HANG assert failure causes hang in drcachesim

[abhinav92003]

Describe the bug
While working on #4341 I noticed that failure of DR_ASSERT causes a hang in drcachesim, which then needs to be terminated manually.

To reproduce

1. Patch the following diff. It causes the assert failure, which reveals the bug.

clients/drcachesim/tests/allasm_aarch64_prefetch.asm
@@ -163,6 +163,8 @@ _start:
         prfum    pstl3strm, [x1]
         prfum    pstl3strm, [x1]
 
+        prfum    28, [x1]
+
         // Exit.
         mov      w0, #1            // stdout
         adr      x1, helloworld

2. On an AArch64 machine, run the following commands

cmake -DDEBUG=ON  -DBUILD_TESTS=ON <path-to-source>
make -j
./bin64/drrun -t drcachesim -- suite/tests/bin/allasm_aarch64_prefetch

3. This will cause an assert failure at

   dynamorio/clients/drcachesim/tracer/instru.cpp

   Line 147 in cdc24cb

   DR_ASSERT_MSG(false, "Unsupported AArch64 prefetch operation.");

ASSERT FAILURE: /home/abhinavas/dr/src/i4328-1/clients/drcachesim/tracer/instru.cpp:147: false (Unsupported AArch64 prefetch operation.)

But the process does not exit.

ps -x output:

abhinavas@tx1:~$ ps -x
  PID TTY      STAT   TIME COMMAND
...
50632 pts/4    R+     0:02 /home/abhinavas/dr/build/i4328-1/bin64/../clients/bin64/drcachesim -dr /home/abhinavas/dr/build/i4328-1/bin64/.. -dr_ops  -tracer /home/abhinavas/dr/build/i4328-1/bin64/../clients/lib64/debug/libdrmemtrace.so -tracer_alt /home/abhinavas/dr/build/i4328
50633 pts/4    Z+     0:00 [allasm_aarch64_] <defunct>
...

The test app (allasm_aarch64_prefetch) is shown as defunct and the drcachesim process is still running. Based on man:

Processes marked <defunct> are dead processes (so-called "zombies")
       that remain because their parent has not destroyed them properly.

• What happens when you run without any client?
  No issues.
• What happens when you run with debug build ("-debug" flag to drrun/drconfig/drinject)?
  Same issue with -dr_debug

Expected behavior
drcachesim and test app should exit properly after the assert failure

Versions

• What version of DynamoRIO are you using?
  At commit cdc24cb

• What operating system version are you running on?
  Debian GNU/Linux 9.5 (stretch) on AArch64

• Is your application 32-bit or 64-bit?
  64-bit

Additional context
I found that after adding -dr_ops -unsafe_build_ldstex and setarch, the processes exit as expected.

./bin64/drrun -t drcachesim  -dr_ops -unsafe_build_ldstex --  setarch `uname -m` -R suite/tests/bin/allasm_aarch64_prefetch

I confirmed that both the additional options are required.

[derekbruening]

Xref #4296. Normally this would be due to the pipe not being closed, which would be dependent on a clean exit, and unrelated to -unsafe_build_ldstex, which affects whether the app hangs during code cache execution.