net/http: server sets TCPConn writebuf through ConnContext hook will block the send of a big response body

[chenjie199234]

What version of Go are you using (go version)?

1.17.6

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

linux amd64

What did you do?

&http.Server{
	ReadTimeout:    time.Second,
	IdleTimeout:    time.Second,
	MaxHeaderBytes: 1024,
	ConnContext: func(ctx context.Context, conn net.Conn) context.Context {
		(conn.(*net.TCPConn)).SetReadBuffer(2048)
		(conn.(*net.TCPConn)).SetWriteBuffer(2048)
		return ctx
	},
}
use this server and write 12K+ data to response's body
but only 4096 data send
same code,but set the buf to 4096,all data will be send
but when i send 1M+ data when the buf is set to 4096,the send data truncate too
why?

this is the tcpdump file
wireshark filter is tcp.stream eq 18
pod-capture.cap.zip

What did you expect to see?

all data should be send

What did you see instead?

it will block on tcp write,and only part of the data will be send

[mknyszek]

CC @neild

[chenjie199234]

@mknyszek
here is one request's trace info from pprof
it works on G1069312,and it finally blocks on internal/pool.(*FD).Write():283,no more trace info in this 60s.
trace.zip

[chenjie199234]

here is one request's profile info from pprof
profile.zip

[chenjie199234]

this is my self trace info
the start is the time when i get the request
the end is the time when the Write return

[TRACE] {"trace_id":"1643164511883031560_7626180989541976002","deep":1,"start":1643164511883037729,"end":1643165625828797709,"host_name":"supports-service-77cc9b447f-4btjv","role":"server","from_app":"unknown","from_ip":"10.136.207.227:23366","from_method":"unknown","from_path":"unknown","to_app":"psp-prod.supports","to_ip":"10.136.192.250","to_method":"POST","to_path":"/supports.gamelog/SearchLog","err_msg":"","err_code":0}

[chenjie199234]

if delete

(conn.(*net.TCPConn)).SetReadBuffer(2048)
(conn.(*net.TCPConn)).SetWriteBuffer(2048)

every thing works fine
here is the trace from pprof after i delete it
trace-delete-set-buf.zip
why?