Modify GDBRemoteCommunication::ScopedTimeout to not ever decrease a timeout

[clayborg]

Summary

We use GDBRemoteCommunication::ScopedTimeout in many places to change the packet timeout that is used for individual packets. If someone modifies the default timeout manually or the GDB remote server requests a longer timeout in a 'q' packet, then don't ever reduce a timeout for a packet since that could make things fail.

This patch checks the timeout to ensure the new timeout is larger before it modifies the timeout. The GDBRemoteCommunication::ScopedTimeout object also remembers if it did update the timeout and will restore the old timeout only if it did.

[labath]

looks good, thank you.

lldb/trunk/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.h

public:
   public:
     ScopedTimeout(GDBRemoteCommunication &gdb_comm,
                   std::chrono::seconds timeout);
     ~ScopedTimeout();
 
   private:
     GDBRemoteCommunication &m_gdb_comm;
     std::chrono::seconds m_saved_timeout;
+    // Don't ever reduce the timeout for a packet, only increase it. If the
+    // requested timeout if less than the current timeout, we don't set it
+    // and won't need to restore it.
+    bool m_timeout_modified;
   };
 
   GDBRemoteCommunication(const char *comm_name, const char *listener_name);
 
   ~GDBRemoteCommunication() override;
 
   PacketResult GetAck();
 

lldb/trunk/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp

#endif
 
   return error;
 }
 
 void GDBRemoteCommunication::DumpHistory(Stream &strm) { m_history.Dump(strm); }
 
 GDBRemoteCommunication::ScopedTimeout::ScopedTimeout(
     GDBRemoteCommunication &gdb_comm, std::chrono::seconds timeout)
-    : m_gdb_comm(gdb_comm) {
+  : m_gdb_comm(gdb_comm), m_timeout_modified(false) {
+    auto curr_timeout = gdb_comm.GetPacketTimeout();
+    // Only update the timeout if the timeout is greater than the current
+    // timeout. If the current timeout is larger, then just use that.
+    if (curr_timeout < timeout) {
+      m_timeout_modified = true;
       m_saved_timeout = m_gdb_comm.SetPacketTimeout(timeout);
     }
+}
 
 GDBRemoteCommunication::ScopedTimeout::~ScopedTimeout() {
+  // Only restore the timeout if we set it in the constructor.
+  if (m_timeout_modified)
     m_gdb_comm.SetPacketTimeout(m_saved_timeout);
 }
 
 // This function is called via the Communications class read thread when bytes
 // become available
 // for this connection. This function will consume all incoming bytes and try to
 // parse whole
 // packets as they become available. Full packets are placed in a queue, so that
 // all packet