os: Rename error behavior changed from Go 1.7 to Go 1.8 when source file doesn't exist

[zeebo]

What version of Go are you using (go version)?

go1.8

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH=""
GORACE=""
GOROOT="/Users/jeff/external/golang"
GOTOOLDIR="/Users/jeff/external/golang/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/tn/09dglcts0b9gc5111hnf0_dm0000gn/T/go-build647286002=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

run this program under go1.7 and go1.8

package main

import (
	"fmt"
	"os"
)

func main() {
	os.Mkdir("destination", 0755)
	os.Create("destination/nonempty")
	err := os.Rename("doesnt-exist", "destination")
	fmt.Println(os.IsNotExist(err), err)
}

https://play.golang.org/p/v4gGML3VCP

What did you expect to see?

the same output

What did you see instead?

$ go1.7 run main.go
true rename doesnt-exist destination: no such file or directory
$ go1.8 run main.go
false rename doesnt-exist destination: file exists

Fixing issue #14527 in CL https://golang.org/cl/31358 caused this (it checks if the destination is a directory before attempting the rename). There are many places in my code base where I optimistically attempt to migrate some old directory path to a new one, and all of them will have to include a check that the old directory exists first to maintain idempotency.

[bradfitz]

Yes, this was intentional.

https://golang.org/doc/go1.8#os says:

On Unix systems, os.Rename will now return an error when used to rename a directory to an existing empty directory. Previously it would fail when renaming to a non-empty directory but succeed when renaming to an empty directory. This makes the behavior on Unix correspond to that of other systems.

The fix that works on both Go 1.7 and Go 1.8 is to write instead:

	err := os.Rename("doesnt-exist", "destination/doesnt-exist")

[zeebo]

You misunderstand. The difference is that in go1.7 the error that is returned is that the source does not exist, and in go1.8 the error that is returned is that the destination already exists. That semantic change caused bugs for me, and so I was hoping to maintain the property that if you attempt to rename with a source that doesn't exist, that check is performed first. The mv command has this property, for example.

[zeebo]

To be clear, I'm fine with go1.8 erroring because the destination already exists, it would just be good if it errored that the source does not exist first to maintain compatibility with go1.7.

[bradfitz]

Gotcha.

But now that the behavior has changed in a released version of Go, I'm not sure how much good (or harm) it does to change it again. We've never documented the precedence of errors between the two possible failure cases.

The only reliable thing to do is stat your source file first, before the rename.

I can reopen for a decision, but I don't see a great answer.

[bradfitz]

/cc @rsc @ianlancetaylor

[WGH-]

The only reliable thing to do is stat your source file first, before the rename.

Doesn't sound very reliable due to potential race condition, though.

[bradfitz]

Then do it afterwards, or afterwards also to disambiguate between the two possible errors. Like I said, we've never documented which of the src error vs dest error is returned first if both are bogus.

[rsc]

I think this is not important enough to fix for Go 1.8.1. I don't mind if someone feels strongly and wants to add an Lstat of oldname for Go 1.9.

[gopherbot]

CL https://golang.org/cl/40577 mentions this issue.