Empty path should match the "/" PathPattern

This commit makes both slash "/" and empty "" request paths match the
"/" `PathPattern`.

Issue: SPR-15653

Author: aclement

spring-web/src/main/java/org/springframework/web/util/pattern/PathPattern.java

@@ -156,15 +156,15 @@ public String getPatternString() {
 		return this.patternString;
 	}
 
-
 	/**
 	 * Whether this pattern matches the given path.
 	 * @param pathContainer the candidate path to attempt to match against
 	 * @return {@code true} if the path matches this pattern
 	 */
 	public boolean matches(PathContainer pathContainer) {
 		if (this.head == null) {
-			return !hasLength(pathContainer);
+			return !hasLength(pathContainer) || 
+				(this.matchOptionalTrailingSeparator && pathContainerIsJustSeparator(pathContainer));
 		}
 		else if (!hasLength(pathContainer)) {
 			if (this.head instanceof WildcardTheRestPathElement || this.head instanceof CaptureTheRestPathElement) {
@@ -187,7 +187,9 @@ else if (!hasLength(pathContainer)) {
 	@Nullable
 	public PathMatchInfo matchAndExtract(PathContainer pathContainer) {
 		if (this.head == null) {
-			return hasLength(pathContainer) ? null : PathMatchInfo.EMPTY;
+			return hasLength(pathContainer) &&
+				!(this.matchOptionalTrailingSeparator && pathContainerIsJustSeparator(pathContainer))
+				? null : PathMatchInfo.EMPTY;
 		}
 		else if (!hasLength(pathContainer)) {
 			if (this.head instanceof WildcardTheRestPathElement || this.head instanceof CaptureTheRestPathElement) {
@@ -708,5 +710,10 @@ private boolean hasLength(@Nullable PathContainer container) {
 	private static int scoreByNormalizedLength(PathPattern pattern) {
 		return -pattern.getNormalizedLength();
 	}
+	
+	private boolean pathContainerIsJustSeparator(PathContainer pathContainer) {
+		return pathContainer.value().length() == 1 && 
+				pathContainer.value().charAt(0) == separator;
+	}
 
 }

spring-web/src/test/java/org/springframework/web/util/pattern/PathPatternTests.java

@@ -77,6 +77,7 @@ public void matching_LiteralPathElement() {
 	@Test
 	public void basicMatching() {
 		checkMatches("", "");
+		checkMatches("", "/");
 		checkMatches("", null);
 		checkNoMatch("/abc", "/");
 		checkMatches("/", "/");
@@ -1113,9 +1114,13 @@ public void parameters() {
 		result = matchAndExtract("/abc/{var}","/abc/one");
 		assertEquals("one",result.getUriVariables().get("var"));
 		assertNull(result.getMatrixVariables().get("var"));
+		
+		result = matchAndExtract("","");
+		assertNotNull(result);
+		result = matchAndExtract("","/");
+		assertNotNull(result);
 	}
 
-
 	private PathPattern.PathMatchInfo matchAndExtract(String pattern, String path) {
 		 return parse(pattern).matchAndExtract(PathPatternTests.toPathContainer(path));
 	}