github地址 https://github.com/theanarkh/read-libuv-code

文件监听的原理是，第一次先执行stat函数获取文件基本信息，然后在stat的回调函数里设置定时器，定时器超时后会执行stat，然后获取stat信息，再次执行stat回调函数重新设置定时器，如此反复，如果stat不一样就执行用户的回调。

#include "uv.h"
#include "uv-common.h"

#include <assert.h>
#include <stdlib.h>
#include <string.h>

struct poll_ctx {
  uv_fs_poll_t* parent_handle; /* NULL if parent has been stopped or closed */
  int busy_polling;
  unsigned int interval;
  uint64_t start_time;
  uv_loop_t* loop;
  uv_fs_poll_cb poll_cb;
  uv_timer_t timer_handle;
  uv_fs_t fs_req; /* TODO(bnoordhuis) mark fs_req internal */
  uv_stat_t statbuf;
  // 字符串的值追加在结构体后面
  char path[1]; /* variable length */
};

static int statbuf_eq(const uv_stat_t* a, const uv_stat_t* b);
static void poll_cb(uv_fs_t* req);
static void timer_cb(uv_timer_t* timer);
static void timer_close_cb(uv_handle_t* handle);

static uv_stat_t zero_statbuf;

// 初始化uv_fs_poll_t结构
int uv_fs_poll_init(uv_loop_t* loop, uv_fs_poll_t* handle) {
  uv__handle_init(loop, (uv_handle_t*)handle, UV_FS_POLL);
  return 0;
}


int uv_fs_poll_start(uv_fs_poll_t* handle,
                     uv_fs_poll_cb cb,
                     const char* path,
                     unsigned int interval) {
  struct poll_ctx* ctx;
  uv_loop_t* loop;
  size_t len;
  int err;

  if (uv__is_active(handle))
    return 0;

  loop = handle->loop;
  len = strlen(path);
  // 分配一块内存存上下文结构体和path对应的字符串
  ctx = uv__calloc(1, sizeof(*ctx) + len);

  if (ctx == NULL)
    return UV_ENOMEM;
  // 初始化上下文结构
  ctx->loop = loop;
  // 内容改变时的回调
  ctx->poll_cb = cb;
  // 多久检测一次内容是否变化
  ctx->interval = interval ? interval : 1;
  // 开始的时间点
  ctx->start_time = uv_now(loop);
  // 上下文对应的handle结构
  ctx->parent_handle = handle;
  memcpy(ctx->path, path, len + 1);
  // 初始化一个定时器
  err = uv_timer_init(loop, &ctx->timer_handle);
  if (err < 0)
    goto error;
  // 设置UV_HANDLE_INTERNAL标记位
  ctx->timer_handle.flags |= UV_HANDLE_INTERNAL;
  //清除UV_HANDLE_REF标记
  uv__handle_unref(&ctx->timer_handle);
  // 异步获取path对应的文件的信息，获取到后执行poll_cb
  err = uv_fs_stat(loop, &ctx->fs_req, ctx->path, poll_cb);
  if (err < 0)
    goto error;
  // 挂载上下文到handle
  handle->poll_ctx = ctx;
  // 激活该handle，但不增加handle的active数
  uv__handle_start(handle);

  return 0;

error:
  // 出错则释放分配的内存
  uv__free(ctx);
  return err;
}

// 停止poll
int uv_fs_poll_stop(uv_fs_poll_t* handle) {
  struct poll_ctx* ctx;

  if (!uv__is_active(handle))
    return 0;

  ctx = handle->poll_ctx;
  assert(ctx != NULL);
  assert(ctx->parent_handle != NULL);
  // 解除关联
  ctx->parent_handle = NULL;
  handle->poll_ctx = NULL;

  /* Close the timer if it's active. If it's inactive, there's a stat request
   * in progress and poll_cb will take care of the cleanup.
   */
  // 停止定时器，设置回调为time_close_cb，设置状态为closing
  if (uv__is_active(&ctx->timer_handle))
    uv_close((uv_handle_t*)&ctx->timer_handle, timer_close_cb);
  
  uv__handle_stop(handle);

  return 0;
}

// 获取path
int uv_fs_poll_getpath(uv_fs_poll_t* handle, char* buffer, size_t* size) {
  struct poll_ctx* ctx;
  size_t required_len;

  if (!uv__is_active(handle)) {
    *size = 0;
    return UV_EINVAL;
  }

  ctx = handle->poll_ctx;
  assert(ctx != NULL);

  required_len = strlen(ctx->path);
  if (required_len >= *size) {
    *size = required_len + 1;
    return UV_ENOBUFS;
  }

  memcpy(buffer, ctx->path, required_len);
  *size = required_len;
  buffer[required_len] = '\0';

  return 0;
}


void uv__fs_poll_close(uv_fs_poll_t* handle) {
  uv_fs_poll_stop(handle);
}

// 定时器到期执行的回调
static void timer_cb(uv_timer_t* timer) {
  struct poll_ctx* ctx;

  ctx = container_of(timer, struct poll_ctx, timer_handle);
  assert(ctx->parent_handle != NULL);
  assert(ctx->parent_handle->poll_ctx == ctx);
  ctx->start_time = uv_now(ctx->loop);
  // 再次获取stat信息
  if (uv_fs_stat(ctx->loop, &ctx->fs_req, ctx->path, poll_cb))
    abort();
}

