Describe the bug

When Z_FEATURE_FRAGMENTATION=0 (common on constrained MCUs — fragmentation requires heap allocation), _z_transport_tx_send_fragment() in src/transport/common/tx.c silently drops the message but returns _Z_RES_OK. The return value propagates all the way up through z_put() and z_publisher_put(), so the application sees success while the payload was never transmitted.

This is a silent failure: no error is returned, only an _Z_INFO log line that is invisible at default log levels.

Affected code

src/transport/common/tx.c, the #else branch compiled when Z_FEATURE_FRAGMENTATION=0:

static z_result_t _z_transport_tx_send_fragment(...) {
    _ZP_UNUSED(ztc);
    _ZP_UNUSED(n_msg);
    // ...
    _Z_INFO("Sending the message required fragmentation feature that is deactivated.");
    return _Z_RES_OK;   // payload was dropped — caller is told OK
}

This stub is reached from _z_transport_tx_send_n_msg_inner() whenever _z_network_message_encode() fails to fit the payload in the TX buffer and batch_has_data == false.

To reproduce

1. Build zenoh-pico with Z_FEATURE_FRAGMENTATION=0
2. Call z_put() (or z_publisher_put()) with a payload larger than Z_BATCH_UNICAST_SIZE
3. Observe: return value is 0 (Z_OK) but no message is received on the subscriber

Expected behavior

z_put() should return a non-zero error code (e.g. _Z_ERR_TRANSPORT_NO_SPACE) so the caller can detect and handle the failure.

Proposed fix

Two-line change in the disabled-fragmentation stub:

// Before
_Z_INFO("Sending the message required fragmentation feature that is deactivated.");
return _Z_RES_OK;

// After
_Z_ERROR("Message exceeds transport batch size. Enable Z_FEATURE_FRAGMENTATION to send large payloads.");
return _Z_ERR_TRANSPORT_NO_SPACE;

_Z_ERR_TRANSPORT_NO_SPACE is already defined in result.h (value -98).

System info

Affects all platforms built with Z_FEATURE_FRAGMENTATION=0. Particularly impactful on constrained MCU targets (Arduino, Zephyr small configs, bare-metal) where fragmentation is often disabled to avoid heap dependency.

AI-assisted — authored with Claude, reviewed by Komada.