spacedriveapp/spacedrive · error · anyhow::Error

Failed to write length prefix: {}

Error message

Failed to write length prefix: {}

What it means

Writing the 4-byte big-endian length prefix required by the remote multiplexer failed on the unidirectional stream. As with all stream writes here, the error reflects connection state - connection lost or the peer reset the stream - rather than local I/O.

Source

Thrown at core/src/service/network/transports/sync.rs:133

					node_id,
					conn: new_conn.clone(),
				});
			}

			new_conn
		};

		// Open a unidirectional stream and send the message
		let mut send = conn
			.open_uni()
			.await
			.map_err(|e| anyhow::anyhow!("Failed to open stream: {}", e))?;

		// Write length prefix (required by multiplexer)
		let len = bytes.len() as u32;
		send.write_all(&len.to_be_bytes())
			.await
			.map_err(|e| anyhow::anyhow!("Failed to write length prefix: {}", e))?;

		// Write message bytes
		send.write_all(&bytes).await.map_err(|e| {
			warn!(
				device_uuid = %target_device,
				error = %e,
				"Failed to write sync message to stream"
			);
			anyhow::anyhow!("Failed to write message: {}", e)
		})?;

		send.finish()
			.map_err(|e| anyhow::anyhow!("Failed to finish stream: {}", e))?;

		tracing::info!(
			"Sync message sent successfully to device {} ({} bytes via uni stream)",
			target_device,
			bytes.len()

View on GitHub (pinned to 6dfeccf211)

Solutions

1. Retry the send on a new connection, evicting any cached entry first
2. Check the cached connection's close_reason before reuse
3. Inspect peer logs for stream reset reasons if persistent

Err(e) if e.to_string().contains("Failed to write length prefix") => {
    // connection lost at stream start: evict cache entry, reconnect, resend once
    active_connections.write().await.remove(&cache_key);
    retry_send_on_fresh_connection(device, node_id, bytes).await
}

When it happens

Trigger: Cached or fresh connection dropping right at stream open; peer resetting uni streams it cannot match to a protocol; mobile peer suspended mid-send.

Common situations: Flaky links; peer standby; peer restart with the same identity invalidating the cached connection.

Related errors

• Failed to open bidirectional stream: {}
• Failed to send length: {}
• Failed to send request: {}
• Failed to write message: {}
• Failed to finish stream: {}

AI-assisted analysis of spacedriveapp/spacedrive@6dfeccf211 (2026-08-16). Data as JSON: /api/errors/aff81cfdafe11a96. Report an issue: GitHub.