OK, I've tested it in clean multipass VMs and test-snapd-rsync-core20.rsync does not work on Bionic, Focal and later. Interestingly it ships a local copy of libc6 inside that could be the problem and it worked on Focal for some time due to accidentally matching the host's libc6.
...
Reading target:/usr/lib/debug/snap/test-snapd-rsync-core20/11/lib/x86_64-linux-gnu//libc-2.31.so from remote target...
Program received signal SIGSEGV, Segmentation fault.
0x000055eb84b7b2d4 in time@plt ()
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007f070255d100 0x00007f070257f7c4 Yes (*) target:/lib64/ld-linux-x86-64.so.2 No linux-vdso.so.1
0x00007f07025514f0 0x00007f07025557e8 Yes (*) target:/snap/test-snapd-rsync-core20/11/usr/lib/x86_64-linux-gnu/libacl.so.1
0x00007f0702543720 0x00007f070254a92d Yes (*) target:/snap/test-snapd-rsync-core20/11/usr/lib/x86_64-linux-gnu/libpopt.so.0
0x00007f0702374630 0x00007f07024e908f Yes (*) target:/snap/test-snapd-rsync-core20/11/lib/x86_64-linux-gnu/libc.so.6
(*): Shared library is missing debugging information.
For the record test-snapd-rsync-core18.rsync does not ship and internal libc copy and does work on all releases I tried (Bionic, Focal).
OK, I've tested it in clean multipass VMs and test-snapd- rsync-core20. rsync does not work on Bionic, Focal and later. Interestingly it ships a local copy of libc6 inside that could be the problem and it worked on Focal for some time due to accidentally matching the host's libc6.
... /usr/lib/ debug/snap/ test-snapd- rsync-core20/ 11/lib/ x86_64- linux-gnu/ /libc-2. 31.so from remote target...
Reading target:
Program received signal SIGSEGV, Segmentation fault. /lib64/ ld-linux- x86-64. so.2
No linux-vdso.so.1 /snap/test- snapd-rsync- core20/ 11/usr/ lib/x86_ 64-linux- gnu/libacl. so.1 /snap/test- snapd-rsync- core20/ 11/usr/ lib/x86_ 64-linux- gnu/libpopt. so.0 /snap/test- snapd-rsync- core20/ 11/lib/ x86_64- linux-gnu/ libc.so. 6
0x000055eb84b7b2d4 in time@plt ()
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007f070255d100 0x00007f070257f7c4 Yes (*) target:
0x00007f07025514f0 0x00007f07025557e8 Yes (*) target:
0x00007f0702543720 0x00007f070254a92d Yes (*) target:
0x00007f0702374630 0x00007f07024e908f Yes (*) target:
(*): Shared library is missing debugging information.
For the record test-snapd- rsync-core18. rsync does not ship and internal libc copy and does work on all releases I tried (Bionic, Focal).