net/http: TestTransportFlushesRequestHeader failures with `close of closed channel`

[gopherbot]

#!watchflakes
post <- pkg == "net/http" && test == "TestTransportFlushesRequestHeader" && `close of closed channel`

Issue created automatically to collect these failures.

Example (log):

--- FAIL: TestTransportFlushesRequestHeader (0.00s)
    --- FAIL: TestTransportFlushesRequestHeader/h2 (0.03s)
        clientserver_test.go:210: server log: http2: panic serving 127.0.0.1:35148: close of closed channel
            goroutine 10358 [running]:
            net/http.(*http2serverConn).runHandler.func1()
            	/workdir/go/src/net/http/h2_bundle.go:6104 +0x145
            panic({0x86b5e0, 0x9a8600})
            	/workdir/go/src/runtime/panic.go:884 +0x213
            net/http_test.testTransportFlushesRequestHeader.func1({0x600010ab?, 0x7a1969aaa5a5a59a?}, 0x40000000000000?)
            	/workdir/go/src/net/http/transport_test.go:4192 +0x1d
            net/http.HandlerFunc.ServeHTTP(0x0?, {0x9ac530?, 0xc00061a168?}, 0xc0002fd6a0?)
            	/workdir/go/src/net/http/server.go:2122 +0x2f
            net/http.serverHandler.ServeHTTP({0x8e42d6?}, {0x9ac530, 0xc00061a168}, 0xc000a78a00)
            	/workdir/go/src/net/http/server.go:2936 +0x316
            net/http.initALPNRequest.ServeHTTP({{0x9acba8?, 0xc002020ff0?}, 0xc000c9f880?, {0xc00032cf00?}}, {0x9ac530, 0xc00061a168}, 0xc000a78a00)
            	/workdir/go/src/net/http/server.go:3545 +0x245
            net/http.(*http2serverConn).runHandler(0xc000c08460?, 0xc0002fd7d0?, 0x6e6786?, 0xc0007d2bd0?)
            	/workdir/go/src/net/http/h2_bundle.go:6111 +0xc2
            created by net/http.(*http2serverConn).scheduleHandler
            	/workdir/go/src/net/http/h2_bundle.go:6046 +0x245
        transport_test.go:4205: stream error: stream ID 1; INTERNAL_ERROR; received from peer

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "net/http" && test == "TestTransportFlushesRequestHeader"

2023-11-07 17:04 linux-amd64 go@46fb7816 net/http.TestTransportFlushesRequestHeader (log)

--- FAIL: TestTransportFlushesRequestHeader (0.00s)
    --- FAIL: TestTransportFlushesRequestHeader/h2 (0.03s)
        clientserver_test.go:210: server log: http2: panic serving 127.0.0.1:35148: close of closed channel
            goroutine 10358 [running]:
            net/http.(*http2serverConn).runHandler.func1()
            	/workdir/go/src/net/http/h2_bundle.go:6104 +0x145
            panic({0x86b5e0, 0x9a8600})
            	/workdir/go/src/runtime/panic.go:884 +0x213
            net/http_test.testTransportFlushesRequestHeader.func1({0x600010ab?, 0x7a1969aaa5a5a59a?}, 0x40000000000000?)
            	/workdir/go/src/net/http/transport_test.go:4192 +0x1d
            net/http.HandlerFunc.ServeHTTP(0x0?, {0x9ac530?, 0xc00061a168?}, 0xc0002fd6a0?)
            	/workdir/go/src/net/http/server.go:2122 +0x2f
            net/http.serverHandler.ServeHTTP({0x8e42d6?}, {0x9ac530, 0xc00061a168}, 0xc000a78a00)
            	/workdir/go/src/net/http/server.go:2936 +0x316
            net/http.initALPNRequest.ServeHTTP({{0x9acba8?, 0xc002020ff0?}, 0xc000c9f880?, {0xc00032cf00?}}, {0x9ac530, 0xc00061a168}, 0xc000a78a00)
            	/workdir/go/src/net/http/server.go:3545 +0x245
            net/http.(*http2serverConn).runHandler(0xc000c08460?, 0xc0002fd7d0?, 0x6e6786?, 0xc0007d2bd0?)
            	/workdir/go/src/net/http/h2_bundle.go:6111 +0xc2
            created by net/http.(*http2serverConn).scheduleHandler
            	/workdir/go/src/net/http/h2_bundle.go:6046 +0x245
        transport_test.go:4205: stream error: stream ID 1; INTERNAL_ERROR; received from peer

— watchflakes

[bcmills]

(attn @dneil)

Note that commit 46fb781 is on release-branch.go1.20 (#59483).