGA needs, with Error* support. The
following patches will make use of it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220525144140.591926-6-marcandre.lureau@redhat.com>

qga/cutils.c

@@ -0,0 +1,33 @@
+/*
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+#include "cutils.h"
+
+#include "qapi/error.h"
+
+/**
+ * qga_open_cloexec:
+ * @name: the pathname to open
+ * @flags: as in open()
+ * @mode: as in open()
+ *
+ * A wrapper for open() function which sets O_CLOEXEC.
+ *
+ * On error, -1 is returned.
+ */
+int qga_open_cloexec(const char *name, int flags, mode_t mode)
+{
+    int ret;
+
+#ifdef O_CLOEXEC
+    ret = open(name, flags | O_CLOEXEC, mode);
+#else
+    ret = open(name, flags, mode);
+    if (ret >= 0) {
+        qemu_set_cloexec(ret);
+    }
+#endif
+
+    return ret;
+}