lnwire: make htlc_maximum_msat mandatory in channel_update

[erickcestari]

Updates ChannelUpdate1 parsing to make htlc_maximum_msat mandatory, aligning with the updated BOLT specification and fixing a compatibility issue with other Lightning implementations.

The BOLT specification was updated to make the htlc_maximum_msat field mandatory in channel_update messages. The must_be_one bit in message_flags that previously indicated this field's presence is now a constant value that should be ignored by receivers.

During differential fuzzing between LND and C-Lightning, a compatibility issue was discovered:

• LND: Accepts 130-byte channel_update messages (missing htlc_maximum_msat)
• C-Lightning: Rejects these messages as malformed (expecting 138 bytes with mandatory htlc_maximum_msat)

context: lightning/bolts#999

[gemini-code-assist]

Summary of Changes

Hello @erickcestari, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the ChannelUpdate1 message parsing in the lnwire package to align with the latest BOLT specification. The primary change makes the htlc_maximum_msat field mandatory during message decoding. This ensures consistency with the updated protocol and resolves a known compatibility issue with other Lightning implementations, such as C-Lightning, which already enforce the presence of this field.

Highlights

• BOLT Specification Alignment: The ChannelUpdate1 decoding logic has been updated to reflect the latest BOLT specification, which mandates the presence of the htlc_maximum_msat field in channel_update messages.
• Mandatory htlc_maximum_msat: The htlc_maximum_msat field is now unconditionally read during the decoding of ChannelUpdate1 messages, removing the previous conditional check based on MessageFlags.
• Compatibility Fix: This modification addresses a compatibility issue where LND would accept 130-byte channel_update messages (missing htlc_maximum_msat), while C-Lightning would reject them as malformed, expecting the 138-byte message with the mandatory field.

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[gemini-code-assist]

Code Review

This PR updates ChannelUpdate1 to make htlc_maximum_msat mandatory during decoding, which aligns with the latest BOLT specs. The change itself is correct, but as I've noted in my review comment, it's incomplete. Other parts of the file need to be updated to ensure consistency between the encoding and decoding paths, which is critical to prevent bugs.

[Roasbeef]

FWIW re already reject these by default:

lnd/netann/channel_update.go

Lines 279 to 284 in 0c2f045

	// The maxHTLC flag is mandatory.
	if !msg.MessageFlags.HasMaxHtlc() {
	return fmt.Errorf("max htlc flag not set for channel "+
	"update %v", spew.Sdump(msg))
	}

[erickcestari]

Ooops. Closing this pr...