TLV reader: Improve floating-point support

1. For some reason the Get(float&) signature was declared in the .h file
   but was never defined. Define it as well. Disallow reading a 64-bit
   floating point number to float, to avoid silent truncation / loss of
   precision.
2. It is undefined behaviour to use unions for type-punning in C++. Use
   raw memcpy calls instead, which is defined by the C++ standard
   (std::bit_cast is not available).

   Compilers will generally be able to optimize this memcpy away, unless
   a freestanding implementation (with possibly non-standard memcpy) is
   used.

This is a backport from an analogous commit to CHIP:
project-chip/connectedhomeip#8531

src/lib/core/WeaveTLVReader.cpp

@@ -541,6 +541,45 @@ WEAVE_ERROR TLVReader::Get(uint64_t& v)
     return WEAVE_NO_ERROR;
 }
 
+namespace {
+float BitCastToFloat(const uint64_t elemLenOrVal)
+{
+    float f;
+    auto u32 = static_cast<uint32_t>(elemLenOrVal);
+    memcpy(&f, &u32, sizeof(f));
+    return f;
+}
+} // namespace
+
+// Note: Unlike the integer Get functions, this code avoids doing conversions
+// between float and double wherever possible, because these conversions are
+// relatively expensive on platforms that use soft-float instruction sets.
+
+/**
+ * Get the value of the current element as a single-precision floating point number.
+ *
+ * @param[out]  v                       Receives the value associated with current TLV element.
+ *
+ * @retval #WEAVE_NO_ERROR              If the method succeeded.
+ * @retval #WEAVE_ERROR_WRONG_TLV_TYPE  If the current element is not a TLV floating point type, or
+ *                                      the reader is not positioned on an element.
+ *
+ */
+WEAVE_ERROR TLVReader::Get(float & v)
+{
+    switch (ElementType())
+    {
+    case kTLVElementType_FloatingPointNumber32:
+    {
+        v = BitCastToFloat(mElemLenOrVal);
+        break;
+    }
+    default:
+        return WEAVE_ERROR_WRONG_TLV_TYPE;
+    }
+    return WEAVE_NO_ERROR;
+}
+
 /**
  * Get the value of the current element as a double-precision floating point number.
  *
@@ -557,24 +596,14 @@ WEAVE_ERROR TLVReader::Get(double& v)
     {
     case kTLVElementType_FloatingPointNumber32:
     {
-        union
-        {
-            uint32_t u32;
-            float f;
-        } cvt;
-        cvt.u32 = (uint32_t)mElemLenOrVal;
-        v = cvt.f;
+        v = BitCastToFloat(mElemLenOrVal);
         break;
     }
     case kTLVElementType_FloatingPointNumber64:
     {
-        union
-        {
-            uint64_t u64;
-            double d;
-        } cvt;
-        cvt.u64 = mElemLenOrVal;
-        v = cvt.d;
+        double d;
+        memcpy(&d, &mElemLenOrVal, sizeof(d));
+        v = d;
         break;
     }
     default:

src/test-apps/TestTLV.cpp

@@ -580,11 +580,12 @@ void ReadEncoding1(nlTestSuite *inSuite, TLVReader& reader)
 
         TestNext<TLVReader>(inSuite, reader2);
 
-        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), (float)17.9);
+        TestGet<TLVReader, float>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
+        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
 
         TestNext<TLVReader>(inSuite, reader2);
 
-        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), (double)17.9);
+        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), 17.9);
 
         TestEndAndCloseContainer(inSuite, reader, reader2);
     }
@@ -2946,11 +2947,12 @@ void TestWeaveTLVReaderDup(nlTestSuite *inSuite)
 
         TestNext<TLVReader>(inSuite, reader2);
 
-        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), (float)17.9);
+        TestGet<TLVReader, float>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
+        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
 
         TestNext<TLVReader>(inSuite, reader2);
 
-        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), (double)17.9);
+        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), 17.9);
 
         TestEndAndCloseContainer(inSuite, reader, reader2);
     }
@@ -2974,6 +2976,11 @@ void TestWeaveTLVReaderErrorHandling(nlTestSuite *inSuite)
     err = reader.Get(val);
     NL_TEST_ASSERT(inSuite, err == WEAVE_ERROR_WRONG_TLV_TYPE);
 
+    // Get(float&)
+    float numF;
+    err = reader.Get(numF);
+    NL_TEST_ASSERT(inSuite, err == WEAVE_ERROR_WRONG_TLV_TYPE);
+
     // Get(double&)
     double numD;
     err = reader.Get(numD);
@@ -3020,6 +3027,38 @@ void TestWeaveTLVReaderErrorHandling(nlTestSuite *inSuite)
     NL_TEST_ASSERT(inSuite, err == WEAVE_ERROR_WRONG_TLV_TYPE);
     free((void *)data);
 }
+
+/**
+ *  Test that Weave TLV reader returns an error when a read is requested that
+ *  would truncate the output.
+ */
+void TestWeaveTLVReaderTruncatedReads(nlTestSuite * inSuite)
+{
+    uint8_t buf[2048];
+    TLVWriter writer;
+    TLVReader reader;
+
+    WEAVE_ERROR err;
+    float outF;
+
+    // Setup: Write some values into the buffer
+    writer.Init(buf, sizeof(buf));
+    writer.ImplicitProfileId = TestProfile_2;
+
+    err = writer.Put(AnonymousTag, double(12.5));
+    NL_TEST_ASSERT(inSuite, err == WEAVE_NO_ERROR);
+
+    // Test reading values from the buffer
+    reader.Init(buf, sizeof(buf));
+
+    TestNext<TLVReader>(inSuite, reader);
+
+    TestGet<TLVReader, double>(inSuite, reader, kTLVType_FloatingPointNumber, AnonymousTag, 12.5);
+
+    err = reader.Get(outF);
+    NL_TEST_ASSERT(inSuite, err == WEAVE_ERROR_WRONG_TLV_TYPE);
+}
+
 /**
  *  Test Weave TLV Reader in a use case
  */
@@ -3046,7 +3085,7 @@ void TestWeaveTLVReaderInPractice(nlTestSuite *inSuite)
 
     TestNext<TLVReader>(inSuite, reader);
 
-    TestGet<TLVReader, double>(inSuite, reader, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_1, 4000000000ULL), (float) 1.0);
+    TestGet<TLVReader, float>(inSuite, reader, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_1, 4000000000ULL), (float) 1.0);
 }
 
 void TestWeaveTLVReader_NextOverContainer_ProcessElement(nlTestSuite *inSuite, TLVReader& reader, void *context)
@@ -3147,6 +3186,8 @@ void CheckWeaveTLVReader(nlTestSuite *inSuite, void *inContext)
 
     TestWeaveTLVReaderErrorHandling(inSuite);
 
+    TestWeaveTLVReaderTruncatedReads(inSuite);
+
     TestWeaveTLVReaderInPractice(inSuite);
 
     TestWeaveTLVReader_NextOverContainer(inSuite);