rpc: preserve global xpubs and proprietary fields in joinpsbts #35516

1. 
   thomasbuilds commented at 7:12 PM on June 11, 2026: contributor

   joinpsbts collects the global xpubs of all the joined PSBTs into merged_psbt, but returns a separately constructed shuffled_psbt into which only the inputs, outputs, and unknown fields are copied. The collected PSBT_GLOBAL_XPUB records are silently dropped, and PSBT_GLOBAL_PROPRIETARY records are not collected at all.

   The xpub collection was added in #17034, which was written against a joinpsbts that still returned merged_psbt, but was merged after #16512 had introduced the shuffled_psbt rebuild, so the collected xpubs have never reached the result.

   Shuffle the inputs and outputs of merged_psbt in place instead of rebuilding a new PSBT, so that all global data is preserved, and union the global proprietary records in the merge loop, matching the combinepsbt behavior from #34893.

2. rpc: preserve global xpubs and proprietary fields in joinpsbts

   joinpsbts collects the global xpubs of all the joined PSBTs into
   merged_psbt, but returns a separately constructed shuffled_psbt into
   which only the inputs, outputs, and unknown fields are copied. The
   collected PSBT_GLOBAL_XPUB records are silently dropped, and
   PSBT_GLOBAL_PROPRIETARY records are not collected at all.
   
   The xpub collection was added in #17034, which was written against a
   joinpsbts that still returned merged_psbt, but was merged after #16512
   had introduced the shuffled_psbt rebuild, so the collected xpubs have
   never reached the result.
   
   Shuffle the inputs and outputs of merged_psbt in place instead of
   rebuilding a new PSBT, so that all global data is preserved, and union
   the global proprietary records in the merge loop, matching the
   combinepsbt behavior from #34893.

   31f23167b3
3. test: check joinpsbts preserves global xpubs and proprietary fields faa2500a88
4. DrahtBot added the label RPC/REST/ZMQ on Jun 11, 2026
5. 
   DrahtBot commented at 7:12 PM on June 11, 2026: contributor

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   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35516.

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   Conflicts

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6. 
   fanquake commented at 12:35 PM on June 12, 2026: member

   cc @achow101 @w0xlt @theStack

7. 
   achow101 commented at 9:49 PM on June 12, 2026: member

   Concept ACK

8. 
   g-maxxx commented at 7:15 PM on July 4, 2026: none

   ACK faa2500

9. DrahtBot requested review from achow101 on Jul 4, 2026
10. 
    thomasbuilds commented at 7:57 PM on July 6, 2026: contributor

    #35665 should land first. Merged alone, this PR would make the duplicate global xpub issue fixed there reachable through joinpsbts: the xpubs are still merged origin-by-origin, so joining PSBTs that provide different origins for the same xpub would serialize duplicate PSBT_GLOBAL_XPUB keys that no RPC can parse. I will rebase on top once it's in.

Review Requested
achow101