serve webdav: fix MOVE overwrite failing without Overwrite header

Per RFC 4918 section 10.6, when the Overwrite header is omitted from a
COPY or MOVE request the resource MUST treat the request as if
Overwrite: T had been sent.

The upstream golang.org/x/net/webdav library mishandles this for MOVE
by checking == "T" instead of != "F", so an absent header is treated
as Overwrite: F and the request fails with 412 Precondition Failed.

Normalise the header to T in the rclone WebDAV server before
delegating to the upstream handler when the client did not send one.
This restores RFC-compliant default behaviour and can be removed once
the upstream fix in golang/go#66059 lands and the golang.org/x/net
dependency is bumped.

Fixes #9496

(cherry picked from commit cfb9a10a3d)

cmd/serve/webdav/webdav.go

@@ -391,6 +391,12 @@ func (w *WebDAV) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 		w.serveDir(rw, r, remote)
 		return
 	}
+	// Work around bug in x/net/webdav which handles Overwrite incorrectly
+	// See: https://github.com/golang/go/issues/66059
+	// Remove when the above bug is fixed.
+	if (r.Method == "COPY" || r.Method == "MOVE") && r.Header.Get("Overwrite") == "" {
+		r.Header.Set("Overwrite", "T")
+	}
 	// Add URL Prefix back to path since webdavhandler needs to
 	// return absolute references.
 	r.URL.Path = w.opt.HTTP.BaseURL + r.URL.Path

cmd/serve/webdav/webdav_test.go

@@ -271,3 +271,89 @@ func TestRc(t *testing.T) {
 		"vfs_cache_mode": "off",
 	})
 }
+
+// startWritableServer starts a webdav server backed by a fresh temp
+// directory and returns the server URL. It is used by the Overwrite tests
+// which need to exercise mutating verbs such as MKCOL and MOVE.
+func startWritableServer(t *testing.T) string {
+	t.Helper()
+
+	f, err := fs.NewFs(context.Background(), t.TempDir())
+	require.NoError(t, err)
+
+	opt := Opt
+	opt.HTTP.ListenAddr = []string{testBindAddress}
+
+	w, err := newWebDAV(context.Background(), f, &opt, &vfscommon.Opt, &proxy.Opt)
+	require.NoError(t, err)
+	go func() {
+		require.NoError(t, w.Serve())
+	}()
+	t.Cleanup(func() {
+		assert.NoError(t, w.Shutdown())
+	})
+
+	return w.server.URLs()[0]
+}
+
+func mkcol(t *testing.T, baseURL, path string) {
+	t.Helper()
+	req, err := http.NewRequest("MKCOL", baseURL+path, nil)
+	require.NoError(t, err)
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	_ = resp.Body.Close()
+	require.Equal(t, http.StatusCreated, resp.StatusCode, "MKCOL %s", path)
+}
+
+// TestMoveDefaultsToOverwrite is a regression test for
+// https://github.com/rclone/rclone/issues/9496
+//
+// RFC 4918 section 10.6 requires that when the Overwrite header is omitted
+// from a COPY or MOVE request, the resource MUST behave as if Overwrite: T
+// had been sent. The upstream golang.org/x/net/webdav library mis-handles
+// the MOVE case (see https://github.com/golang/go/issues/66059), so rclone
+// normalises the header before delegating so the default matches the RFC.
+func TestMoveDefaultsToOverwrite(t *testing.T) {
+	testURL := startWritableServer(t)
+
+	mkcol(t, testURL, "dir1")
+	mkcol(t, testURL, "dir2")
+
+	// MOVE without Overwrite header: per RFC 4918 the default is T, so the
+	// existing destination must be replaced and the server must return 2xx.
+	req, err := http.NewRequest("MOVE", testURL+"dir2", nil)
+	require.NoError(t, err)
+	req.Header.Set("Destination", testURL+"dir1")
+
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	defer func() { _ = resp.Body.Close() }()
+
+	assert.NotEqual(t, http.StatusPreconditionFailed, resp.StatusCode,
+		"MOVE without Overwrite header must not return 412; RFC 4918 default is Overwrite: T")
+	assert.True(t, resp.StatusCode >= 200 && resp.StatusCode < 300,
+		"expected 2xx, got %d", resp.StatusCode)
+}
+
+// TestMoveOverwriteFalseStillRejects ensures the rclone normalisation only
+// fills in a missing Overwrite header and never overrides an explicit
+// Overwrite: F sent by the client.
+func TestMoveOverwriteFalseStillRejects(t *testing.T) {
+	testURL := startWritableServer(t)
+
+	mkcol(t, testURL, "dir1")
+	mkcol(t, testURL, "dir2")
+
+	req, err := http.NewRequest("MOVE", testURL+"dir2", nil)
+	require.NoError(t, err)
+	req.Header.Set("Destination", testURL+"dir1")
+	req.Header.Set("Overwrite", "F")
+
+	resp, err := http.DefaultClient.Do(req)
+	require.NoError(t, err)
+	defer func() { _ = resp.Body.Close() }()
+
+	assert.Equal(t, http.StatusPreconditionFailed, resp.StatusCode,
+		"MOVE with explicit Overwrite: F must still return 412 when destination exists")
+}