Call to btrfs device ready ioctl

That helps when we build btrfs RAID0 and fixes following race condition
in tests:

[    1.608465] booster: loading module btrfs
[    1.695250] Btrfs loaded, crc32c=crc32c-intel, zoned=yes, fsverity=yes
[    1.695410] booster: udev event {Action:add KObj:/devices/virtual/misc/btrfs-control Env:map[ACTION:add DEVNAME:btrfs-control DEVPATH:/devices/virtual/misc/btrfs-control MAJOR:10 MINOR:234 SEQNUM:1156 SUBSYSTEM:misc]}
[    1.698089] booster: udev event {Action:add KObj:/module/btrfs Env:map[ACTION:add DEVPATH:/module/btrfs SEQNUM:1157 SUBSYSTEM:module]}
[    1.699492] booster: mounting /dev/sda1->/booster.root, fs=btrfs, flags=0x0, options=
[    1.700392] BTRFS: device fsid 5eaa0c1c-e1dc-4be7-9b03-9f1ed5a87289 devid 1 transid 8 /dev/sda1 scanned by init (129)
[    1.701508] booster: udev event {Action:add KObj:/devices/virtual/bdi/btrfs-1 Env:map[ACTION:add DEVPATH:/devices/virtual/bdi/btrfs-1 SEQNUM:1158 SUBSYSTEM:bdi]}
[    1.703030] BTRFS info (device sda1): using crc32c (crc32c-intel) checksum algorithm
[    1.703796] BTRFS info (device sda1): using free space tree
[    1.704984] BTRFS error (device sda1): devid 2 uuid c815d0ba-796d-4b7c-a5a9-3e6852c6ed0a is missing
[    1.705846] BTRFS error (device sda1): failed to read the system array: -2
[    1.706784] BTRFS error (device sda1): open_ctree failed
[    1.707380] booster: mount(/dev/sda1): invalid argument
[    1.707924] booster: mounting /dev/sda2->/booster.root, fs=btrfs, flags=0x0, options=
[    1.708708] booster: udev event {Action:remove KObj:/devices/virtual/bdi/btrfs-1 Env:map[ACTION:remove DEVPATH:/devices/virtual/bdi/btrfs-1 SEQNUM:1159 SUBSYSTEM:bdi]}
[    1.708780] BTRFS: device fsid 5eaa0c1c-e1dc-4be7-9b03-9f1ed5a87289 devid 2 transid 8 /dev/sda2 scanned by init (129)
[    1.711416] booster: udev event {Action:add KObj:/devices/virtual/bdi/btrfs-2 Env:map[ACTION:add DEVPATH:/devices/virtual/bdi/btrfs-2 SEQNUM:1160 SUBSYSTEM:bdi]}
[    1.711548] BTRFS info (device sda2): using crc32c (crc32c-intel) checksum algorithm
[    1.714017] BTRFS info (device sda2): using free space tree
[    1.715232] BTRFS error (device sda2): devid 1 uuid 58a3e858-0b27-4f7c-8aa5-0dfed774177a is missing
[    1.716119] BTRFS error (device sda2): failed to read the system array: -2
[    1.717021] BTRFS error (device sda2): open_ctree failed
[    1.717599] booster: mount(/dev/sda2): invalid argument
[    1.718186] booster: udev event {Action:remove KObj:/devices/virtual/bdi/btrfs-2 Env:map[ACTION:remove DEVPATH:/devices/virtual/bdi/btrfs-2 SEQNUM:1161 SUBSYSTEM:bdi]}
    btrfs_test.go:17:
        	Error Trace:	/home/anatol/sources/golang/booster/tests/btrfs_test.go:17
        	Error:      	Received unexpected error:
        	            	EOF
        	Test:       	TestBtrfsRaid0
2023/02/28 17:53:15 Got error while waiting for Qemu process completion: signal: killed
--- FAIL: TestBtrfsRaid0 (41.28s)

Issue #194

init/main.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unsafe"
 
 	"github.com/yookoala/realpath"
 	"golang.org/x/sys/unix"
@@ -368,6 +369,12 @@ func mountRootFs(dev, fstype string) error {
 	wg := loadModules(fstype)
 	wg.Wait()
 
+	if fstype == "btrfs" {
+		if err := waitForBtrfsDevicesReady(dev); err != nil {
+			return err
+		}
+	}
+
 	rootMountingMutex.Lock()
 	defer rootMountingMutex.Unlock()
 
@@ -390,6 +397,27 @@ func mountRootFs(dev, fstype string) error {
 	return nil
 }
 
+// Wait until all devices of a multiple-device filesystem are scanned and registered within the kernel module
+func waitForBtrfsDevicesReady(dev string) error {
+	controlFile, err := os.OpenFile("/dev/btrfs-control", os.O_RDWR, 0)
+	if err != nil {
+		return err
+	}
+	defer controlFile.Close()
+
+	/* this should be 4k */
+	var btrfsIoctlVolArgs struct {
+		fs   int64
+		name [4088]uint8
+	}
+
+	copy(btrfsIoctlVolArgs.name[:], dev)
+
+	var BTRFS_IOCTL_MAGIC uintptr = 0x94
+	BTRFS_IOC_DEVICES_READY := ior(BTRFS_IOCTL_MAGIC, 39, unsafe.Sizeof(btrfsIoctlVolArgs))
+	return ioctl(controlFile.Fd(), BTRFS_IOC_DEVICES_READY, uintptr(unsafe.Pointer(&btrfsIoctlVolArgs)))
+}
+
 func mountFlags() (uintptr, string) {
 	rootMountFlags, options := sunderMountFlags(rootFlags, rootAutodiscoveryMountFlags)
 	if rootRo {