[PoC] spawn a GDB server on panics and hardfaults

[japaric]

when the --gdb flag is passed, probe-run will spawn a GDB server when
the program panics or encounters a hardware fault (HardFault)

a GDB client can then connect to inspect the failure

blockers

• AFAICT, there's no way to clean shutdown the GDB server implementation I used here. we may want to consider using gdbstub instead. another advantage of using gdbstub is that we could forbid operations like finish, step and next which don't make sense in postmortem context as it's no longer possible to continue executing the program

open questions

• should we spawn a GDB server on stack overflows? a stack overflow corrupts the top frame so it becomes impossible to compute a backtrace. however, it's still possible to read CPU registers though it may be hard to map these back to meaningful variables as the frame command won't work in this context (see code block below)

$ arm-none-eabi-gdb -q target/thumbv7em-none-eabihf/debug/overflow
Reading symbols from target/thumbv7em-none-eabihf/debug/overflow...
(gdb) target remote :1337
Remote debugging using :1337
HardFaultTrampoline () at asm.S:19
19      asm.S: No such file or directory.
(gdb) backtrace
#0  HardFaultTrampoline () at asm.S:19
#1  <signal handler called>
Backtrace stopped: Cannot access memory at address 0x1ffff9a8
(gdb) frame 2
No frame at level 2.
(gdb) info registers
r0             0x7                 7
r1             0x1                 1
r2             0x2003fcdb          537132251
r3             0x2003fcdb          537132251
r4             0x1ffff9bc          536869308
r5             0x0                 0
r6             0x0                 0
r7             0x200039cc          536885708
r8             0x0                 0
r9             0x0                 0
r10            0x0                 0
r11            0x0                 0
r12            0x200079dc          536902108
sp             0x1ffff990          0x1ffff990
lr             0xfffffff9          4294967289
pc             0x16c8              0x16c8 <HardFaultTrampoline>
cpsr           0x21000003          553648131

alternative / possible extension

• allow running the GDB server and defmt logs concurrently.
  • the problem with this if using gdb-server is that its API is hard-coded to use std::sync::Mutex which is not guaranteed to be a fair mutex on all platforms. the defmt logs are collected in a tight loop, via RTT, and that loop also needs to access the mutex that gdb-server uses. the like outcome is that the RTT loop "hogs" the mutex and starves the gdb-server thread. a gdbstub implementation that uses a fair mutex (e.g. parking_lot::FairMutex) would not run into such problem
  • the downside of this feature is that the throughput of the RTT logs would go down. as the gdb feature is opt-in that may not be much of an issue as long the downside is properly documented