net/http: apparent deadlock in TestServerFlushAndHijack

[bcmills]

goroutine 1835 [sync.Cond.Wait, 2 minutes]:
runtime.gopark(0x7ff63d6bf390?, 0x4000308a00?, 0x0?, 0x0?, 0xe7791f701?)
	C:/workdir/go/src/runtime/proc.go:366 +0xe8 fp=0x4000369800 sp=0x40003697e0 pc=0x7ff63d6594b8
runtime.goparkunlock(...)
	C:/workdir/go/src/runtime/proc.go:372
sync.runtime_notifyListWait(0x4000246050, 0x0)
	C:/workdir/go/src/runtime/sema.go:513 +0x178 fp=0x4000369850 sp=0x4000369800 pc=0x7ff63d682918
sync.(*Cond).Wait(0x4000246040)
	C:/workdir/go/src/sync/cond.go:56 +0xd8 fp=0x4000369890 sp=0x4000369850 pc=0x7ff63d68f348
net/http.(*connReader).abortPendingRead(0x40002fa000)
	C:/workdir/go/src/net/http/server.go:720 +0xc8 fp=0x40003698e0 sp=0x4000369890 pc=0x7ff63d894348
net/http.(*conn).hijackLocked(0x4000111cc0)
	C:/workdir/go/src/net/http/server.go:315 +0x34 fp=0x4000369960 sp=0x40003698e0 pc=0x7ff63d892ce4
net/http.(*response).Hijack(0x400030b0a0)
	C:/workdir/go/src/net/http/server.go:2034 +0x104 fp=0x40003699e0 sp=0x4000369960 pc=0x7ff63d89ab74
net/http_test.TestServerFlushAndHijack.func1({0x7ff63db36590?, 0x400030b0a0}, 0x7ff63d6bf390?)
	C:/workdir/go/src/net/http/serve_test.go:4405 +0x90 fp=0x4000369a50 sp=0x40003699e0 pc=0x7ff63d953740
net/http.HandlerFunc.ServeHTTP(0x0?, {0x7ff63db36590?, 0x400030b0a0?}, 0x7ff63d661cfc?)
	C:/workdir/go/src/net/http/server.go:2084 +0x3c fp=0x4000369a80 sp=0x4000369a50 pc=0x7ff63d89afbc
net/http.serverHandler.ServeHTTP({0x40002fa000?}, {0x7ff63db36590, 0x400030b0a0}, 0x400022ab00)
	C:/workdir/go/src/net/http/server.go:2916 +0x408 fp=0x4000369b40 sp=0x4000369a80 pc=0x7ff63d89e628
net/http.(*conn).serve(0x4000111cc0, {0x7ff63db36ae8, 0x40004dabd0})
	C:/workdir/go/src/net/http/server.go:1966 +0x550 fp=0x4000369fa0 sp=0x4000369b40 pc=0x7ff63d899cf0
net/http.(*Server).Serve.func3()
	C:/workdir/go/src/net/http/server.go:3071 +0x34 fp=0x4000369fd0 sp=0x4000369fa0 pc=0x7ff63d89f0c4
runtime.goexit()
	C:/workdir/go/src/runtime/asm_arm64.s:1259 +0x4 fp=0x4000369fd0 sp=0x4000369fd0 pc=0x7ff63d686d84
created by net/http.(*Server).Serve
	C:/workdir/go/src/net/http/server.go:3071 +0x448

