Add new function qemu_file_get_to_fd() that allows reading data from
QEMUFile and writing it straight into a given fd.

This will be used later in VFIO migration code.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 migration/qemu-file.c | 34 ++++++++++++++++++++++++++++++++++
 migration/qemu-file.h |  1 +
 2 files changed, 35 insertions(+)

diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index 2d5f74ffc2..79303c9d34 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -940,3 +940,37 @@ QIOChannel *qemu_file_get_ioc(QEMUFile *file)
 {
     return file->ioc;
 }
+
+/*
+ * Read size bytes from QEMUFile f and write them to fd.
+ */
+int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size)
+{
+    while (size) {
+        size_t pending = f->buf_size - f->buf_index;
+        ssize_t rc;
+
+        if (!pending) {
+            rc = qemu_fill_buffer(f);
+            if (rc < 0) {
+                return rc;
+            }
+            if (rc == 0) {
+                return -1;
+            }
+            continue;
+        }
+
+        rc = write(fd, f->buf + f->buf_index, MIN(pending, size));
+        if (rc < 0) {
+            return rc;
+        }
+        if (rc == 0) {
+            return -1;
+        }
+        f->buf_index += rc;
+        size -= rc;
+    }
+
+    return 0;
+}
diff --git a/migration/qemu-file.h b/migration/qemu-file.h
index fa13d04d78..9d0155a2a1 100644
--- a/migration/qemu-file.h
+++ b/migration/qemu-file.h
@@ -148,6 +148,7 @@ int qemu_file_shutdown(QEMUFile *f);
 QEMUFile *qemu_file_get_return_path(QEMUFile *f);
 void qemu_fflush(QEMUFile *f);
 void qemu_file_set_blocking(QEMUFile *f, bool block);
+int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size);
 
 void ram_control_before_iterate(QEMUFile *f, uint64_t flags);
 void ram_control_after_iterate(QEMUFile *f, uint64_t flags);
-- 
2.26.3