// 获取到stat后执行的回调
static void poll_cb(uv_fs_t* req) {
  uv_stat_t* statbuf;
  struct poll_ctx* ctx;
  uint64_t interval;

  ctx = container_of(req, struct poll_ctx, fs_req);

  if (ctx->parent_handle == NULL) { /* handle has been stopped or closed */
    uv_close((uv_handle_t*)&ctx->timer_handle, timer_close_cb);
    uv_fs_req_cleanup(req);
    return;
  }

  if (req->result != 0) {
    if (ctx->busy_polling != req->result) {
      ctx->poll_cb(ctx->parent_handle,
                   req->result,
                   &ctx->statbuf,
                   &zero_statbuf);
      ctx->busy_polling = req->result;
    }
    goto out;
  }

  statbuf = &req->statbuf;
  // 第一次不执行回调，因为没有可对比的stat，第二次及后续的操作才可能执行回调，因为第一次执行的时候置busy_polling=1
  if (ctx->busy_polling != 0)
    // 出错或者stat发生了变化则执行回调
    if (ctx->busy_polling < 0 || !statbuf_eq(&ctx->statbuf, statbuf))
      ctx->poll_cb(ctx->parent_handle, 0, &ctx->statbuf, statbuf);
  // 保存当前获取到的stat信息，置1
  ctx->statbuf = *statbuf;
  ctx->busy_polling = 1;

out:
  uv_fs_req_cleanup(req);

  if (ctx->parent_handle == NULL) { /* handle has been stopped by callback */
    uv_close((uv_handle_t*)&ctx->timer_handle, timer_close_cb);
    return;
  }

  /* Reschedule timer, subtract the delay from doing the stat(). */
  /*
    假设在开始时间点为1，interval为10的情况下执行了stat，stat完成执行并执行poll_cb回调的时间点是
    3，那么定时器的超时时间则为10-3=7，即7个单位后就要触发超时，而不是10，是因为stat阻塞消耗了3个单位的
    时间，所以下次执行超时回调函数时说明从start时间点开始算，已经经历了x单位各interval，然后超时回调里又
    执行了stat函数，再到执行stat回调，这个时间点即now=start+x单位个interval+stat消耗的时间。得出now-start
    为interval的x倍+stat消耗，即对interval取余可得到stat消耗，所以
    当前轮，定时器的超时时间为interval - ((now-start) % interval)
  */
  interval = ctx->interval;
  interval -= (uv_now(ctx->loop) - ctx->start_time) % interval;

  if (uv_timer_start(&ctx->timer_handle, timer_cb, interval, 0))
    abort();
}

// 释放上下文结构体的内存
static void timer_close_cb(uv_handle_t* handle) {
  uv__free(container_of(handle, struct poll_ctx, timer_handle));
}


static int statbuf_eq(const uv_stat_t* a, const uv_stat_t* b) {
  return a->st_ctim.tv_nsec == b->st_ctim.tv_nsec
      && a->st_mtim.tv_nsec == b->st_mtim.tv_nsec
      && a->st_birthtim.tv_nsec == b->st_birthtim.tv_nsec
      && a->st_ctim.tv_sec == b->st_ctim.tv_sec
      && a->st_mtim.tv_sec == b->st_mtim.tv_sec
      && a->st_birthtim.tv_sec == b->st_birthtim.tv_sec
      && a->st_size == b->st_size
      && a->st_mode == b->st_mode
      && a->st_uid == b->st_uid
      && a->st_gid == b->st_gid
      && a->st_ino == b->st_ino
      && a->st_dev == b->st_dev
      && a->st_flags == b->st_flags
      && a->st_gen == b->st_gen;
}


#if defined(_WIN32)

#include "win/internal.h"
#include "win/handle-inl.h"

void uv__fs_poll_endgame(uv_loop_t* loop, uv_fs_poll_t* handle) {
  assert(handle->flags & UV_HANDLE_CLOSING);
  assert(!(handle->flags & UV_HANDLE_CLOSED));
  uv__handle_close(handle);
}

做技术的同学，欢迎关注公众号编程杂技。分享技术，交流技术。