Unison can't sync properly existing files over MTPFS

[ericzolf]

I've tried the following (and this is repeatable on multiple devices, with different Android versions, e.g. LineageOS 15.1 on Samsung Tablet vs. Android 8.1.0 on BQ Phone):

simple-mtpfs dirA
mkdir -p dirA/testdir
mkdir dirB
echo Android > dirA/testdirA/test.txt
echo PC > dirB/testdirB/test.txt

then setup unison to sync testdirA with testdirB.

1. first sync -> unison shows a conflict, decide in favour of the PC, sync, all looks well
2. 2nd sync -> unison states:

test.txt Error: Two or more files on a case-sensitive system have names identical except for case. They cannot be synchronized to a case-insensitive file system. (File 'test.txt')

$ ll dirA/testdirA
total 1
-rw-r--r--. 1 ericl ericl 3 Jul 14 10:35 test.txt
-rw-r--r--. 1 ericl ericl 3 Jul 14 10:35 test.txt

(notice the size of 3 for both files)

3. on the Android device, there is only one test.txt file and it has the right content "PC"
4. un-plug, re-plug, re-mount the Android device:

$ ll -ia dirA/testdirA
total 1
3 -rw-r--r--. 1 ericl ericl 8 Jul 14 10:34 test.txt
4 -rw-r--r--. 1 ericl ericl 3 Jul 14 10:35 .unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp
$ cat dirA/testdirA/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp 
cat: /tmp/bla/rdtmp/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp: Numerical result out of range
$ cat dirA/testdirA/rdtmp/test.txt 
PC

(the file .unison.test... definitely isn't seen on Android)

While doing the last 3 commands (1x ls + 2 x cat) shows in debug mode the following:

unique: 187, opcode: LOOKUP (1), nodeid: 1, insize: 46, pid: 6798
LOOKUP /testdirA
getattr /testdirA
   NODEID: 2
   unique: 187, success, outsize: 144
unique: 188, opcode: OPENDIR (27), nodeid: 2, insize: 48, pid: 6798
opendir flags: 0x18800 /testdirA
   opendir[0] flags: 0x18800 /testdirA
   unique: 188, success, outsize: 32
unique: 189, opcode: READDIR (28), nodeid: 2, insize: 80, pid: 6798
readdir[0] from 0
   unique: 189, success, outsize: 136
unique: 190, opcode: LOOKUP (1), nodeid: 2, insize: 101, pid: 6798
LOOKUP /testdirA/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp
getattr /testdirA/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp
   NODEID: 4
   unique: 190, success, outsize: 144
unique: 191, opcode: LOOKUP (1), nodeid: 2, insize: 49, pid: 6798
LOOKUP /testdirA/test.txt
getattr /testdirA/test.txt
   NODEID: 3
   unique: 191, success, outsize: 144
unique: 192, opcode: READDIR (28), nodeid: 2, insize: 80, pid: 6798
   unique: 192, success, outsize: 16
unique: 193, opcode: RELEASEDIR (29), nodeid: 2, insize: 64, pid: 0
releasedir[0] flags: 0x0
   unique: 193, success, outsize: 16
unique: 194, opcode: LOOKUP (1), nodeid: 1, insize: 46, pid: 6805
LOOKUP /testdirA
getattr /testdirA
   NODEID: 2
   unique: 194, success, outsize: 144
unique: 195, opcode: LOOKUP (1), nodeid: 2, insize: 101, pid: 6805
LOOKUP /testdirA/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp
getattr /testdirA/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp
   NODEID: 4
   unique: 195, success, outsize: 144
unique: 196, opcode: OPEN (14), nodeid: 4, insize: 48, pid: 6805
open flags: 0x8000 /testdirA/.unison.test.txt.a8cbe6a01ee35a5b7a3843a1058af622.unison.tmp
Error 2: PTP Layer error 02fe: LIBMTP_Get_File_To_File_Descriptor(): Could not get file from device.
Error 2: Error 02fe: PTP Data Expected
fuse: bad error value: 2
   unique: 196, error: -34 (Numerical result out of range), outsize: 16
unique: 197, opcode: LOOKUP (1), nodeid: 1, insize: 46, pid: 6813
LOOKUP /testdirA
getattr /testdirA
   NODEID: 2
   unique: 197, success, outsize: 144
unique: 198, opcode: LOOKUP (1), nodeid: 2, insize: 49, pid: 6813
LOOKUP /testdirA/test.txt
getattr /testdirA/test.txt
   NODEID: 3
   unique: 198, success, outsize: 144
unique: 199, opcode: OPEN (14), nodeid: 3, insize: 48, pid: 6813
open flags: 0x8000 /testdirA/test.txt
   open[4] flags: 0x8000 /testdirA/test.txt
   unique: 199, success, outsize: 32
unique: 200, opcode: READ (15), nodeid: 3, insize: 80, pid: 6813
read[4] 4096 bytes from 0 flags: 0x8000
   read[4] 3 bytes from 0
   unique: 200, success, outsize: 19
unique: 201, opcode: GETATTR (3), nodeid: 3, insize: 56, pid: 6813
getattr /testdirA/test.txt
   unique: 201, success, outsize: 120
unique: 202, opcode: READ (15), nodeid: 3, insize: 80, pid: 6813
read[4] 4096 bytes from 0 flags: 0x8000
   read[4] 3 bytes from 0
   unique: 202, success, outsize: 19
unique: 203, opcode: FLUSH (25), nodeid: 3, insize: 64, pid: 6813
flush[4]
   unique: 203, success, outsize: 16
unique: 204, opcode: RELEASE (18), nodeid: 3, insize: 64, pid: 0
release[4] flags: 0x8000
   unique: 204, success, outsize: 16

In my opinion, there are potentially 2 errors:

1. overwriting a file didn't work properly
2. simple-tmpfs seems to keep some kind of (wrong) cache across re-mounting a file system

Perhaps there is a smart combination of options that would make it work, but there are so many of them that at least a hint which one(s) could be helpful would be really nice.

[ericzolf]

After a reboot of the PC, the issue is gone, so it sounds really like a caching issue. I tried again with direct_io but the result was the same (debug log: mtpfs_debug.txt