Unity IAP插件的iOS接入，服务器出现21002问题。
之前的老的框架，之前的项目一直都很正常的运行，这次接入iOS支付的时候，服务器出现了21002错误。这个问题让我头疼了1天。
原因很简单，正常的iOS内购，苹果给的收据，由客户端进行base64编码之后，直接发送给服务器，然后服务器向苹果的对账服务器请求验证。只需要按苹果要求组装好json字符串即可。
而网上关于设置 content_type = "application/json; charset=utf-8"，或则将空格换成+号的说法，其实和这个21002半毛钱关系都没有。
据我的测试可得知，content_type设不设置都没关系，而空格换成+号也是属于客户端与服务器之间的传输协议问题。如果是http请求，可能被替换，但是如果是用socket连接，这个一点都不是问题。
问题的唯一可能存在的地方，就是这个base64数据块有问题。可能是base64编码数据被破坏了，还有种可能就是base64编码的内容被破坏了。
后一种情况听起来貌似匪夷所思。但事实上，我这次碰到的就是这种情况。为什么呢？因为Unity IAP插件发送给客户端的是一个Json字符串！而收据作为这个Json的一部分，已经被编码了。客户端再次编码整个字符串，就完全是南辕北辙了。
{"Store":"AppleAppStore","TransactionID":"2000000039643941","Payload":"MIITvQYJKoZIhvcNAQcCoIITrjCCE6oCAQExCzAJBgUrDgMCGgUAMIIDXgYJKoZIhvcNAQcBoIIDTwSCA0sxggNHMAoCAQgCAQEEAhYAMAoCARQCAQEEAgwAMAsCAQECAQEEAwIBADALAgEDAgEBBAMMATAwCwIBCwIBAQQDAgEAMAsCAQ8CAQEEAwIBADALAgEQAgEBBAMCAQAwCwIBGQIBAQQDAgEDMAwCAQoCAQEEBBYCNCswDAIBDgIBAQQEAgIAnjANAgENAgEBBAUCAwJLHDANAgETAgEBBAUMAzEuMDAOAgEJAgEBBAYCBFAyNTYwGAIBBAIBAgQQGPbrdE+kSdWjkiiQITX29TAbAgEAAgEBBBMMEVByb2R1Y3Rpb25TYW5kYm94MBwCAQUCAQEEFKsl44zcXE0xasiszPzJ/c8xkJGBMB4CAQwCAQEEFhYUMjAyMi0wNC0yNFQwNToyNzo1N1owHgIBEgIBAQQWFhQyMDEzLTA4LTAxVDA3OjAwOjAwWjAoAgECAgEBBCAMHmNvbS53b293YXlnYW1lcy5vbmVwZXJjZW50LmlvczAwAgEHAgEBBCi9PlIOqxn1OinWJJ574+GdEl2cHzM6UE6+iRMPDPPC001AYaF9QZVRMD4CAQYCAQEENjlvoEjy8RZrVJjJ1HXGjAre7PTJ8a8E6AneOQYppRF3qbnZ7t0wd1GRVEACKfLCHvD..."}这儿可以看到，Json字符串的Payload部分就是收据，而且还是经过Base64编码的。因此。只需要把这个部分读出来发送给服务器即可。
另外，说到21002，还有个可能，就是服务器与苹果对账服务器的连接，不能是长连接。每一次请求完就关闭。如果使用那种Keep Alive模式，试图与苹果对账服务器保持长连接的话，你会得到随机的21002错误。
而如果想要验证自己的代码是否有问题，可以直接使用下面的收据（已经base64编码）试试。如果能得到解密字符串，就代表代码是成功的。
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