x/net/webdav: MOVE without an Overwrite header does not default to T

[samcrang]

Go version

go version go1.22.0 darwin/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN='/Users/sam/src/go/bin'
GOCACHE='/Users/sam/Library/Caches/go-build'
GOENV='/Users/sam/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/sam/src/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/sam/src/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/Cellar/go/1.22.0/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/Cellar/go/1.22.0/libexec/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/sam/src/spike/go-webdav/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/c8/njvpx3cn1k7g0zjkf50rk6200000gn/T/go-build779784624=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I have a simple WebDAV server:

package main

import (
	"net/http"

	"golang.org/x/net/webdav"
)

func main() {
	http.Handle("/", &webdav.Handler{
		FileSystem: webdav.Dir("."),
		LockSystem: webdav.NewMemLS(),
	})
	http.ListenAndServe(":8080", nil)
}

I'm using the builtin macOS WebDAV client (from Finder.app: Go > Network). I then connect to the WebDAV and try to move a file to a destination that already exists (ie overwriting the destination file with the source file).

What did you see happen?

$  pwd
/Volumes/127.0.0.1
$  touch foo bar
$  ls
bar     foo     go.mod  go.sum  main.go
$  mv foo bar
mv: rename foo to bar: Invalid argument

What did you expect to see?

It should overwrite the destination file with the source file and remove the source file. There should be no error.

I did some investigation and discovered that the macOS WebDAV client doesn't appear to set the Overwrite header when doing a MOVE operation. I grabbed the traffic with WireShark:

MOVE /foo HTTP/1.1
Host: 127.0.0.1:8080
Destination: http://127.0.0.1:8080/bar
Accept: */*
Content-Length: 0
Connection: keep-alive
User-Agent: WebDAVFS/3.0.0 (03008000) Darwin/23.3.0 (x86_64)

HTTP/1.1 412 Precondition Failed
Date: Fri, 01 Mar 2024 15:43:51 GMT
Content-Length: 19
Content-Type: text/plain; charset=utf-8

Precondition Failed

According to the WebDAV spec:

If the overwrite header is not included in a COPY or MOVE request, then the resource must treat the request as if it has an overwrite header of value "T".

However, the current implementation doesn't seem to do this. Instead it returns a 412 because it seems to assume the Overwrite header will always exist.

I think one solution would be to replace r.Header.Get("Overwrite") == "T" with r.Header.Get("Overwrite") != "F". I forked x/net and it seemed to work for me—if you're happy with this as a solution I'm happy to raise a PR.

However, it's also worth noting that the specification says:

If a COPY or MOVE is not performed due to the value of the Overwrite header, the method must fail with a 412 (Precondition Failed) status code. The server must do authorization checks before checking this or any conditional header.

I interpret this to mean "any value for Overwrite other that T or F should be rejected". I guess this means that the use of r.Header.Get("Overwrite") != "F" would also be technically incorrect too but it'd seem unlikely to cause any problems(?).

Also as an aside the "Overwite should default to T" behavior appears in the WebDAV modules for Apache and nginx.