We experienced some deadlocks when we used multiple threads for logging
using `scan-builds` intercept-build tool when we used multiple threads by
e.g. logging `make -j16`
```
(gdb) bt
#0 0x00007f2bb3aff110 in __lll_lock_wait () from /lib/x86_64-linux-gnu/libpthread.so.0
#1 0x00007f2bb3af70a3 in pthread_mutex_lock () from /lib/x86_64-linux-gnu/libpthread.so.0
#2 0x00007f2bb3d152e4 in ?? ()
#3 0x00007ffcc5f0cc80 in ?? ()
#4 0x00007f2bb3d2bf5b in ?? () from /lib64/ld-linux-x86-64.so.2
#5 0x00007f2bb3b5da27 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#6 0x00007f2bb3b5dbe0 in exit () from /lib/x86_64-linux-gnu/libc.so.6
#7 0x00007f2bb3d144ee in ?? ()
#8 0x746e692f706d742f in ?? ()
#9 0x692d747065637265 in ?? ()
#10 0x2f653631326b3034 in ?? ()
#11 0x646d632e35353532 in ?? ()
#12 0x0000000000000000 in ?? ()
```
I think the gcc's exit call caused the injected `libear.so` to be unloaded
by the `ld`, which in turn called the `void on_unload() __attribute__((destructor))`.
That tried to acquire an already locked mutex which was left locked in the
`bear_report_call()` call, that probably encountered some error and
returned early when it forgot to unlock the mutex.
All of these are speculation since from the backtrace I could not verify
if frames 2 and 3 are in fact corresponding to the `libear.so` module.
But I think it's a fairly safe bet.
So, hereby I'm releasing the held mutex on *all paths*, even if some failure
happens.
PS: I would use lock_guards, but it's C.
Reviewed-by: NoQ
Differential Revision: https://reviews.llvm.org/D118439
d919d027ba