reverseproxy: Pull latest proxy changes from stdlib

[francislavoie]

I went through the commits that touched stdlib's reverseproxy.go file in the past couple of years (only a dozen or so), and copied over all the changes that are to code that was copied into Caddy.

The commits I pulled changes from (hopefully apparent from looking at those commits what the changes in this PR are doing):

• golang/go@2cc3473
• golang/go@a5cea06
• golang/go@ecdbffd
• golang/go@2189852
• golang/go@ca3c0df
• golang/go@9c017ff

This may also fix #4247 because of the change to copyResponse to set mlw.flushPending = true right away. /cc @leafac if you'd be willing to try out this PR (you can grab a build from the CI artifacts, or build from this commit with xcaddy) to see if it fixes your usecase.

[leafac]

Oh, nice. I’ll give it a try. Thank you very much!

[mholt]

These changes look great, thanks Francis. The divergence from bugfixes upstream was something I was worried about. I'll wait to approve this until @leafac reports back whether it resolves his issue.

[mholt]

Should we wait for you, @leafac? (Thanks for trying it out, btw)

[leafac]

Yeah, sorry about taking too long to answer. I’ll test this first thing when I get to work. 😃

[leafac]

👍 I tested and I believe it’s working now 🙌

Thank you everyone for your work. You’re awesome!

---

Here’s what I did in more detail:

1. I went to this page which includes an artifact called caddy_macOS_go1.16_9c0cd66…. I downloaded it.
2. I ran this version using my https://github.com/leafac/caddy-express-sse. This is the output I got:

$ curl -Nv https://localhost    
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/cert.pem
  CApath: none
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-ECDSA-AES256-GCM-SHA384
* ALPN, server accepted to use h2
* Server certificate:
*  subject: [NONE]
*  start date: Aug 12 08:37:31 2021 GMT
*  expire date: Aug 12 20:37:31 2021 GMT
*  subjectAltName: host "localhost" matched cert's "localhost"
*  issuer: CN=Caddy Local Authority - ECC Intermediate
*  SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x15300f800)
> GET / HTTP/2
> Host: localhost
> User-Agent: curl/7.64.1
> Accept: */*
> 
* Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
< HTTP/2 200 
< content-type: text/event-stream; charset=utf-8
< date: Thu, 12 Aug 2021 08:37:59 GMT
< server: Caddy
< x-powered-by: Express
< 

Note how the headers came right away.

[francislavoie]

Excellent! Thanks for confirming @leafac 🎉

[mholt]

Thanks for doing this, Francis! And thanks for testing it out, @leafac!