From b1269c91211deab8f94c3cf85accb51e5a71ca5c Mon Sep 17 00:00:00 2001
From: shaobero <shaobero@adobe.com>
Date: Thu, 4 Nov 2021 17:57:12 +0000
Subject: [PATCH] ICU-21823 Adding changes to fix charset detection incase of
 no match

---
 icu4c/source/i18n/csdetect.cpp          |  5 +++++
 icu4c/source/test/cintltst/ucsdetst.c   |  1 +
 icu4c/source/test/intltest/csdetest.cpp | 23 +++++++++++++++++++++++
 icu4c/source/test/intltest/csdetest.h   |  1 +
 4 files changed, 30 insertions(+)

diff --git a/icu4c/source/i18n/csdetect.cpp b/icu4c/source/i18n/csdetect.cpp
index 84f0776542d..d866eb66286 100644
--- a/icu4c/source/i18n/csdetect.cpp
+++ b/icu4c/source/i18n/csdetect.cpp
@@ -270,6 +270,11 @@ const CharsetMatch * const *CharsetDetector::detectAll(int32_t &maxMatchesFound,
 
     maxMatchesFound = resultCount;
 
+    if (maxMatchesFound == 0) {
+        status = U_INVALID_CHAR_FOUND;
+        return NULL;
+    }
+
     return resultArray;
 }
 
diff --git a/icu4c/source/test/cintltst/ucsdetst.c b/icu4c/source/test/cintltst/ucsdetst.c
index f65f8d79aa4..b8d3b5f20b8 100644
--- a/icu4c/source/test/cintltst/ucsdetst.c
+++ b/icu4c/source/test/cintltst/ucsdetst.c
@@ -402,6 +402,7 @@ static void TestBufferOverflow(void) {
     }
 
     for (idx = 0; idx < UPRV_LENGTHOF(testStrings); idx++) {
+        status = U_ZERO_ERROR;
         ucsdet_setText(csd, testStrings[idx], -1, &status);
         match = ucsdet_detect(csd, &status);
 
diff --git a/icu4c/source/test/intltest/csdetest.cpp b/icu4c/source/test/intltest/csdetest.cpp
index 66f9340b83a..95f19d43d1a 100644
--- a/icu4c/source/test/intltest/csdetest.cpp
+++ b/icu4c/source/test/intltest/csdetest.cpp
@@ -109,6 +109,10 @@ void CharsetDetectionTest::runIndexedTest( int32_t index, UBool exec, const char
             if (exec) Ticket6954Test();
             break;
 
+       case 10: name = "Ticket21823Test";
+            if (exec) Ticket21823Test();
+            break;
+
         default: name = "";
             break; //needed to end loop
     }
@@ -839,3 +843,22 @@ void CharsetDetectionTest::Ticket6954Test() {
     TEST_ASSERT(strcmp(name1, "windows-1252")==0);
 #endif
 }
+
+
+// Ticket 21823 - Issue with Charset Detector for ill-formed input strings. 
+//                Its fix involves returning a failure based error code 
+//                (U_INVALID_CHAR_FOUND) incase no charsets appear to match the input data.
+void CharsetDetectionTest::Ticket21823Test() {
+    UErrorCode status = U_ZERO_ERROR;
+    std::string str = "\x80";
+    UCharsetDetector* csd = ucsdet_open(&status);
+
+    ucsdet_setText(csd, str.data(), str.length(), &status);
+    const UCharsetMatch* match = ucsdet_detect(csd, &status);
+
+    if (match == NULL) {
+        TEST_ASSERT(U_FAILURE(status));
+    }
+
+    ucsdet_close(csd);
+}
diff --git a/icu4c/source/test/intltest/csdetest.h b/icu4c/source/test/intltest/csdetest.h
index 72d1ca92aea..8893b88e591 100644
--- a/icu4c/source/test/intltest/csdetest.h
+++ b/icu4c/source/test/intltest/csdetest.h
@@ -33,6 +33,7 @@ public:
     virtual void IBM420Test();
     virtual void Ticket6394Test();
     virtual void Ticket6954Test();
+    virtual void Ticket21823Test();
 
 private:
     void checkEncoding(const UnicodeString &testString,
-- 
2.50.1