TLVReader: Add test for Expect() and handle corner case better

Expect(tag) implies that we should actually be positioned on an element.
Also make a note about the position of the reader after Next(...)

src/lib/core/TLVReader.cpp

@@ -585,7 +585,8 @@ CHIP_ERROR TLVReader::Next()
 
 CHIP_ERROR TLVReader::Expect(Tag expectedTag)
 {
-    VerifyOrReturnError(mElemTag == expectedTag, CHIP_ERROR_UNEXPECTED_TLV_ELEMENT);
+    VerifyOrReturnError(GetType() != kTLVType_NotSpecified, CHIP_ERROR_WRONG_TLV_TYPE);
+    VerifyOrReturnError(GetTag() == expectedTag, CHIP_ERROR_UNEXPECTED_TLV_ELEMENT);
     return CHIP_NO_ERROR;
 }
 
@@ -598,8 +599,8 @@ CHIP_ERROR TLVReader::Next(Tag expectedTag)
 
 CHIP_ERROR TLVReader::Expect(TLVType expectedType, Tag expectedTag)
 {
-    ReturnErrorOnFailure(Expect(expectedTag));
     VerifyOrReturnError(GetType() == expectedType, CHIP_ERROR_WRONG_TLV_TYPE);
+    VerifyOrReturnError(GetTag() == expectedTag, CHIP_ERROR_UNEXPECTED_TLV_ELEMENT);
     return CHIP_NO_ERROR;
 }
 

src/lib/core/TLVReader.h

@@ -167,7 +167,10 @@ class DLL_EXPORT TLVReader
      * Advances the TLVReader object to the next TLV element to be read, asserting the tag of
      * the new element.
      *
-     * This is a convenience method that combines the behavior of Next() and Expect().
+     * This is a convenience method that combines the behavior of Next() and Expect(...).
+     *
+     * Note that if this method returns an error, the reader may or may not have been advanced already.
+     * In use cases where this is important, separate calls to Next() and Expect(...) should be made.
      *
      * @retval #CHIP_NO_ERROR              If the reader was successfully positioned on a new element
      *                                     matching the expected parameters.
@@ -179,6 +182,7 @@ class DLL_EXPORT TLVReader
      * Checks that the TLV reader is positioned at an element with the expected tag.
      *
      * @retval #CHIP_NO_ERROR              If the reader is positioned on the expected element.
+     * @retval #CHIP_ERROR_WRONG_TLV_TYPE  If the reader is not positioned on an element.
      * @retval #CHIP_ERROR_UNEXPECTED_TLV_ELEMENT
      *                                      If the tag associated with the new element does not match the
      *                                      value of the @p expectedTag argument.
@@ -189,7 +193,10 @@ class DLL_EXPORT TLVReader
      * Advances the TLVReader object to the next TLV element to be read, asserting the type and tag of
      * the new element.
      *
-     * This is a convenience method that combines the behavior of Next() and Expect().
+     * This is a convenience method that combines the behavior of Next() and Expect(...).
+     *
+     * Note that if this method returns an error, the reader may or may not have been advanced already.
+     * In use cases where this is important, separate calls to Next() and Expect(...) should be made.
      *
      * @retval #CHIP_NO_ERROR              If the reader was successfully positioned on a new element
      *                                     matching the expected parameters.

src/lib/core/tests/TestTLV.cpp

@@ -3628,6 +3628,59 @@ void TestTLVReaderErrorHandling(nlTestSuite * inSuite)
     chip::Platform::MemoryFree(const_cast<uint8_t *>(data));
 }
 
