[network-diagnostic] limit number of entries in Child Table TLV (#4163)

This commit fixes Thread Certification Test Case Router_5_7_3 where
Child Table TLV was damaged.

This commit also ensures that only Base TLV format is used.

The bug was introduced introduced in d796b0.

src/core/common/tlvs.hpp

@@ -58,6 +58,15 @@ OT_TOOL_PACKED_BEGIN
 class Tlv
 {
 public:
+    /**
+     * Length values.
+     *
+     */
+    enum
+    {
+        kBaseTlvMaxLength = 254, ///< The maximum length of the Base TLV format.
+    };
+
     /**
      * This method returns the Type value.
      *

src/core/thread/network_diagnostic.cpp

@@ -233,9 +233,13 @@ otError NetworkDiagnostic::AppendChildTable(Message &aMessage)
 
     count = Get<ChildTable>().GetNumChildren(ChildTable::kInStateValid);
 
-    if (count > (CHAR_BIT * sizeof(uint8_t) / sizeof(ChildTableEntry)))
+    // The length of the Child Table TLV may exceed the outgoing link's MTU (1280B).
+    // As a workaround we limit the number of entries in the Child Table TLV,
+    // also to avoid using extended TLV format. The issue is processed by the
+    // Thread Group (SPEC-894).
+    if (count > (Tlv::kBaseTlvMaxLength / sizeof(ChildTableEntry)))
     {
-        count = CHAR_BIT * sizeof(uint8_t) / sizeof(ChildTableEntry);
+        count = Tlv::kBaseTlvMaxLength / sizeof(ChildTableEntry);
     }
 
     tlv.SetLength(static_cast<uint8_t>(count * sizeof(ChildTableEntry)));
@@ -244,6 +248,8 @@ otError NetworkDiagnostic::AppendChildTable(Message &aMessage)
 
     for (ChildTable::Iterator iter(GetInstance(), ChildTable::kInStateValid); !iter.IsDone(); iter++)
     {
+        VerifyOrExit(count--);
+
         Child &child = *iter.GetChild();
 
         timeout = 0;