Polish contribution

This commit polishes an external contribution, ensuring that not just
spaces are encoded as underscores, and that underscores are encoded
as non-printable.

See gh-30252

Author: poutsma

Diff for: spring-web/src/main/java/org/springframework/http/ContentDisposition.java

@@ -23,6 +23,7 @@
 import java.time.format.DateTimeParseException;
 import java.util.ArrayList;
 import java.util.Base64;
+import java.util.BitSet;
 import java.util.List;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -58,6 +59,19 @@ public final class ContentDisposition {
 	private static final String INVALID_HEADER_FIELD_PARAMETER_FORMAT =
 			"Invalid header field parameter format (as defined in RFC 5987)";
 
+	private static final BitSet PRINTABLE = new BitSet(256);
+
+
+	static {
+		// RFC 2045, Section 6.7, and RFC 2047, Section 4.2
+		for (int i=33; i<= 126; i++) {
+			PRINTABLE.set(i);
+		}
+		PRINTABLE.set(61, false); // =
+		PRINTABLE.set(63, false); // ?
+		PRINTABLE.set(95, false); // _
+	}
+
 
 	@Nullable
 	private final String type;
@@ -545,7 +559,7 @@ private static String decodeQuotedPrintableFilename(String filename, Charset cha
 		int index = 0;
 		while (index < value.length) {
 			byte b = value[index];
-			if (b == '_') {
+			if (b == '_') { // RFC 2047, section 4.2, rule (2)
 				baos.write(' ');
 				index++;
 			}
@@ -583,7 +597,10 @@ private static String encodeQuotedPrintableFilename(String filename, Charset cha
 		sb.append(charset.name());
 		sb.append("?Q?");
 		for (byte b : source) {
-			if (isPrintable(b)) {
+			if (b == 32) { // RFC 2047, section 4.2, rule (2)
+				sb.append('_');
+			}
+			else if (isPrintable(b)) {
 				sb.append((char) b);
 			}
 			else {
@@ -599,7 +616,11 @@ private static String encodeQuotedPrintableFilename(String filename, Charset cha
 	}
 
 	private static boolean isPrintable(byte c) {
-		return (c >= '!' && c <= '<') || (c >= '@' && c <= '~') || c == '>';
+		int b = c;
+		if (b < 0) {
+			b = 256 + b;
+		}
+		return PRINTABLE.get(b);
 	}
 
 	private static String encodeQuotedPairs(String filename) {

Diff for: spring-web/src/test/java/org/springframework/http/ContentDispositionTests.java

@@ -331,7 +331,12 @@ void parseFormattedWithQuestionMark() {
 		ContentDisposition cd = ContentDisposition.attachment()
 				.filename(filename, StandardCharsets.UTF_8)
 				.build();
-		String[] parts = cd.toString().split("; ");
+		String result = cd.toString();
+		assertThat(result).isEqualTo("attachment; " +
+						"filename=\"=?UTF-8?Q?filename_with_=3F=E9=97=AE=E5=8F=B7.txt?=\"; " +
+						"filename*=UTF-8''filename%20with%20%3F%E9%97%AE%E5%8F%B7.txt");
+
+		String[] parts = result.split("; ");
 
 		String quotedPrintableFilename = parts[0] + "; " + parts[1];
 		assertThat(ContentDisposition.parse(quotedPrintableFilename).getFilename())
@@ -341,4 +346,5 @@ void parseFormattedWithQuestionMark() {
 		assertThat(ContentDisposition.parse(rfc5987Filename).getFilename())
 				.isEqualTo(filename);
 	}
+
 }