VPP recycles the graph node when the interface is deleted. Most tx graph nodes have format_tx_function. tapcli does not. If tapcli happens to recycle a tx graph node which has format_tx_function, problem will arise when showing the trace buffer which was collected for the interface that tapcli recycles. So this is what I did.

1. turn on trace for the graph node that has format_tx_function. In this case, I select vhost-user interface.
2. send traffic through the graph node's tx interface.
3. delete the interface that we turned trace on. The trace buffer still has the trace packets for the deleted interface in the system.
4. create a tapcli interface. It recycles the just deleted interface's graph node index.
5. show trace crashes on the format trace function due to tapcli does not have the format_tx_trace function. The reason tapcli is called to display the trace buffer because it recycles vhost-user's tx graph node index.

vpp# trace add vhost-user-input 100
trace add vhost-user-input 100

send traffic through the vhost-user interface

vpp# delete vhost VirtualEthernet0/0/0
delete vhost VirtualEthernet0/0/0
vpp# tap connect tap0
tap connect tap0
tapcli-0
vpp# sh trace
sh trace

Thread 1 "vpp_main" received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) set pagination off
(gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x00007ffff6830eb9 in do_percent (va=0x7fffb60ca988, fmt=0x7ffff6d3cb46 "%U", _s=<synthetic pointer>) at /home/sluong/vpp1901/vpp/src/vppinfra/format.c:373
#2 va_format (s=0x7fffb6dfb62c '' <repeats 19 times>, " Start of thread 0 vpp_main ", '' <repeats 19 times>, "\nNo packets in trace buffer\n", '' <repeats 19 times>, " Start of thread 1 vpp_wk_0 ", '' <repeats 19 times>, "\nNo packets in trace buffer\n", '-' <repeats 12 times>..., fmt=<optimized out>, va=va@entry=0x7fffb60ca988) at /home/sluong/vpp1901/vpp/src/vppinfra/format.c:404
#3 0x00007ffff68308d7 in format (s=<optimized out>, fmt=fmt@entry=0x7ffff6d3cb43 "\n %U") at /home/sluong/vpp1901/vpp/src/vppinfra/format.c:423
#4 0x00007ffff6d1fd35 in format_vlib_trace (s=<optimized out>, va=<optimized out>) at /home/sluong/vpp1901/vpp/src/vlib/trace.c:162
#5 0x00007ffff6830eb9 in do_percent (va=0x7fffb60cab78, fmt=0x7ffff6d3cb94 "%U\n\n", _s=<synthetic pointer>) at /home/sluong/vpp1901/vpp/src/vppinfra/format.c:373
#6 va_format (s=0x7fffb614553c ":00:17:319002: ethernet-input\n frame: flags 0x1, hw-if-index 2, sw-if-index 2\n IP4: 62:54:00:00:04:01 -> 52:54:00:00:04:01\n00:00:17:319005: l2-input\n l2-input: sw_if_index 2 dst 52:54:00:00:04:01 s"..., fmt=<optimized out>, va=va@entry=0x7fffb60cab78) at /home/sluong/vpp1901/vpp/src/vppinfra/format.c:404
#7 0x00007ffff68308d7 in format (s=<optimized out>, fmt=fmt@entry=0x7ffff6d3cb8a "Packet %d\n%U\n\n") at /home/sluong/vpp1901/vpp/src/vppinfra/format.c:423
#8 0x00007ffff6d22ff9 in cli_show_trace_buffer (vm=0x7ffff6f4b3c0 <vlib_global_main>, input=<optimized out>, cmd=<optimized out>) at /home/sluong/vpp1901/vpp/src/vlib/trace.c:297
#9 0x00007ffff6ced315 in vlib_cli_dispatch_sub_commands (vm=vm@entry=0x7ffff6f4b3c0 <vlib_global_main>, cm=cm@entry=0x7ffff6f4b5c0 <vlib_global_main+512>, input=input@entry=0x7fffb60caf60, parent_command_index=<optimized out>) at /home/sluong/vpp1901/vpp/src/vlib/cli.c:644
#10 0x00007ffff6ced6da in vlib_cli_dispatch_sub_commands (vm=vm@entry=0x7ffff6f4b3c0 <vlib_global_main>, cm=cm@entry=0x7ffff6f4b5c0 <vlib_global_main+512>, input=input@entry=0x7fffb60caf60, parent_command_index=parent_command_index@entry=0) at /home/sluong/vpp1901/vpp/src/vlib/cli.c:605
#11 0x00007ffff6ceda90 in vlib_cli_input (vm=0x7ffff6f4b3c0 <vlib_global_main>, input=input@entry=0x7fffb60caf60, function=function@entry=0x7ffff6d2bf80 <unix_vlib_cli_output>, function_arg=function_arg@entry=0) at /home/sluong/vpp1901/vpp/src/vlib/cli.c:744
#12 0x00007ffff6d2dee5 in unix_cli_process_input (cm=0x7ffff6f4b140 <unix_cli_main>, cli_file_index=0) at /home/sluong/vpp1901/vpp/src/vlib/unix/cli.c:2420
#13 unix_cli_process (vm=0x7ffff6f4b3c0 <vlib_global_main>, rt=0x7fffb60ba000, f=<optimized out>) at /home/sluong/vpp1901/vpp/src/vlib/unix/cli.c:2536
#14 0x00007ffff6d01636 in vlib_process_bootstrap (_a=<optimized out>) at /home/sluong/vpp1901/vpp/src/vlib/main.c:1440
#15 0x00007ffff683a18c in clib_calljmp () from /home/sluong/vpp1901/vpp/build-root/install-vpp-native/vpp/lib/libvppinfra.so.19.01.3
#16 0x00007fffb5bffb20 in ?? ()
#17 0x00007ffff6d07379 in vlib_process_startup (f=0x0, p=0x7fffb60ba000, vm=0x7ffff6f4b3c0 <vlib_global_main>) at /home/sluong/vpp1901/vpp/src/vlib/main.c:1462
#18 dispatch_process (vm=0x7ffff6f4b3c0 <vlib_global_main>, p=0x7fffb60ba000, last_time_stamp=0, f=0x0) at /home/sluong/vpp1901/vpp/src/vlib/main.c:1507