[trace] clear any existing tracing sessions before relaunching the binary

[wallace]

Summary

There's a bug caused when a process is relaunched: the target, which
doesn't change, keeps the Trace object from the previous process, which
is already defunct, and causes segmentation faults when it's attempted
to be used.
A fix is to clean up the Trace object when the target is disposing of
the previous process during relaunches.

A way to reproduce this:

lldb a.out
b main
r
process trace start
c
r
process trace start

[wallace]

improve test

[jj10306]

nice catch, lgtm!

lldb/source/Target/Target.cpp

void Target::CleanupProcess() {
   DisableAllWatchpoints(false);
   ClearAllWatchpointHitCounts();
   ClearAllWatchpointHistoricValues();
   m_latest_stop_hook_id = 0;
 }
 
 void Target::DeleteCurrentProcess() {
   if (m_process_sp) {
+    // We dispose any active tracing sessions on the current process
+    m_trace_sp.reset();
     m_section_load_history.Clear();
     if (m_process_sp->IsAlive())
       m_process_sp->Destroy(false);
 
     m_process_sp->Finalize();
 
     CleanupProcess();
 

lldb/test/API/commands/trace/TestTraceStartStop.py

\[  0\] {ADDRESS_REGEX}    xorl'''])
 
         self.expect("c")
         # Now the process has finished, so the commands should fail
         self.expect("thread trace start", error=True,
             substrs=["error: Process must be launched"])
 
         self.expect("thread trace stop", error=True,
             substrs=["error: Process must be launched"])
+
+        # We should be able to trace the program if we relaunch it
+        # For this, we'll trace starting at a different point in the new
+        # process.
+        self.expect("breakpoint disable")
+        self.expect("b main.cpp:4")
+        self.expect("r")
+        self.expect("thread trace start")
+        # We can reconstruct the single instruction executed in the first line
+        self.expect("si")
+        self.expect("thread trace dump instructions -c 1",
+            patterns=[f'''thread #1: tid = .*
+  a.out`main \+ 11 at main.cpp:4'''])