internal: non-strict RFC etag support

For some caldav providers, it wasn't possible to unmarshal unquoted text for etag.
https://datatracker.ietf.org/doc/html/rfc2616#section-3.11 points to wrap text with quotes, but in reality it's sometimes ignored.

internal: test for ETag UnmarshalText, ETag String, ETag unmarshal and marshal

internal/elements.go

@@ -377,7 +377,15 @@ type GetETag struct {
 type ETag string
 
 func (etag *ETag) UnmarshalText(b []byte) error {
-	s, err := strconv.Unquote(string(b))
+	s := string(b)
+	// support for providers with non-strict https://datatracker.ietf.org/doc/html/rfc2616#section-3.11 implementation
+	if !strings.HasPrefix(s, "\"") {
+		*etag = ETag(s)
+		return nil
+	}
+
+	var err error
+	s, err = strconv.Unquote(s)
 	if err != nil {
 		return fmt.Errorf("webdav: failed to unquote ETag: %v", err)
 	}

internal/elements_test.go

@@ -63,3 +63,153 @@ func TestTimeRoundTrip(t *testing.T) {
 		t.Fatalf("invalid round-trip:\ngot= %s\nwant=%s", got, want)
 	}
 }
+
+func TestETag_UnmarshalText(t *testing.T) {
+	type args struct {
+		b []byte
+	}
+	tests := []struct {
+		name     string
+		etag     ETag
+		args     args
+		wantErr  bool
+		wantETag ETag
+	}{
+		{
+			name: "double quoted string",
+			etag: "",
+			args: args{
+				b: []byte("\"1692394723948\""),
+			},
+			wantErr:  false,
+			wantETag: "1692394723948",
+		},
+		{
+			name: "empty double quoted string",
+			etag: "",
+			args: args{
+				b: []byte("\"\""),
+			},
+			wantErr:  false,
+			wantETag: "",
+		},
+		{
+			name: "unquoted string",
+			etag: "",
+			args: args{
+				b: []byte("1692394723948"),
+			},
+			wantErr:  false,
+			wantETag: "1692394723948",
+		},
+		{
+			name: "empty string",
+			etag: "",
+			args: args{
+				b: []byte(""),
+			},
+			wantErr:  false,
+			wantETag: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := tt.etag.UnmarshalText(tt.args.b); (err != nil) != tt.wantErr {
+				t.Errorf("UnmarshalText() error = %v, wantErr %v", err, tt.wantErr)
+			}
+
+			if tt.etag != tt.wantETag {
+				t.Errorf("UnmarshalText() want %s, got %s", tt.wantETag, tt.etag)
+			}
+		})
+	}
+}
+
+func TestETag_String(t *testing.T) {
+	tests := []struct {
+		name string
+		etag ETag
+		want string
+	}{
+		{
+			name: "string with double-quote",
+			etag: "162392347123",
+			want: "\"162392347123\"",
+		},
+		{
+			name: "empty string with double-quote",
+			etag: "",
+			want: "\"\"",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.etag.String(); got != tt.want {
+				t.Errorf("String() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestETag_UnmarshalAndMarshalText(t *testing.T) {
+	tests := []struct {
+		name string
+		// initial ETag
+		etag ETag
+		// first value for ETag UnmarshalText
+		unmarshalValue []byte
+		// expected MarshalText() result after UnmarshalText()
+		wantMarshalled []byte
+		// expected UnmarshalText() result after MarshalText() with unmarshalValue
+		wantUnmarshalledETag ETag
+	}{
+		{
+			name:                 "string",
+			etag:                 "",
+			unmarshalValue:       []byte("162392347123"),
+			wantMarshalled:       []byte("\"162392347123\""),
+			wantUnmarshalledETag: "162392347123",
+		},
+		{
+			name:                 "double-quoted",
+			etag:                 "",
+			unmarshalValue:       []byte("\"162392347123\""),
+			wantMarshalled:       []byte("\"162392347123\""),
+			wantUnmarshalledETag: "162392347123",
+		},
+		{
+			name:                 "empty string",
+			etag:                 "",
+			unmarshalValue:       []byte(""),
+			wantMarshalled:       []byte("\"\""),
+			wantUnmarshalledETag: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := tt.etag.UnmarshalText(tt.unmarshalValue); err != nil {
+				t.Errorf("UnmarshalText() = %v", err)
+			}
+
+			m, err := tt.etag.MarshalText()
+			if err != nil {
+				t.Errorf("MarshalText() = %v", err)
+			}
+
+			if !bytes.Equal(m, tt.wantMarshalled) {
+				t.Errorf("MarshalText() want %s, got %s", tt.wantMarshalled, m)
+			}
+
+			if err := tt.etag.UnmarshalText(m); err != nil {
+				t.Errorf("UnmarshalText() got error after MarshalText() = %v", err)
+			}
+
+			if tt.etag != tt.wantUnmarshalledETag {
+				t.Errorf("UnmarshalText() after MarshalText() want %s, got %s", tt.wantUnmarshalledETag, tt.etag)
+			}
+		})
+	}
+}