ICU-22579 Fix Null deref while Unicode Set only has string

icu4c/source/common/rbbiscan.cpp

@@ -1224,6 +1224,7 @@ void RBBIRuleScanner::scanSet() {
     UErrorCode localStatus = U_ZERO_ERROR;
     LocalPointer<UnicodeSet> uset(new UnicodeSet(), localStatus);
     if (U_FAILURE(localStatus)) {
+        error(localStatus);
         return;
     }
     uset->applyPatternIgnoreSpace(fRB->fRules, pos, fSymbolTable, localStatus);
@@ -1240,7 +1241,11 @@ void RBBIRuleScanner::scanSet() {
     // Verify that the set contains at least one code point.
     //
     U_ASSERT(uset.isValid());
-    if (uset->isEmpty()) {
+    UnicodeSet tempSet(*uset);
+    // Use tempSet to handle the case that the UnicodeSet contains
+    // only string element, such as [{ab}] and treat it as empty set.
+    tempSet.removeAllStrings();
+    if (tempSet.isEmpty()) {
         // This set is empty.
         //  Make it an error, because it almost certainly is not what the user wanted.
         //  Also, avoids having to think about corner cases in the tree manipulation code

icu4c/source/test/intltest/rbbitst.cpp

@@ -144,6 +144,7 @@ void RBBITest::runIndexedTest( int32_t index, UBool exec, const char* &name, cha
     TESTCASE_AUTO(TestRandomAccess);
     TESTCASE_AUTO(TestExternalBreakEngineWithFakeTaiLe);
     TESTCASE_AUTO(TestExternalBreakEngineWithFakeYue);
+    TESTCASE_AUTO(TestBug22579);
     TESTCASE_AUTO(TestBug22581);
     TESTCASE_AUTO(TestBug22584);
     TESTCASE_AUTO(TestBug22585);
@@ -5895,6 +5896,14 @@ void RBBITest::TestBug22584() {
     RuleBasedBreakIterator bi2(ruleStr, pe, ec);
 }
 
+void RBBITest::TestBug22579() {
+    // Test not causing null deref in cloneTree
+    UnicodeString ruleStr = u"[{ab}];";
+    UParseError pe {};
+    UErrorCode ec {U_ZERO_ERROR};
+
+    RuleBasedBreakIterator bi(ruleStr, pe, ec);
+}
 void RBBITest::TestBug22581() {
     // Test duplicate variable setting will not leak the rule compilation
     UnicodeString ruleStr = u"$foo=[abc]; $foo=[xyz]; $foo;";

icu4c/source/test/intltest/rbbitst.h

@@ -98,6 +98,7 @@ class RBBITest: public IntlTest {
     void TestRandomAccess();
     void TestExternalBreakEngineWithFakeTaiLe();
     void TestExternalBreakEngineWithFakeYue();
+    void TestBug22579();
     void TestBug22581();
     void TestBug22584();
     void TestBug22585();

icu4j/main/core/src/main/java/com/ibm/icu/text/RBBIRuleScanner.java

@@ -1062,7 +1062,11 @@ void scanSet() {
 
         // Verify that the set contains at least one code point.
         //
-        if (uset.isEmpty()) {
+        // Use tempSet to handle the case that the UnicodeSet contains
+        // only string element, such as [{ab}] and treat it as empty set.
+        UnicodeSet tempSet = new UnicodeSet(uset);
+        tempSet.removeAllStrings();
+        if (tempSet.isEmpty()) {
             // This set is empty.
             //  Make it an error, because it almost certainly is not what the user wanted.
             //  Also, avoids having to think about corner cases in the tree manipulation code

icu4j/main/core/src/test/java/com/ibm/icu/dev/test/rbbi/RBBITest.java

@@ -946,6 +946,20 @@ public void TestUnpairedSurrogate() {
         assertEquals("Rules does not match", rules, bi.toString());
     }
 
+    @Test
+    public void TestBug22579() {
+        try {
+            new RuleBasedBreakIterator("[{ab}];");
+            fail("TestBug22579: RuleBasedBreakIterator() failed to throw an exception with only string in an Unicode set.");
+        }
+        catch (IllegalArgumentException e) {
+            // expected exception with only string inside an Unicode set.
+        }
+        catch (Exception e) {
+            fail("TestBug22579: Unexpected exception while new RuleBasedBreakIterator() with only string in an Unicode Set: " + e);
+        }
+
+    }
     @Test
     public void TestBug22585() {
         try {