又拍云缩略图etag值与下载后的文件md5值不一致

在又拍云上传的文件，原文件 md5 值与响应头中的 etag 值一致，比如

$curl -I https://awen.me/uploads/IMG_4224.JPG -v

得到的 etag 值是

< Cache-Control: public, must-revalidate, max-age=1036800
Cache-Control: public, must-revalidate, max-age=1036800
< ETag: "955B00347688C48932AF1D746D196002"
ETag: "955B00347688C48932AF1D746D196002"
< Last-Modified: Thu, 22 Jun 2017 02:54:44 GMT
Last-Modified: Thu, 22 Jun 2017 02:54:44 GMT
< X-Oss-Request-Id: 594B54428881834B04B7662E
X-Oss-Request-Id: 594B54428881834B04B7662E
< X-Oss-Hash-Crc64ecma: 16915821891617572766
X-Oss-Hash-Crc64ecma: 16915821891617572766
< Content-Md5: lVsANHaIxIkyrx10bRlgAg==
Content-Md5: lVsANHaIxIkyrx10bRlgAg==
< Accept-Ranges: bytes
Accept-Ranges: bytes

下载后得到的结果与原图是一样的

   ➜  Downloads wget -c  https://file.awen.me/blog/2017-06-22-025442.jpg!awen)
--2017-06-22 16:30:51--  https://file.awen.me/blog/2017-06-22-025442.jpg!awen)
Connecting to 127.0.0.1:1087... connected.
Proxy request sent, awaiting response... 200 OK
Length: 142041 (139K) [image/jpeg]
Saving to: ‘2017-06-22-025442.jpg!awen)’

2017-06-22-025442.jpg!awen)                       100%[========================================================================================>] 138.71K  --.-KB/s    in 0.1s

2017-06-22 16:30:52 (1.06 MB/s) - ‘2017-06-22-025442.jpg!awen)’ saved [142041/142041]

➜  Downloads md5 2017-06-22-025442.jpg!awen)
MD5 (2017-06-22-025442.jpg!awen) = 955b00347688c48932af1d746d196002

但是我们继续看下，请求缩略图

➜  Downloads curl -I  https://file.awen.me/blog/2017-06-22-025442.jpg!awen)\!500 -v
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 1087 (#0)
* Establish HTTP proxy tunnel to file.awen.me:443
> CONNECT file.awen.me:443 HTTP/1.1
> Host: file.awen.me:443
> User-Agent: curl/7.51.0
> Proxy-Connection: Keep-Alive
>
< HTTP/1.1 200 Connection established
HTTP/1.1 200 Connection established
<

* Proxy replied OK to CONNECT request
* TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
* Server certificate: file.awen.me
* Server certificate: Let's Encrypt Authority X3
* Server certificate: DST Root CA X3
> HEAD /blog/2017-06-22-025442.jpg!awen)!500 HTTP/1.1
> Host: file.awen.me
> User-Agent: curl/7.51.0
> Accept: */*
>
< HTTP/1.1 200 OK
HTTP/1.1 200 OK
< Server: marco/1.4
Server: marco/1.4
< Date: Thu, 22 Jun 2017 08:32:10 GMT
Date: Thu, 22 Jun 2017 08:32:10 GMT
< Content-Type: image/png
Content-Type: image/png
< Content-Length: 175029
Content-Length: 175029
< Connection: keep-alive
Connection: keep-alive
< X-Source: C/200, G/200
X-Source: C/200, G/200
< Last-Modified: Thu, 22 Jun 2017 02:54:44 GMT
Last-Modified: Thu, 22 Jun 2017 02:54:44 GMT
< ETag: "b8beb4fa909148b01e2ba893ee569735"
ETag: "b8beb4fa909148b01e2ba893ee569735"
< Accept-Ranges: bytes
Accept-Ranges: bytes
< Expires: Tue, 04 Jul 2017 08:32:10 GMT
Expires: Tue, 04 Jul 2017 08:32:10 GMT
< Cache-Control: public, must-revalidate, max-age=1036800
Cache-Control: public, must-revalidate, max-age=1036800
< X-Request-Id: a95f28964d2c3e6bafbbf482f89c721b
X-Request-Id: a95f28964d2c3e6bafbbf482f89c721b
< Via: S.pcw-cn-hkg-167, T.89163.M.1, V.pcw-cn-hkg-163, M.pcw-cn-hkg-163
Via: S.pcw-cn-hkg-167, T.89163.M.1, V.pcw-cn-hkg-163, M.pcw-cn-hkg-163
< Strict-Transport-Security: max-age=15552000; includeSubDomains; preload
Strict-Transport-Security: max-age=15552000; includeSubDomains; preload

<
* Curl_http_done: called premature == 0
* Connection #0 to host 127.0.0.1 left intact

etag 值是 b8beb4fa909148b01e2ba893ee569735，然后我们下载后看下 md5值

➜  Downloads md5 2017-06-22-025442.jpg!awen)\!500
MD5 (2017-06-22-025442.jpg!awen)!500) = 887ea813a8397e783ff396aedecafaf7

不一致，原因是因为又拍云的缩略图是动态生成的，在动态制作的国产中增加了很多参数导致 md5值发生了变化。