+void TestTLVReaderExpect(nlTestSuite * inSuite)
+{
+    // Prepare some test data
+    uint8_t buffer[20];
+    TLVWriter writer;
+    writer.Init(buffer, sizeof(buffer));
+    TLVType outerContainer;
+    NL_TEST_ASSERT_SUCCESS(inSuite, writer.StartContainer(AnonymousTag(), kTLVType_Structure, outerContainer));
+    NL_TEST_ASSERT_SUCCESS(inSuite, writer.PutBoolean(ContextTag(23), false));
+    NL_TEST_ASSERT_SUCCESS(inSuite, writer.EndContainer(outerContainer));
+
+    TLVReader reader;
+    reader.Init(buffer, writer.GetLengthWritten());
+
+    // Positioned before the first element
+    NL_TEST_ASSERT(inSuite, reader.GetType() == kTLVType_NotSpecified);
+
+    NL_TEST_ASSERT(inSuite, reader.Expect(AnonymousTag()) == CHIP_ERROR_WRONG_TLV_TYPE);
+    NL_TEST_ASSERT(inSuite, reader.Expect(ContextTag(23)) == CHIP_ERROR_WRONG_TLV_TYPE);
+    NL_TEST_ASSERT(inSuite, reader.Expect(kTLVType_Boolean, AnonymousTag()) == CHIP_ERROR_WRONG_TLV_TYPE);
+
+    // Positioned on kTLVType_Structure / AnonymousTag(),
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.Next());
+    NL_TEST_ASSERT(inSuite, reader.GetType() == kTLVType_Structure);
+    NL_TEST_ASSERT(inSuite, reader.GetTag() == AnonymousTag());
+
+    NL_TEST_ASSERT(inSuite, reader.Expect(ContextTag(23)) == CHIP_ERROR_UNEXPECTED_TLV_ELEMENT);
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.Expect(AnonymousTag()));
+
+    NL_TEST_ASSERT(inSuite, reader.Expect(kTLVType_SignedInteger, AnonymousTag()) == CHIP_ERROR_WRONG_TLV_TYPE);
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.Expect(kTLVType_Structure, AnonymousTag()));
+
+    // Positioned before first struct element
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.EnterContainer(outerContainer));
+    NL_TEST_ASSERT(inSuite, reader.GetType() == kTLVType_NotSpecified);
+
+    NL_TEST_ASSERT(inSuite, reader.Expect(AnonymousTag()) == CHIP_ERROR_WRONG_TLV_TYPE);
+    NL_TEST_ASSERT(inSuite, reader.Expect(ContextTag(23)) == CHIP_ERROR_WRONG_TLV_TYPE);
+    NL_TEST_ASSERT(inSuite, reader.Expect(kTLVType_Boolean, AnonymousTag()) == CHIP_ERROR_WRONG_TLV_TYPE);
+
+    // Positioned on kTLVType_Boolean / ContextTag(23)
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.Next());
+    NL_TEST_ASSERT(inSuite, reader.GetType() == kTLVType_Boolean);
+    NL_TEST_ASSERT(inSuite, reader.GetTag() == ContextTag(23));
+
+    NL_TEST_ASSERT(inSuite, reader.Expect(AnonymousTag()) == CHIP_ERROR_UNEXPECTED_TLV_ELEMENT);
+    NL_TEST_ASSERT(inSuite, reader.Expect(ContextTag(42)) == CHIP_ERROR_UNEXPECTED_TLV_ELEMENT);
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.Expect(ContextTag(23)));
+
+    NL_TEST_ASSERT(inSuite, reader.Expect(kTLVType_SignedInteger, ContextTag(23)) == CHIP_ERROR_WRONG_TLV_TYPE);
+    NL_TEST_ASSERT_SUCCESS(inSuite, reader.Expect(kTLVType_Boolean, ContextTag(23)));
+}
+
 /**
  *  Test that CHIP TLV reader returns an error when a read is requested that
  *  would truncate the output.
@@ -3789,6 +3842,8 @@ void CheckTLVReader(nlTestSuite * inSuite, void * inContext)
 
     TestTLVReaderErrorHandling(inSuite);
 
+    TestTLVReaderExpect(inSuite);
+
     TestTLVReaderTruncatedReads(inSuite);
 
     TestTLVReaderInPractice(inSuite);