[meshcop] support a zero-length network name for backwards compatibility (#8241)

Add a configuration option to support a zero-length network name
within the OT stack.

src/core/api/thread_api.cpp

@@ -202,6 +202,12 @@ otError otThreadSetNetworkName(otInstance *aInstance, const char *aNetworkName)
 
     VerifyOrExit(AsCoreType(aInstance).Get<Mle::MleRouter>().IsDisabled(), error = kErrorInvalidState);
 
+#if !OPENTHREAD_CONFIG_ALLOW_EMPTY_NETWORK_NAME
+    // Thread interfaces support a zero length name internally for backwards compatibility, but new names
+    // must be at least one valid character long.
+    VerifyOrExit(nullptr != aNetworkName && aNetworkName[0] != '\0', error = kErrorInvalidArgs);
+#endif
+
     error = AsCoreType(aInstance).Get<MeshCoP::NetworkNameManager>().SetNetworkName(aNetworkName);
     AsCoreType(aInstance).Get<MeshCoP::ActiveDatasetManager>().Clear();
     AsCoreType(aInstance).Get<MeshCoP::PendingDatasetManager>().Clear();

src/core/config/misc.h

@@ -631,4 +631,14 @@
 #define OPENTHREAD_CONFIG_NEIGHBOR_DISCOVERY_AGENT_ENABLE 0
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_ALLOW_EMPTY_NETWORK_NAME
+ *
+ * Define as 1 to enable support for an empty network name (zero-length: "")
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_ALLOW_EMPTY_NETWORK_NAME
+#define OPENTHREAD_CONFIG_ALLOW_EMPTY_NETWORK_NAME 0
+#endif
+
 #endif // CONFIG_MISC_H_

src/core/meshcop/meshcop_tlvs.cpp

@@ -138,7 +138,7 @@ void NetworkNameTlv::SetNetworkName(const NameData &aNameData)
 
 bool NetworkNameTlv::IsValid(void) const
 {
-    return GetLength() >= 1 && IsValidUtf8String(mNetworkName, GetLength());
+    return IsValidUtf8String(mNetworkName, GetLength());
 }
 
 void SteeringDataTlv::CopyTo(SteeringData &aSteeringData) const

src/core/meshcop/network_name.cpp

@@ -71,6 +71,9 @@ Error NetworkName::Set(const char *aNameString)
     // chars. The `+ 1` ensures that a `aNameString` with length
     // longer than `kMaxSize` is correctly rejected (returning error
     // `kErrorInvalidArgs`).
+    // Additionally, no minimum length is verified in order to ensure
+    // backwards compatibility with previous versions that allowed
+    // a zero-length name.
 
     Error    error;
     NameData data(aNameString, kMaxSize + 1);
@@ -89,7 +92,7 @@ Error NetworkName::Set(const NameData &aNameData)
     NameData data   = aNameData;
     uint8_t  newLen = static_cast<uint8_t>(StringLength(data.GetBuffer(), data.GetLength()));
 
-    VerifyOrExit((0 < newLen) && (newLen <= kMaxSize), error = kErrorInvalidArgs);
+    VerifyOrExit(newLen <= kMaxSize, error = kErrorInvalidArgs);
 
     data.SetLength(newLen);
 

tests/unit/test_network_name.cpp

@@ -72,7 +72,8 @@ void TestNetworkName(void)
     SuccessOrQuit(networkName.Set(MeshCoP::NameData(kName2, sizeof(kName2))));
     CompareNetworkName(networkName, kName2);
 
-    VerifyOrQuit(networkName.Set(MeshCoP::NameData(kEmptyName, 0)) == kErrorInvalidArgs);
+    SuccessOrQuit(networkName.Set(MeshCoP::NameData(kEmptyName, 0)));
+    CompareNetworkName(networkName, kEmptyName);
 
     SuccessOrQuit(networkName.Set(MeshCoP::NameData(kLongName, sizeof(kLongName))));
     CompareNetworkName(networkName, kLongName);
@@ -80,8 +81,6 @@ void TestNetworkName(void)
     VerifyOrQuit(networkName.Set(MeshCoP::NameData(kLongName, sizeof(kLongName) - 1)) == kErrorAlready,
                  "failed to detect duplicate");
 
-    VerifyOrQuit(networkName.Set(kEmptyName) == kErrorInvalidArgs);
-
     SuccessOrQuit(networkName.Set(MeshCoP::NameData(kName1, sizeof(kName1))));
 
     VerifyOrQuit(networkName.Set(MeshCoP::NameData(kTooLongName, sizeof(kTooLongName))) == kErrorInvalidArgs,