// call frame copied from old .h file and renamed
//  Fields:
//    1) For Java frame (interpreted and compiled),
//       lineno    - bci of the method being executed or -1 if bci is not available
//       method_id - jmethodID of the method being executed
//    2) For native method
//       lineno    - (-3)
//       method_id - jmethodID of the method being executed
typedef struct {
    jint lineno;                      // numberline number in the source file
    jmethodID method_id;              // method executed in this frame
} ASGCT_CallFrame;
// call trace copied from old .h file and renamed
// Fields:
//   env_id     - ID of thread which executed this trace.
//   num_frames - number of frames in the trace.
//                (< 0 indicates the frame is not walkable).
//   frames     - the ASGCT_CallFrames that make up this trace. Callee followed by callers.
typedef struct {
    JNIEnv *env_id;                   // Env where trace was recorded
    jint num_frames;                  // number of frames in this trace
    ASGCT_CallFrame *frames;          // frames
} ASGCT_CallTrace;
// These name match the names reported by the forte quality kit
// 這些枚舉是對(duì)應(yīng)到 ASGCT_CallTrace 中的 num_frames 的返回值的
// 舉個(gè)例子，當(dāng) JVM 在進(jìn)行 GC 時(shí)，返回值中
// ASGCT_CallTrace.num_frames == ticks_GC_active
enum {
  ticks_no_Java_frame         =  0,
  ticks_no_class_load         = -1,
  ticks_GC_active             = -2,
  ticks_unknown_not_Java      = -3,
  ticks_not_walkable_not_Java = -4,
  ticks_unknown_Java          = -5,
  ticks_not_walkable_Java     = -6,
  ticks_unknown_state         = -7,
  ticks_thread_exit           = -8,
  ticks_deopt                 = -9,
  ticks_safepoint             = -10
};

//   trace    - trace data structure to be filled by the VM.
//   depth    - depth of the call stack trace.
//   ucontext - ucontext_t of the LWP
void AsyncGetCallTrace(ASGCT_CallTrace *trace, jint depth, void* ucontext)

void AsyncGetCallTrace(ASGCT_CallTrace *trace, jint depth, void* ucontext) {
  JavaThread* thread;
  // 1. 首先判斷 jniEnv 是否為空，或者 jniEnv 對(duì)應(yīng)的線程是否有效，或者該線程是否已經(jīng)退出，
  // 任一條件滿足，則返回 ticks_thread_exit(對(duì)應(yīng)為核心數(shù)據(jù)結(jié)構(gòu)中枚舉類型所示）
  if (trace->env_id == NULL ||
    (thread = JavaThread::thread_from_jni_environment(trace->env_id)) == NULL ||
    thread->is_exiting()) {
    // bad env_id, thread has exited or thread is exiting
    trace->num_frames = ticks_thread_exit; // -8
    return;
  }
  if (thread->in_deopt_handler()) {
    // thread is in the deoptimization handler so return no frames
    trace->num_frames = ticks_deopt; // -9
    return;
  }
  // 2. 這里對(duì) jniEnv 所指線程是否是當(dāng)前線程進(jìn)行斷言，如果不相等則直接報(bào)錯(cuò)
  assert(JavaThread::current() == thread,
         "AsyncGetCallTrace must be called by the current interrupted thread");
  // 3. JVMTI_EVENT_CLASS_LOAD 事件必須 enable，否則直接返回 ticks_no_class_load
  if (!JvmtiExport::should_post_class_load()) {
    trace->num_frames = ticks_no_class_load; // -1
    return;
  }
  // 4. 當(dāng)前 heap 必須沒(méi)有進(jìn)行 GC ，否則直接返回 ticks_GC_active
  if (Universe::heap()->is_gc_active()) {
    trace->num_frames = ticks_GC_active; // -2
    return;
  }
  // 5. 根據(jù)線程的當(dāng)前狀態(tài)來(lái)獲取對(duì)應(yīng)的線程棧，只有在線程的狀態(tài)為 _thread_in_vm/_thread_in_vm_trans 
  // 和 _thread_in_Java/_thread_in_Java_trans 時(shí)才會(huì)進(jìn)行線程棧的爬取
  switch (thread->thread_state()) {
  case _thread_new:
  case _thread_uninitialized:
  case _thread_new_trans:
    // We found the thread on the threads list above, but it is too
    // young to be useful so return that there are no Java frames.
    trace->num_frames = 0;
    break;
  case _thread_in_native:
  case _thread_in_native_trans:
  case _thread_blocked:
  case _thread_blocked_trans:
  case _thread_in_vm:
  case _thread_in_vm_trans:
    {
      frame fr;
      // 首先獲取當(dāng)前線程的棧頂棧幀
      // param isInJava == false - indicate we aren't in Java code
      if (!thread->pd_get_top_frame_for_signal_handler(&fr, ucontext, false)) {
        trace->num_frames = ticks_unknown_not_Java;  // -3 unknown frame
      } else {
        //  該線程如果沒(méi)有任何的 Java 棧幀，直接返回 0 幀 
        if (!thread->has_last_Java_frame()) {
          trace->num_frames = 0; // No Java frames
        } else {
          trace->num_frames = ticks_not_walkable_not_Java;    // -4 non walkable frame by default

