os: return value of Fd can be closed unexpectedly by finalizer

[gopherbot]

by 2852914@wessie.info:

What does 'go version' print?

go version devel +53ebc70d4e9f Sat Nov 01 08:28:09 2014 -0700 linux/amd64
&
go version go1.3.3 linux/amd64

What steps reproduce the problem?

http://play.golang.org/p/gzyJ0vJPEK

What happened?

bad file descriptor after a few loops (i <= 500)

What should have happened instead?

loop indefinitely, the reproduction has no success case and only exits on the bad file
descriptor.

Please provide any additional information below.

It seems tty isn't live after the log.Fatal line, and err isn't live on the log.Fatal
line

output of `go build -gcflags "-live -l" dom.go`:

tip:
./dom.go:24: live at call to convI2E: autotmp_0007 autotmp_0000 tty
./dom.go:24: live at call to writebarrieriface: autotmp_0007 autotmp_0000 tty
./dom.go:24: live at call to Fatal: autotmp_0000 tty
./dom.go:38: live at call to convT2E: autotmp_0003 autotmp_0007
./dom.go:38: live at call to writebarrieriface: autotmp_0003 autotmp_0007
./dom.go:38: live at call to convT2E: autotmp_0003 autotmp_0007
./dom.go:38: live at call to writebarrieriface: autotmp_0003 autotmp_0007
./dom.go:38: live at call to Printf: autotmp_0003

1.3.3:
./dom.go:24: live at call to convI2E: autotmp_0006 autotmp_0000 tty
./dom.go:24: live at call to Fatal: autotmp_0000 tty
./dom.go:38: live at call to Printf: autotmp_0002

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.4.

[ianlancetaylor]

Comment 2:

I agree that the problem is that tty is not live.  The *os.File to which it points is
eventually collected, and the finalizer closes the file descriptor.  There is nothing
keeping tty live, so it is hard to blame the compiler.  This would be more obvious if
the function were
func ttyFd() uintptr {
    tty, _ := os.Open("/dev/tty")
    return tty.Fd()
}
I think our choices are 1) document that you must be careful to keep the *os.File live
if you call the Fd method; 2) have the Fd method clear the finalizer, and document that
the caller of Fd is responsible for closing the descriptor, somehow--actually this would
probably require adding a new function to close a descriptor.

Labels changed: added release-go1.4maybe, removed release-go1.4.

[davecheney]

Comment 3:

This bug is going on my list of why os.File shouldn't have a finaliser.
I also like the idea that calling .Fd() disables the finaliser.

[rsc]

Comment 4:

Easy fix: keep tty live, such as by calling tty.Close when you are done with the fd.
If you really want just an fd, use syscall.Open.

Status changed to WorkingAsIntended.

[gopherbot]

Comment 5:

CL https://golang.org/cl/162680043 mentions this issue.

[rsc]

Comment 6:

This issue was closed by revision 1cdd9b4.

Status changed to Fixed.