Add ability to have user specify custom upfront shutdown script on channel open

[benthecarman]

This is a nice to have feature so you don't need to do something wacky with your channel manager if you want to open a channel with a specific shutdown script.

[codecov-commenter]

Codecov Report

Patch coverage: 88.17% and project coverage change: -0.05 ⚠️

Comparison is base (bc54441) 91.57% compared to head (b32bd8a) 91.53%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2218      +/-   ##
==========================================
- Coverage   91.57%   91.53%   -0.05%     
==========================================
  Files         104      104              
  Lines       51553    51566      +13     
  Branches    51553    51566      +13     
==========================================
- Hits        47212    47201      -11     
- Misses       4341     4365      +24     

Impacted Files	Coverage Δ
lightning-background-processor/src/lib.rs	83.56% <ø> (ø)
lightning/src/ln/shutdown_tests.rs	96.49% <23.07%> (-1.57%)	⬇️
lightning/src/ln/channel.rs	90.00% <95.45%> (-0.03%)	⬇️
lightning-invoice/src/utils.rs	96.97% <100.00%> (ø)
lightning/src/ln/chanmon_update_fail_tests.rs	98.61% <100.00%> (ø)
lightning/src/ln/channelmanager.rs	88.82% <100.00%> (+<0.01%)	⬆️
lightning/src/ln/functional_test_utils.rs	92.79% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	98.25% <100.00%> (ø)
lightning/src/ln/payment_tests.rs	97.57% <100.00%> (ø)
lightning/src/ln/priv_short_conf_tests.rs	97.59% <100.00%> (ø)
... and 1 more

... and 3 files with indirect coverage changes

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[TheBlueMatt]

This is already done via SignerProvider::get_shutdown_script, but if we want to move it to make it explicit it should be in the handshake config rather than via a new parameter, I think.

[benthecarman]

Similar reasoning here: #2219 (comment)

This PR is less useful I'll admit because we could just set the upfront shutdown option to false and decide the closing address at close time.

[benthecarman]

I thought about putting it in the handshake config, however this is something you can set for the node globally I believe and that could result in a user reusing an address across multiple channels

[wpaulino]

I thought about putting it in the handshake config, however this is something you can set for the node globally I believe and that could result in a user reusing an address across multiple channels

It's currently already reused across multiple channels (which should change with #2174), but you can override the config you use for outbound channels.

[TheBlueMatt]

Can you specify the intended use-case exactly here, at least as far as its different from #2219? I'm missing a bit of why you'd want to hard-code the shutdown script for a specific channel at open, in a way that isn't generic (ie "get an address of mine and use that") but is somehow specific to that channel.

[benthecarman]

Can you specify the intended use-case exactly here, at least as far as its different from #2219? I'm missing a bit of why you'd want to hard-code the shutdown script for a specific channel at open, in a way that isn't generic (ie "get an address of mine and use that") but is somehow specific to that channel.

tbh #2219 is all we really need. With this PR was just trying to cover all the times you could override the shutdown script. If you want to punt on this that is fine, feel free to close

[TheBlueMatt]

Yea, absent a strong motivating use-case I'm inclined to punt on this, I'm not convinced its as useful as 2219 and is just one more place we can override the shutdown script I'd rather skip.