          // 如果存在合法的棧幀，則填充 trace 中的 frames 和 num_frames
          forte_fill_call_trace_given_top(thread, trace, depth, fr);
          ...
        }
      }
    }
    break;
  case _thread_in_Java:
  case _thread_in_Java_trans:
    {
      frame fr;
      // param isInJava == true - indicate we are in Java code
      if (!thread->pd_get_top_frame_for_signal_handler(&fr, ucontext, true)) {
        trace->num_frames = ticks_unknown_Java;  // -5 unknown frame
      } else {
        trace->num_frames = ticks_not_walkable_Java;  // -6, non walkable frame by default
        forte_fill_call_trace_given_top(thread, trace, depth, fr);
      }
    }
    break;
  default:
    // Unknown thread state
    trace->num_frames = ticks_unknown_state; // -7
    break;
  }
}

bool JavaThread::pd_get_top_frame_for_signal_handler(frame* fr_addr,
  void* ucontext, bool isInJava) {
  assert(Thread::current() == this, "caller must be current thread");
  return pd_get_top_frame(fr_addr, ucontext, isInJava);
}

bool JavaThread::pd_get_top_frame(frame* fr_addr, void* ucontext, bool isInJava) {
  assert(this->is_Java_thread(), "must be JavaThread");
  JavaThread* jt = (JavaThread *)this;
  // If we have a last_Java_frame, then we should use it even if
  // isInJava == true.  It should be more reliable than ucontext info.
  if (jt->has_last_Java_frame() && jt->frame_anchor()->walkable()) {
    *fr_addr = jt->pd_last_frame();
    return true;
  }
  // At this point, we don't have a last_Java_frame, so
  // we try to glean some information out of the ucontext
  // if we were running Java code when SIGPROF came in.
  if (isInJava) {
    ucontext_t* uc = (ucontext_t*) ucontext;
    intptr_t* ret_fp;
    intptr_t* ret_sp;
    ExtendedPC addr = os::Linux::fetch_frame_from_ucontext(this, uc,
      &ret_sp, &ret_fp);
    if (addr.pc() == NULL || ret_sp == NULL ) {
      // ucontext wasn't useful
      return false;
    }
    frame ret_frame(ret_sp, ret_fp, addr.pc());
    if (!ret_frame.safe_for_sender(jt)) {
#ifdef COMPILER2
      // C2 uses ebp as a general register see if NULL fp helps
      frame ret_frame2(ret_sp, NULL, addr.pc());
      if (!ret_frame2.safe_for_sender(jt)) {
        // nothing else to try if the frame isn't good
        return false;
      }
      ret_frame = ret_frame2;
#else
      // nothing else to try if the frame isn't good
      return false;
#endif /* COMPILER2 */
    }
    *fr_addr = ret_frame;
    return true;
  }
  // nothing else to try
  return false;
}

static void forte_fill_call_trace_given_top(JavaThread* thd,
                                            ASGCT_CallTrace* trace,
                                            int depth,
                                            frame top_frame) {
  NoHandleMark nhm;
  frame initial_Java_frame;
  Method* method;
  int bci = -1; // assume BCI is not available for method
                // update with correct information if available
  int count;
  count = 0;
  assert(trace->frames != NULL, "trace->frames must be non-NULL");
  // 1. 獲取到棧頂?shù)牡谝粋€(gè) java 棧幀
  // Walk the stack starting from 'top_frame' and search for an initial Java frame.
  find_initial_Java_frame(thd, &top_frame, &initial_Java_frame, &method, &bci);
  // Check if a Java Method has been found.
  if (method == NULL) return;
  //  2. 如果不是合法的方法，直接返回
  if (!method->is_valid_method()) {
    trace->num_frames = ticks_GC_active; // -2
    return;
  }
  vframeStreamForte st(thd, initial_Java_frame, false);
  // 循環(huán)迭代棧上的所有棧幀，一一獲取每個(gè)方法 bci 和 方法 id，這里會(huì)用上從外面?zhèn)魅氲淖畲髼Ｉ?depth
  for (; !st.at_end() && count < depth; st.forte_next(), count++) {
    bci = st.bci();
    method = st.method();
    if (!method->is_valid_method()) {
      // we throw away everything we've gathered in this sample since
      // none of it is safe
      trace->num_frames = ticks_GC_active; // -2
      return;
    }
    // 根據(jù)方法對(duì)象獲取方法 id，如果方法 id 在此時(shí)還未產(chǎn)生，則返回 NULL
    trace->frames[count].method_id = method->find_jmethod_id_or_null();

    // 如果方法是不是 native 方法，則把 lineno 設(shè)置為 bci 的值，否則置 -3 
    if (!method->is_native()) {
      trace->frames[count].lineno = bci;
    } else {
      trace->frames[count].lineno = -3;
    }
  }
  trace->num_frames = count;
  return;
}