Crash "path /tmp is not a directory" on mavsdk-server 3.0.0

[AlexShafir]

Description: NDK crash happens when I try to connect from Android app to Mission Planner ArduCopter SITL in same network.
Surprisingly, it worked ok some time ago. SITL runs correctly when launched inside Mission Planner.
I tried "tcpin" - endless waiting, tcpout - same crash.
I also tested on other device and laptop, and it worked there.
I have disabled Microsoft Defender Firewall, no effect.

Error message: terminating with uncaught exception of type std::__ndk1::__fs::filesystem::filesystem_error: filesystem error: in temp_directory_path: path "/tmp" is not a directory: Not a directory

SITL: Windows 10, Mission Planner, ArduCopter v.4.5.7
Phone: Physical phone with Android 9

Libs:

implementation("io.mavsdk:mavsdk:3.0.0")
implementation("io.mavsdk:mavsdk-server:3.0.0")

Connect code:

val mMavSdkServer = MavsdkServer()
val systemAddress = "tcp://192.168.178.103:5760"
MavsdkEventQueue.executor().execute {
  val mavsdkServerPort = mMavsdkServer.run(systemAddress)
  val mDrone = System("127.0.0.1", mavsdkServerPort)
}

Mavsdk log:

MAVSDK version: v3.5.0
Waiting to discover system on tcp://192.168.178.103:5760...
Connection using tcp:// is deprecated, please use tcpin:// or tcpout://
New system ID: 1 Comp ID: 1
TCP receive error: Interrupted system call, trying to reeconnect...

NDK Crash:

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Revision: '0'
ABI: 'arm64'
pid: 29968, tid: 30037, name: mavsdk-event-qu  >>> com.dev.drone <<<
signal 6 (SIGABRT), code -6 (SI_TKILL), fault addr --------
Abort message: 'terminating with uncaught exception of type std::__ndk1::__fs::filesystem::filesystem_error: filesystem error: in temp_directory_path: path "/tmp" is not a directory: Not a directory'
    x0  0000000000000000  x1  0000000000007555  x2  0000000000000006  x3  0000000000000008
    x4  feff78716e736264  x5  feff78716e736264  x6  feff78716e736264  x7  7f7f7f7f7f7f7f7f
    x8  0000000000000083  x9  2083438910111022  x10 0000000000000000  x11 fffffffc7ffffbdf
    x12 0000000000000001  x13 79726f7463657269  x14 0000000000000008  x15 00000078ee2f2470
    x16 00000079950262b8  x17 0000007994f45ee0  x18 0000000000000010  x19 0000000000007510
    x20 0000000000007555  x21 ffffff80ffffffc8  x22 00000078ee2f2ea0  x23 00000078ee2f2ee0
    x24 00000078ee2f2fc0  x25 00000078ee2f2f08  x26 00000078ee2f4ca0  x27 0020c49ba5e353f7
    x28 0000000000988f2d  x29 00000078ee2f2e10
    sp  00000078ee2f2dd0  lr  0000007994f39354  pc  0000007994f3937c

backtrace:
    #00 pc 000000000002237c  /system/lib64/[libc.so](http://libc.so/) (abort+116)
    #01 pc 00000000000d1cc0  /data/app/com.my.app-rW82WzvYSQQgauARJ7aAWw==/base.apk (offset 0x58000)

[julianoes]

Does this happen in the emulator and/or the real phone?

I suspect it comes from here:
https://github.com/mavlink/MAVSDK/blob/1ff26d163358faa92890eda4452feb1a863896eb/src/mavsdk/core/fs_utils.cpp#L93-L96

[AlexShafir]

Tested on Pixel 8 (API 36) emulator on same machine with SITL.
There I always receive "Java MavSdkException: No system" for setTakeoffAltitude.
MavSdk log:

