migration: support file: uri for source migration

Implement support for a "file:" uri so that a migration can be initiated
directly to a file from QEMU.

Unlike other migration protocol backends, the 'file' protocol cannot
honour non-blocking mode. POSIX file/block storage will always report
ready to read/write, regardless of how slow the underlying storage
will be at servicing the request.

For outgoing migration this limitation is not a serious problem as
the migration data transfer always happens in a dedicated thread.
It may, however, result in delays in honouring a request to cancel
the migration operation.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>

migration/file.c

@@ -0,0 +1,23 @@
+#include "qemu/osdep.h"
+#include "channel.h"
+#include "io/channel-file.h"
+#include "file.h"
+#include "qemu/error-report.h"
+
+
+void file_start_outgoing_migration(MigrationState *s, const char *fname, Error **errp)
+{
+	QIOChannelFile *ioc;
+
+	ioc = qio_channel_file_new_path(fname, O_CREAT|O_TRUNC|O_WRONLY, 0660, errp);
+	if (!ioc) {
+		error_report("Error creating a channel");
+		return;
+	}
+
+	qio_channel_set_name(QIO_CHANNEL(ioc), "migration-file-outgoing");
+	migration_channel_connect(s, QIO_CHANNEL(ioc), NULL, NULL);
+	object_unref(OBJECT(ioc));
+}
+
+

migration/file.h

@@ -0,0 +1,9 @@
+#ifndef QEMU_MIGRATION_FILE_H
+#define QEMU_MIGRATION_FILE_H
+
+void file_start_outgoing_migration(MigrationState *s,
+                                   const char *filename,
+                                   Error **errp);
+
+#endif
+

migration/meson.build

@@ -17,6 +17,7 @@ softmmu_ss.add(files(
   'colo.c',
   'exec.c',
   'fd.c',
+  'file.c',
   'global_state.c',
   'migration-hmp-cmds.c',
   'migration.c',

migration/migration.c

@@ -20,6 +20,7 @@
 #include "migration/blocker.h"
 #include "exec.h"
 #include "fd.h"
+#include "file.h"
 #include "socket.h"
 #include "sysemu/runstate.h"
 #include "sysemu/sysemu.h"
@@ -2523,6 +2524,8 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
         exec_start_outgoing_migration(s, p, &local_err);
     } else if (strstart(uri, "fd:", &p)) {
         fd_start_outgoing_migration(s, p, &local_err);
+    } else if (strstart(uri, "file:", &p)) {
+	file_start_outgoing_migration(s, p, &local_err);
     } else {
         if (!(has_resume && resume)) {
             yank_unregister_instance(MIGRATION_YANK_INSTANCE);