…

goroutine 1831 [IO wait, 2 minutes]:
runtime.gopark(0x4000107890?, 0x0?, 0x98?, 0x87?, 0x4000308848?)
	C:/workdir/go/src/runtime/proc.go:366 +0xe8 fp=0x40002dd860 sp=0x40002dd840 pc=0x7ff63d6594b8
runtime.netpollblock(0x40002683c0?, 0x4?, 0x0?)
	C:/workdir/go/src/runtime/netpoll.go:453 +0x16c fp=0x40002dd8a0 sp=0x40002dd860 pc=0x7ff63d65004c
internal/poll.runtime_pollWait(0x1c35e4c1b60, 0x72)
	C:/workdir/go/src/runtime/netpoll.go:233 +0x9c fp=0x40002dd8c0 sp=0x40002dd8a0 pc=0x7ff63d680fdc
internal/poll.(*pollDesc).wait(0x0?, 0x7ff63d7d9408?, 0x0)
	C:/workdir/go/src/internal/poll/fd_poll_runtime.go:83 +0x2c fp=0x40002dd8f0 sp=0x40002dd8c0 pc=0x7ff63d6beeac
internal/poll.execIO(0x4000308798, 0x7ff63daae1b8)
	C:/workdir/go/src/internal/poll/fd_windows.go:175 +0xcc fp=0x40002dd950 sp=0x40002dd8f0 pc=0x7ff63d6bf89c
internal/poll.(*FD).Read(0x4000308780, {0x4000303000, 0x1000, 0x1000})
	C:/workdir/go/src/internal/poll/fd_windows.go:441 +0x278 fp=0x40002dd9e0 sp=0x40002dd950 pc=0x7ff63d6c0768
net.(*netFD).Read(0x4000308780, {0x4000303000?, 0x40003ffb18?, 0x7ff63d62be48?})
	C:/workdir/go/src/net/fd_posix.go:55 +0x2c fp=0x40002dda30 sp=0x40002dd9e0 pc=0x7ff63d7ca26c
net.(*conn).Read(0x40004f6198, {0x4000303000?, 0x4000079b28?, 0x7ff63d704e74?})
	C:/workdir/go/src/net/net.go:183 +0x38 fp=0x40002dda80 sp=0x40002dda30 pc=0x7ff63d7d5518
net/http.(*persistConn).Read(0x40004a07e0, {0x4000303000?, 0x4000079b18?, 0x7ff63d633250?})
	C:/workdir/go/src/net/http/transport.go:1926 +0x54 fp=0x40002ddae0 sp=0x40002dda80 pc=0x7ff63d8b2ba4
bufio.(*Reader).fill(0x400009a660)
	C:/workdir/go/src/bufio/bufio.go:106 +0x100 fp=0x40002ddb20 sp=0x40002ddae0 pc=0x7ff63d71c0c0
bufio.(*Reader).ReadSlice(0x400009a660, 0x1?)
	C:/workdir/go/src/bufio/bufio.go:371 +0x34 fp=0x40002ddb70 sp=0x40002ddb20 pc=0x7ff63d71cd64
net/http/internal.readChunkLine(0x400030307d?)
	C:/workdir/go/src/net/http/internal/chunked.go:129 +0x24 fp=0x40002ddbc0 sp=0x40002ddb70 pc=0x7ff63d851d44
net/http/internal.(*chunkedReader).beginChunk(0x40004dac30)
	C:/workdir/go/src/net/http/internal/chunked.go:48 +0x2c fp=0x40002ddbf0 sp=0x40002ddbc0 pc=0x7ff63d85178c
net/http/internal.(*chunkedReader).Read(0x40004dac30, {0x40003c4007?, 0x4000079cd8?, 0x7ff63d62b70c?})
	C:/workdir/go/src/net/http/internal/chunked.go:98 +0x140 fp=0x40002ddc80 sp=0x40002ddbf0 pc=0x7ff63d851a70
net/http.(*body).readLocked(0x40003363c0, {0x40003c4007?, 0x1c35e5e03c0?, 0x7ff63d855f35?})
	C:/workdir/go/src/net/http/transfer.go:844 +0x40 fp=0x40002ddcd0 sp=0x40002ddc80 pc=0x7ff63d8a86c0
net/http.(*body).Read(0x0?, {0x40003c4007?, 0x200000101011d20?, 0x1c35e5e03c0?})
	C:/workdir/go/src/net/http/transfer.go:836 +0x13c fp=0x40002ddd40 sp=0x40002ddcd0 pc=0x7ff63d8a859c
net/http.(*bodyEOFSignal).Read(0x4000336400, {0x40003c4007, 0x1f9, 0x1f9})
	C:/workdir/go/src/net/http/transport.go:2768 +0x17c fp=0x40002dddd0 sp=0x40002ddd40 pc=0x7ff63d8b6e7c
io.ReadAll({0x7ff63db338c8, 0x4000336400})
	C:/workdir/go/src/io/io.go:645 +0xe4 fp=0x40002dde60 sp=0x40002dddd0 pc=0x7ff63d6b8d94
net/http_test.TestServerFlushAndHijack(0x4000280820)
	C:/workdir/go/src/net/http/serve_test.go:4420 +0x18c fp=0x40002ddf60 sp=0x40002dde60 pc=0x7ff63d95349c
testing.tRunner(0x4000280820, 0x7ff63daaf4b8)
	C:/workdir/go/src/testing/testing.go:1410 +0x110 fp=0x40002ddfb0 sp=0x40002ddf60 pc=0x7ff63d717190
testing.(*T).Run.func1()
	C:/workdir/go/src/testing/testing.go:1457 +0x30 fp=0x40002ddfd0 sp=0x40002ddfb0 pc=0x7ff63d717f00
runtime.goexit()
	C:/workdir/go/src/runtime/asm_arm64.s:1259 +0x4 fp=0x40002ddfd0 sp=0x40002ddfd0 pc=0x7ff63d686d84
created by testing.(*T).Run
	C:/workdir/go/src/testing/testing.go:1457 +0x328

greplogs --dashboard -md -l -e 'goroutine \d+ \[IO wait, \d+ minutes\]:\n(?:.+\n\t.+\n)*net/http_test\.TestServerFlushAndHijack' --since=2021-03-05

2021-12-02T22:03:11-d3bf627/windows-arm64-10

(CC @neild @ianlancetaylor @bradfitz)

[bcmills]

These stack traces are very similar to (but do not exactly match) the ones in #44783.