Sealevel/Solana relayer produces confusing error when signer key is missing

[danwt]

Summary

When running the relayer for Sealevel/Solana chains without a signer key configured (e.g., missing --chains.<chain>.signer.key flag), the error message is extremely misleading:

ERROR hyperlane_sealevel::mailbox: error: No return data from InboxGetRecipientIsm instruction
    at chains/hyperlane-sealevel/src/mailbox.rs:447
    in hyperlane_sealevel::mailbox::recipient_ism with recipient: 0x...
    in relayer::msg::message_processor::prepare_classic_task with domain: solanatestnet
WARN relayer::msg::pending_message: Repreparing message: Error fetching ISM address, error: ContractError(StringError("No return data from InboxGetRecipientIsm instruction"))

This error gives no indication that the actual problem is a missing signer key. We spent many hours debugging this before realizing the root cause was simply a missing --chains.solanatestnet.signer.key flag.

Root Cause

The issue is in the code flow:

1. build_mailbox() in hyperlane-base/src/settings/chains.rs calls sealevel_signer() which returns Option<Keypair>
2. The optional keypair is passed to SealevelMailbox::new() as Option<SealevelKeypair>
3. The mailbox is created successfully with payer: None
4. Later, when relaying a message, methods like get_account_metas(), simulate_instruction(), and get_recipient_ism() are called
5. These methods require a payer, and when None, they fail with ChainCommunicationError::SignerUnavailable
6. However, the simulation failure cascades into "No return data from InboxGetRecipientIsm instruction" which is completely opaque

Expected Behavior

The relayer should fail immediately at startup with a clear error message like:

Error: Sealevel chain 'solanatestnet' requires a signer key. 
Please configure: --chains.solanatestnet.signer.key <hex_private_key> or --defaultSigner.key <hex_private_key>

Proposed Fix

Add validation in build_mailbox() to require a signer for Sealevel chains:

ChainConnectionConf::Sealevel(conf) => {
    let keypair = self.sealevel_signer().await.context(ctx)?;
    
    // Sealevel requires a signer for mailbox operations
    let keypair = keypair.ok_or_else(|| {
        eyre!(
            "Sealevel chain '{}' requires a signer key. \
             Please configure: --chains.{}.signer.key <hex_private_key> \
             or --defaultSigner.key <hex_private_key>",
            self.domain.name(),
            self.domain.name()
        )
    })?;
    
    // ... rest of mailbox construction
}

This is consistent with how Fuel chains handle the signer requirement (see fuel_signer() which explicitly requires a signer and fails fast).

Impact

• Severity: Medium (causes significant debugging time waste)
• Affected chains: All Sealevel/Solana chains
• Affected operations: Relaying messages (mailbox.process)

Environment

• Rust agent
• Sealevel/Solana chains