net: TestZeroByteRead breaks Plan 9 build

[mikioh]

See https://build.golang.org/log/17b76202a3b136ca6f462aa408094e95db6152f5

panic: test timed out after 3m0s

goroutine 129 [running]:
panic(0x1b43c0, 0x105ae6f0)
    /tmp/workdir/go/src/runtime/panic.go:500 +0x32f
testing.startAlarm.func1()
    /tmp/workdir/go/src/testing/testing.go:855 +0xf4
created by time.goFunc
    /tmp/workdir/go/src/time/sleep.go:129 +0x35

goroutine 1 [chan receive]:
testing.(*T).Run(0x105adc00, 0x1e1186, 0x10, 0x207738, 0x27601)
    /tmp/workdir/go/src/testing/testing.go:584 +0x2c3
testing.RunTests.func1(0x104ce380)
    /tmp/workdir/go/src/testing/testing.go:730 +0x97
testing.tRunner(0x104ce380, 0x104f7ecc)
    /tmp/workdir/go/src/testing/testing.go:547 +0x8b
testing.RunTests(0x2072f8, 0x2a2fe0, 0x93, 0x93, 0x1)
    /tmp/workdir/go/src/testing/testing.go:736 +0x331
testing.(*M).Run(0x104f7f7c, 0x304be000)
    /tmp/workdir/go/src/testing/testing.go:680 +0x78
net.TestMain(0x104f7f7c)
    /tmp/workdir/go/src/net/main_test.go:48 +0x25
main.main()
    net/_test/_testmain.go:388 +0xff

goroutine 126 [syscall]:
syscall.Syscall6(0x4, 0x2b4468, 0x0, 0xffffffff, 0xffffffff, 0x0, 0x0, 0x3, 0x2, 0x3e9db, ...)
    /tmp/workdir/go/src/syscall/asm_plan9_386.s:57 +0x5
syscall.Pread(0x4, 0x0, 0x0, 0x0, 0xffffffff, 0xffffffff, 0x710a3, 0x0, 0x0)
    /tmp/workdir/go/src/syscall/zsyscall_plan9_386.go:228 +0x72
syscall.Read(0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /tmp/workdir/go/src/syscall/syscall_plan9.go:124 +0x54
os.(*File).read(0x105ae6d8, 0x0, 0x0, 0x0, 0x4da80, 0x0, 0x0)
    /tmp/workdir/go/src/os/file_plan9.go:247 +0x49
os.(*File).Read(0x105ae6d8, 0x0, 0x0, 0x0, 0x105ae690, 0x0, 0x0)
    /tmp/workdir/go/src/os/file.go:97 +0x6e
net.(*netFD).Read(0x105baa80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /tmp/workdir/go/src/net/fd_plan9.go:79 +0xf6
net.(*conn).Read(0x105ae6e8, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
    /tmp/workdir/go/src/net/net.go:173 +0x88
net.TestZeroByteRead(0x105adc00)
    /tmp/workdir/go/src/net/net_test.go:402 +0x49d
testing.tRunner(0x105adc00, 0x207738)
    /tmp/workdir/go/src/testing/testing.go:547 +0x8b
created by testing.(*T).Run
    /tmp/workdir/go/src/testing/testing.go:583 +0x2a4

goroutine 25 [syscall]:
syscall.Syscall6(0x3, 0x104d9dec, 0x19, 0xffffffff, 0xffffffff, 0x0, 0x0, 0x104d58d8, 0x2, 0x3ea76, ...)
    /tmp/workdir/go/src/syscall/asm_plan9_386.s:57 +0x5
syscall.Pwrite(0x3, 0x104d9dec, 0x19, 0x20, 0xffffffff, 0xffffffff, 0x19, 0x0, 0x0)
    /tmp/workdir/go/src/syscall/zsyscall_plan9_386.go:245 +0x72
syscall.Write(0x3, 0x104d9dec, 0x19, 0x20, 0x2, 0x0, 0x0)
    /tmp/workdir/go/src/syscall/syscall_plan9.go:128 +0x54
os.(*File).write(0x104de6e0, 0x104d9dec, 0x19, 0x20, 0x104d9e58, 0x0, 0x0)
    /tmp/workdir/go/src/os/file_plan9.go:265 +0x64
os.(*File).Write(0x104de6e0, 0x104d9dec, 0x19, 0x20, 0x19, 0x0, 0x0)
    /tmp/workdir/go/src/os/file.go:138 +0x6e
os.(*File).WriteString(0x104de6e0, 0x104d9e58, 0x19, 0x104d1a2f, 0x0, 0x0)
    /tmp/workdir/go/src/os/file.go:199 +0x9a
net.dialPlan9Blocking(0x28e980, 0x104d19e0, 0x1de47b, 0x3, 0x28e440, 0x0, 0x28e440, 0x104d1a00, 0x0, 0x0, ...)
    /tmp/workdir/go/src/net/ipsock_plan9.go:200 +0x132
net.dialPlan9.func2(0x28e980, 0x104d19e0, 0x1de47b, 0x3, 0x28e440, 0x0, 0x28e440, 0x104d1a00, 0x104fb440)
    /tmp/workdir/go/src/net/ipsock_plan9.go:178 +0x85
created by net.dialPlan9
    /tmp/workdir/go/src/net/ipsock_plan9.go:186 +0x113
FAIL    net 180.052s

[bradfitz]

This isn't a Go 1.7 issue, since Plan 9 is not a First Class Port: https://github.com/golang/go/wiki/PortingPolicy#first-class-ports

/cc @0intro

[bradfitz]

I will also look into fixing this. I didn't mean to assign it to @0intro. I meant to cc him.

[mikioh]

Simply 5bcdd63 forgot to tweak fd_plan9.go.

diff --git a/src/net/fd_plan9.go b/src/net/fd_plan9.go
index a92f514..55e7a03 100644
--- a/src/net/fd_plan9.go
+++ b/src/net/fd_plan9.go
@@ -75,6 +75,9 @@ func (fd *netFD) Read(b []byte) (n int, err error) {
        if err := fd.readLock(); err != nil {
                return 0, err
        }
+       if len(b) == 0 {
+               return 0, nil
+       }
        defer fd.readUnlock()
        if len(p) == 0 {
                // If the caller wanted a zero byte read, return immediately

[bradfitz]

@mikioh, that looks like you're missing the unlock then. Move it down after the defer unlock. But really, just send a CL.