一：背景
1. 講故事
這個月初，有位朋友wx上找到我，說他的api過一段時間后，就會出現只有請求，沒有回應的情況，截圖如下：
從朋友的描述中看樣子程式是被什么東西卡住了，這種卡死的問題解決起來相對簡單，接下來我就用 windbg 給大家分析一下，
二：Windbg 分析
1. Request 請求正在干嘛？
既然朋友說 api 有 request 無 response，那怎么去驗證朋友的話對不對呢？ 我們都知道 .NET 用 HttpContext 來表示一個請求，言外之意就是可以去抓 HttpContext 下的時長屬性，Netext 中有一個 !whttp 命令可以幫助我們，
0:000> !whttp
HttpContext Thread Time Out Running Status Verb Url
000000563bf803b0 42 00:01:50 00:01:24 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x-HN
000000563bf84660 -- 00:01:50 Finished 200 GET http://xxx.com:30003/
000000563c4a0470 51 00:01:50 00:00:12 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx2C
00000056bbf63590 30 00:01:50 00:02:41 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx-B2C
00000056bc82a038 -- 00:01:50 Finished 200 GET http://localhost:30003/
00000056bc84a3e8 44 00:01:50 00:00:51 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x
00000056bc8671c8 46 00:01:50 00:00:45 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx-B2C
000000573bf44698 35 00:01:50 00:02:39 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x
000000573bf483c0 33 00:01:50 00:02:41 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x-HN
000000573bf97e80 40 00:01:50 00:02:32 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=ZJB2C
000000573c583b08 -- 00:01:50 Finished 200 GET http://localhost:30003/
000000573c589ec8 -- 00:01:50 Finished 200 GET http://xxx.com:30003/Wms/xxx/xxx/xxx
000000573c760e28 -- 00:01:50 Finished 200 POST http://xxx.com:30003/Wms/xxx/xxx/xxx
000000573c95f990 48 00:01:50 00:00:31 200 POST http://xxx.com:30003/Wms/Common/xxx?xxx=xxx&xxx=x-HN
00000057bbf4f8e8 31 00:01:50 00:02:12 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x
00000057bc080340 50 00:01:50 00:00:19 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x
000000583c4aee80 43 00:01:50 00:01:11 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx2B
000000583c4d0c50 53 00:01:50 00:00:01 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx2B
00000058bbf8f1a0 34 00:01:50 00:02:22 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx2B
000000593bfe1758 41 00:01:50 00:01:22 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx2C
000000593c892160 -- 00:01:50 Finished 200 GET http://xxx.com:30003/Wms/xxx/xxx/xxxJob
000000593ca813b0 45 00:01:50 00:00:30 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx-HN
000000593caa45d8 -- 00:01:50 Finished 200 GET http://xxx.com:30003/
00000059bc1ad808 32 00:01:50 00:01:45 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=xxx-B2C
00000059bc1c3d70 36 00:01:50 00:01:29 200 POST http://xxx.com:30003/Wms/xxx/xxx?xxx=xxx&xxx=x
25 HttpContext object(s) found matching criteria
從 Running 列可以看到大多請求都已經達到1分鐘以上，這也驗證了朋友所說的卡死問題，按照經驗，可以取 Running 列中最大的 httpContext 所在的執行緒，也就是上面的 30 和 33 號執行緒， 看看它們都在干什么？
2. 探究 Running 最長執行緒
接下來切到 30 和 33 號執行緒，看看它們的執行緒堆疊，
0:000> ~30s
ntdll!NtWaitForSingleObject+0xa:
00007ffd`b81f024a c3 ret
0:030> !clrstack
OS Thread Id: 0x29d0 (30)
Child SP IP Call Site
0000005acc3ac590 00007ffdb81f024a [PrestubMethodFrame: 0000005acc3ac590] xxx.xxx.RedisConnectionHelp.get_Instance()
0000005acc3ac850 00007ffd4dd78911 xxx.xxx.RedisCache..ctor(Int32, System.String)
0000005acc3ac8c0 00007ffd4dd78038 xxx.xxx.CacheByRedis.HashGet[[System.__Canon, mscorlib]](System.String, System.String, Int32)
0000005acc3ac968 00007ffdabef1f7c [StubHelperFrame: 0000005acc3ac968]
0000005acc3ac9c0 00007ffd4dd77f18 xxx.xxx.Cache.xxx.GetCacheNotAreaDataEntity[[System.__Canon, mscorlib]](System.String, System.String, System.String)
...
