abseil-cpp/absl/debugging/internal/stacktrace_emscripten-inl.inc

// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Portable implementation - just use glibc
//
// Note:  The glibc implementation may cause a call to malloc.
// This can cause a deadlock in HeapProfiler.

#ifndef ABSL_DEBUGGING_INTERNAL_STACKTRACE_EMSCRIPTEN_INL_H_
#define ABSL_DEBUGGING_INTERNAL_STACKTRACE_EMSCRIPTEN_INL_H_

#include <emscripten.h>

#include <atomic>
#include <cstring>

#include "absl/base/attributes.h"
#include "absl/debugging/stacktrace.h"

extern "C" {
uintptr_t emscripten_stack_snapshot();
uint32_t emscripten_stack_unwind_buffer(uintptr_t pc, void *buffer,
                                        uint32_t depth);
}

// Sometimes, we can try to get a stack trace from within a stack
// trace, which can cause a self-deadlock.
// Protect against such reentrant call by failing to get a stack trace.
//
// We use __thread here because the code here is extremely low level -- it is
// called while collecting stack traces from within malloc and mmap, and thus
// can not call anything which might call malloc or mmap itself.
static __thread int recursive = 0;

// The stack trace function might be invoked very early in the program's
// execution (e.g. from the very first malloc).
// As such, we suppress usage of backtrace during this early stage of execution.
static std::atomic<bool> disable_stacktraces(true);  // Disabled until healthy.
// Waiting until static initializers run seems to be late enough.
// This file is included into stacktrace.cc so this will only run once.
ABSL_ATTRIBUTE_UNUSED static int stacktraces_enabler = []() {
  // Check if we can even create stacktraces. If not, bail early and leave
  // disable_stacktraces set as-is.
  // clang-format off
  if (!EM_ASM_INT({ return (typeof wasmOffsetConverter !== 'undefined'); })) {
    return 0;
  }
  // clang-format on
  disable_stacktraces.store(false, std::memory_order_relaxed);
  return 0;
}();

template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
static int UnwindImpl(void **result, int *sizes, int max_depth, int skip_count,
                      const void *ucp, int *min_dropped_frames) {
  if (recursive || disable_stacktraces.load(std::memory_order_relaxed)) {
    return 0;
  }
  ++recursive;

  static_cast<void>(ucp);  // Unused.
  constexpr int kStackLength = 64;
  void *stack[kStackLength];

  int size;
  uintptr_t pc = emscripten_stack_snapshot();
  size = emscripten_stack_unwind_buffer(pc, stack, kStackLength);

  int result_count = size - skip_count;
  if (result_count < 0) result_count = 0;
  if (result_count > max_depth) result_count = max_depth;
  for (int i = 0; i < result_count; i++) result[i] = stack[i + skip_count];

  if (IS_STACK_FRAMES) {
    // No implementation for finding out the stack frame sizes yet.
    memset(sizes, 0, sizeof(*sizes) * result_count);
  }
  if (min_dropped_frames != nullptr) {
    if (size - skip_count - max_depth > 0) {
      *min_dropped_frames = size - skip_count - max_depth;
    } else {
      *min_dropped_frames = 0;
    }
  }

  --recursive;

  return result_count;
}

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace debugging_internal {
bool StackTraceWorksForTest() { return true; }
}  // namespace debugging_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_EMSCRIPTEN_INL_H_