net/http: TestTransportPersistConnLeak failure on plan9-386-0intro builder

[bcmills]

#!watchflakes
post <- goos == "plan9" && pkg == "net/http" && test == "TestTransportPersistConnLeak"

2019-11-05T19:06:28-7e71c9c/plan9-386-0intro

http.test 1338092: warning: process exceeds 100 file descriptors
--- FAIL: TestTransportPersistConnLeak (2.96s)
    transport_test.go:1619: goroutine growth: 170 -> 322 -> 220 (delta: 50)
    transport_test.go:1620: too many new goroutines
2019/11/05 14:48:14 http: TLS handshake error from 127.0.0.1:55194: EOF
2019/11/05 14:48:14 http: TLS handshake error from 127.0.0.1:57366: EOF
FAIL
FAIL	net/http	114.358s

See previously #14887, #14375, #4822, #7237.

CC @0intro @millerresearch @fhs

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- goos == "plan9" && pkg == "net/http" && test == "TestTransportPersistConnLeak"

2023-03-02 15:57 plan9-arm go@b958d4a5 net/http.TestTransportPersistConnLeak (log)

http.test 30018: warning: process exceeds 100 file descriptors
fatal error: notesleep not on g0

runtime stack:
runtime.throw({0x4b9fe7, 0x13})
	/boot/workdir/go/src/runtime/panic.go:1075 +0x4c fp=0x3ffffca4 sp=0x3ffffc90 pc=0x3d8a8
runtime.notesleep(0x9395974)
	/boot/workdir/go/src/runtime/lock_sema.go:168 +0x12c fp=0x3ffffcc0 sp=0x3ffffca4 pc=0xb110
runtime.mPark()
	/boot/workdir/go/src/runtime/proc.go:1519 +0x1c fp=0x3ffffccc sp=0x3ffffcc0 pc=0x43684
...
	/boot/workdir/go/src/runtime/chan.go:583 +0x4bc fp=0x95adeb0 sp=0x95ade74 pc=0x5318
runtime.chanrecv1(0x94df980, 0x95adeeb)
	/boot/workdir/go/src/runtime/chan.go:442 +0x20 fp=0x95adec4 sp=0x95adeb0 pc=0x4e2c
testing.(*T).Run(0x964d0e0, {0x4b071e, 0x2}, 0x952c558)
	/boot/workdir/go/src/testing/testing.go:1633 +0x40c fp=0x95adf28 sp=0x95adec4 pc=0x103fb4
net/http_test.run[...](0x964d0e0, 0x4e8eac, {0x95adf8c, 0x1, 0x1})
	/boot/workdir/go/src/net/http/clientserver_test.go:82 +0x23c fp=0x95adf70 sp=0x95adf28 pc=0x423890
net/http_test.TestTransportPersistConnLeak(0x964d0e0)
	/boot/workdir/go/src/net/http/transport_test.go:1772 +0x60 fp=0x95adf94 sp=0x95adf70 pc=0x3e5ef4
testing.tRunner(0x964d0e0, 0x4e8820)

— watchflakes