MAVSDK version: v3.5.0
Waiting to discover system on tcpout://192.168.178.103:5760...
New system ID: 1 Comp ID: 1
Component Autopilot (1) added.
Vehicle type changed (new type: 2, old type: 0)
Discovered 1 component(s)
System discovered
type=1400 audit(0.0:518): avc:  denied  { read } for  name="somaxconn" dev="proc" ino=85190 scontext=u:r:untrusted_app:s0:c215,c256,c512,c768 tcontext=u:object_r:proc_net:s0 tclass=file permissive=0 app=com.my.app
Server started
Server set to listen on 0.0.0.0:35561
mavsdk_server is now running, listening on port 35561
< Java MavSdkException: No system >
MAVLink: info: GPS 1: detected as u-blox at 230400 baud

[AlexShafir]

This crash happens only with mavsdk-server 3.0.0
Last working version for me is mavsdk-server:2.1.6

Maybe this is related bug https://discuss.ardupilot.org/t/sitl-tcp-socket-closure-when-connecting-with-mavsdk/83806

[julianoes]

There I always receive "Java MavSdkException: No system" for setTakeoffAltitude.
MavSdk log:

No system would mean the connection doesn't actually work and once you try to access the system, it crashes, I'd guess.

[AlexShafir]

No system would mean the connection doesn't actually work

Tested mavsdk-server 3.0.0 (mavsdk 3.0.0) on:

• Emulators Android 10, Android 11, NDK crash.
• Android 9 phone, NDK crash.
• Emulator Android 16, No system exception after executing command
• worked on OnePlus 9 Pro with Android 14.

I got a better stacktrace, here is where 'path /tmp is not a directory' happen:

std::__ndk1::__fs::filesystem::__temp_directory_path(std::__ndk1::error_code*)+431
(mavsdk::create_tmp_directory(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> > const&)+37)
(mavsdk::MavlinkComponentMetadata::MavlinkComponentMetadata(mavsdk::SystemImpl&)+1437)
(mavsdk::SystemImpl::SystemImpl(mavsdk::MavsdkImpl&)+888)
mavsdk::System::System(mavsdk::MavsdkImpl&)+62
mavsdk::MavsdkImpl::make_system_with_component(unsigned char, unsigned char)+755

Downgrading to mavsdk-server 2.1.6:

• on emulator Android 11 it works if I add 5 sec delay before command (my emulator is a bit slow)
• works on Android 9 phone

Additionally on Android 11 emulator in MavSdk log following line appears:
type=1400 audit(0.0:518): avc: denied { read } for name="somaxconn" dev="proc" ino=85190 scontext=u:r:untrusted_app:s0:c215,c256,c512,c768 tcontext=u:object_r:proc_net:s0 tclass=file permissive=0 app=com.my.app
It means SELinux on Android restricted access to certain api.

Conclusion:
For mavsdk-server 3.0.0

• Incorrect connection state (No system) on Android 16 emulator
• Worked on OnePlus 9 Pro with Android 14.
• NDK crash 'path /tmp is not a directory' in other cases

Maybe ndk crash is suppressed on Android 16.
And /tmp folder was probably handled better in custom ROM on OnePlus device.

My guess is that 'path /tmp is not a directory' is root cause.

[julianoes]

My problem is that I have no example that I can run to just reproduce this. I believe the example from this repo is broken. If someone were to clean it up, that would help me because I could easily just reproduce things.

I'm not using Java or Android in my day to day, so that's why I'm not very deep in this and can just fix things as they pop up.

[AlexShafir]

I took a look at MavSdk-Java Android sample, broken thing there is expired mapbox key, it should be updated to start working.
Also, MapBox can be replaced with alternatives that do not require key like vtm-android.
I need some to think what to do best.

I created a small Android app that has TCP IP field and following buttons: Connect, Arm, Takeoff, Land and shows status message, altitude.
It can be used to reproduce my bug.
Repo here: https://github.com/AlexShafir/MavSdkExample
I have tested this example and it is working (with mavsdk-server 2.1.6)

[julianoes]

That's great, thanks @AlexShafir. I'll look at this later this week.

[julianoes]

If you can give me some pointers how you could connect to localhost from within the emulator that might help. So far, I haven't had success. All I see is that the button switches to Disconnect.

Ah, now I see it in logcat:

 Waiting to discover system on tcpin://10.0.2.2:5760...

Also tried tcpout. I don't know how TCP works with ArduPilot...