0:030> ~33s
ntdll!NtWaitForMultipleObjects+0xa:
00007ffd`b81f07ba c3 ret
0:033> !clrstack
OS Thread Id: 0x3ad4 (33)
Child SP IP Call Site
0000005accabae90 00007ffdb81f07ba [GCFrame: 0000005accabae90]
0000005accabafb8 00007ffdb81f07ba [HelperMethodFrame_1OBJ: 0000005accabafb8] System.Threading.Monitor.ObjWait(Boolean, Int32, System.Object)
0000005accabb0d0 00007ffdaac60d64 System.Threading.ManualResetEventSlim.Wait(Int32, System.Threading.CancellationToken)
0000005accabb160 00007ffdaac5b4bb System.Threading.Tasks.Task.SpinThenBlockingWait(Int32, System.Threading.CancellationToken)
0000005accabb1d0 00007ffdab5a01d1 System.Threading.Tasks.Task.InternalWait(Int32, System.Threading.CancellationToken)
0000005accabb2a0 00007ffdab59cfa7 System.Threading.Tasks.Task`1[[System.__Canon, mscorlib]].GetResultxxx(Boolean)
0000005accabb2e0 00007ffd4d8d338f xxx.Config.xxx.Config`1[[System.__Canon, mscorlib]].GetConfig(xxx.Config.Model.ConfigListener, System.Func`2<xxx.Config.Request.GetConfigRequest,System.Threading.Tasks.Task`1<System.String>>)
0000005accabb340 00007ffd4d8d2f40 xxx.Config.xxx.Config`1[[System.__Canon, mscorlib]].get_Item(System.String, System.String)
0000005accabb3c0 00007ffd4dd78f7f xxx.Util.BaseConfig.get_GetRedisConn()
0000005accabb440 00007ffd4dd78e9c xxx.xxx.RedisConnectionHelp.GetConnectionString()
0000005accabb4a0 00007ffd4dd789cb xxx.xxx.RedisConnectionHelp..cctor()
0000005accabb940 00007ffdabef6953 [GCFrame: 0000005accabb940]
0000005accabc5b0 00007ffdabef6953 [PrestubMethodFrame: 0000005accabc5b0] xxx.xxx.RedisConnectionHelp.get_Instance()
0000005accabc870 00007ffd4dd78911 xxx.xxx.RedisCache..ctor(Int32, System.String)
0000005accabc8e0 00007ffd4dd78038 xxx.xxx.CacheByRedis.HashGet[[System.__Canon, mscorlib]](System.String, System.String, Int32)
0000005accabc988 00007ffdabef1f7c [StubHelperFrame: 0000005accabc988]
0000005accabc9e0 00007ffd4dd77f18 xxx.Core.Cache.xxx.GetCacheNotAreaDataEntity[[System.__Canon, mscorlib]](System.String, System.String, System.String)
...
上面的資訊不難發現 30 號執行緒正卡在 RedisConnectionHelp.get_Instance() 處，33 號線已經進入了 RedisConnectionHelp.get_Instance() 方法中，最后在 GetConfig() 處等待 Result 的結果，按經驗來說，30 號執行緒看樣子正在鎖等待， 33 號正在等待異步結果，接下來的突破點就是探究下 RedisConnectionHelp.Instance 處代碼，
3. 尋找問題代碼
接下來用反編譯工具 ILSpy 找到問題代碼，
public static class RedisConnectionHelp
{
public static ConnectionMultiplexer Instance
{
get
{
if (_instance == null)
{
lock (Locker)
{
if (_instance == null || !_instance.IsConnected)
{
_instance = GetManager();
}
}
}
return _instance;
}
}
}
30 號執行緒果然是卡在 Locker 處，接下來深挖下 33 號執行緒所執行的 GetManager() 方法，簡化后代碼如下：
public T this[string dataId, string key = ""]
{
get
{
try
{
string config = GetConfig(configListener, new NacosConfigClient(Base.NacosConfiguration).DoGetConfigAsync);
return JsonConvert.DeserializeObject<T>(config);
}
catch (Exception ex)
{
return default(T);
}
}
}
private string GetConfig(ConfigListener listener, Func<GetConfigRequest, Task<string>> action)
{
var text2 = action(new GetConfigRequest
{
DataId = listener.DataId,
Group = listener.Group,
Tenant = text
}).Result;
return text2;
}
internal async Task<string> DoGetConfigAsync(GetConfigRequest request)
{
IRestResponse restResponse = await HttpUtil.Request(currentServerAddr, Method.GET, request.ParamValues(), xxx);
return restResponse.Content;
}
可以看到代碼卡在了 Result 上無限期等待，到這里我就想到了 同步背景關系 ，我看他這個程式是 .NET 4.8 下的 ASP.NET MVC，記得不錯背景關系應該是 AspNetSynchronizationContext，具體死鎖原因可參見我的這篇文章：一句 Task.Result 就死鎖, 這代碼還怎么寫？，解決辦法大概有四種，
- 使用 .ConfigureAwait(false)
- 改成全異步
- 用 Task 再包一層，
- 改成全同步
三：總結
其實本次事故主要還是因為在 同步代碼 中做了 異步代碼.Result 導致的死鎖問題，有非常多的文章都在抨擊這種現象，在 asp.net core 中已經移除了這種同步背景關系的大坑，給到朋友的建議是改成全同步，死鎖問題也隨之消失，
哈哈，真替朋友開心！
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標籤：C#