[sanitizer_common] Allow customization of StartSymbolizerSubprocess

[yln]

Summary

Make SymbolizerProcess::StartSymbolizerSubprocess virtual and protected
to allow subclasses to customize it via "override and call".

[delcypher]

LGTM but I don't own this code so please allow time for the owners to speak up in case they object.

compiler-rt/lib/sanitizer_common/sanitizer_symbolizer_internal.h

 // external symbolizer program, running in a different subprocess.
 // SymbolizerProcess may not be used from two threads simultaneously.
 class SymbolizerProcess {
  public:
   explicit SymbolizerProcess(const char *path, bool use_forkpty = false);
   const char *SendCommand(const char *command);
 
  protected:
+  /// The maximum number of arguments required to invoke a tool process.
+  static const unsigned kArgVMax = 6;
+
+  // Customizable by subclasses.
+  virtual bool StartSymbolizerSubprocess();
+  virtual bool ReadFromSymbolizer(char *buffer, uptr max_length);
+
+ private:
   virtual bool ReachedEndOfOutput(const char *buffer, uptr length) const {
     UNIMPLEMENTED();
   }
 
-  /// The maximum number of arguments required to invoke a tool process.
-  enum { kArgVMax = 6 };
-
   /// Fill in an argv array to invoke the child process.
   virtual void GetArgV(const char *path_to_binary,
                        const char *(&argv)[kArgVMax]) const {
     UNIMPLEMENTED();
   }
 
-  virtual bool ReadFromSymbolizer(char *buffer, uptr max_length);
-
- private:
   bool Restart();
   const char *SendCommandImpl(const char *command);
   bool WriteToSymbolizer(const char *buffer, uptr length);
-  bool StartSymbolizerSubprocess();
 
   const char *path_;
   fd_t input_fd_;
   fd_t output_fd_;
 
   static const uptr kBufferSize = 16 * 1024;
   char buffer_[kBufferSize];
 

compiler-rt/lib/sanitizer_common/sanitizer_symbolizer_mac.cc

public:
   explicit AtosSymbolizerProcess(const char *path, pid_t parent_pid)
       : SymbolizerProcess(path, /*use_forkpty*/ true) {
     // Put the string command line argument in the object so that it outlives
     // the call to GetArgV.
     internal_snprintf(pid_str_, sizeof(pid_str_), "%d", parent_pid);
   }
 
  private:
+  virtual bool StartSymbolizerSubprocess() override {
+    // Configure sandbox before starting atos process.
+    return SymbolizerProcess::StartSymbolizerSubprocess();
+  }
+
   bool ReachedEndOfOutput(const char *buffer, uptr length) const override {
     return (length >= 1 && buffer[length - 1] == '\n');
   }
 
   void GetArgV(const char *path_to_binary,
                const char *(&argv)[kArgVMax]) const override {
     int i = 0;
     argv[i++] = path_to_binary;