cmd: addinvoice --qr to print invoice QR

[hieblmi]

Optionally print the QR code of an invoice, added with lncli addinvoice --amt 1010 --qr

[gemini-code-assist]

Summary of Changes

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This pull request enhances the lncli command-line tool by integrating a QR code generation feature for invoices. Users can now opt to display a scannable QR code for their payment requests directly in the terminal, significantly improving the ease of use for mobile wallet integration and streamlining the payment process.

Highlights

• New Feature: QR Code Display: Introduced a new --qr flag to the addinvoice command, allowing users to display a QR code of the generated payment request directly in the terminal.
• Dependency Update: Added a new Go module dependency, github.com/mdp/qrterminal/v3, to facilitate QR code generation and display.
• Documentation: Updated the release notes for version 0.21.0 to reflect the addition of the --qr flag.

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Changelog

• cmd/commands/cmd_invoice.go
  • Added os and qrterminal imports.
  • Introduced a new cli.BoolFlag named qr to the AddInvoiceCommand.
  • Implemented logic to generate and print a half-block QR code of the PaymentRequest to os.Stdout if the qr flag is set.
• docs/release-notes/release-notes-0.21.0.md
  • Added a new entry documenting the --qr flag for the addinvoice command under the 'New Features' section.
• go.mod
  • Added github.com/mdp/qrterminal/v3 and rsc.io/qr as indirect dependencies.
• go.sum
  • Updated checksums for the newly added github.com/mdp/qrterminal/v3 and rsc.io/qr dependencies.

Activity

• No human activity has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request introduces a useful feature to display a QR code for an invoice via the --qr flag in the addinvoice command. The implementation is straightforward and correct. I have one suggestion to add a comment to the new code block to align with the repository's style guide regarding code documentation.

[ziggie1984]

Have you tested this with a bigger invoice including blinded paths ? I wonder what is the information limit for this QR code representation ?

[ziggie1984]

why are these in a separarte required block ?

[hieblmi]

moved them, fixed.

[hieblmi]

Have you tested this with a bigger invoice including blinded paths ? I wonder what is the information limit for this QR code representation ?

You really have to dial down the font size of your terminal to make the QR code fit onto the screen, but it works.

On the limits:

QR code max: 4,296 alphanumeric characters (version 40, error correction L). Since bech32 invoices are alphanumeric, this is the effective limit.                                                                      
                                                                                                                                                                                                                       
  Blinded route size: There's no hard protocol max, but practically:                                                                                                                                                     
  - A standard invoice: ~300-400 chars                                                                                                                                                                                   
  - With 1 blinded path (2-3 hops): ~600-800 chars                                                                                                                                                                       
  - With 3 blinded paths (3-5 hops each): ~1,200-2,000 chars                                                                                                                                                           
  - Extreme cases: could exceed 3,000+ chars         

  So even large blinded invoices should fit within the QR limit. No need to disable --qr for blinded paths.