contrib: [tracing] fix pointer argument handling in mempool_monitor.py #33086

1. 
   deadmanoz commented at 8:39 AM on July 29, 2025: contributor

   The BPF code was incorrectly passing pointer variables by value to bpf_usdt_readarg(), causing the function to fail silently and resulting in transaction hashes showing as zeros and reason strings displaying empty strings.

   This fix adds the missing reference operator (&) when passing pointer variables to bpf_usdt_readarg(), allowing the function to properly write the pointer values and enabling correct display of transaction hashes and removal/rejection reasons.

   Fixes the regression introduced in ec47ba349d where bpf_usdt_readarg_p was replaced with bpf_usdt_readarg but the calling convention wasn't properly updated for pointer arguments.

   Before: "0000000000000000000000000000000000000000000000000000000000000000" tx hashes, and missing reasons (empty strings) for removal.

   <img width="1683" height="1330" alt="Screenshot 2025-07-29 at 4 30 03 PM" src="https://github.com/user-attachments/assets/71ba88be-dbcc-43a6-bfe7-bd49ae082b13" />

   After: tx hashes show, reasons for removal showing.

   <img width="1682" height="1330" alt="Screenshot 2025-07-29 at 4 29 23 PM" src="https://github.com/user-attachments/assets/03738c75-5526-4c1e-82c2-ba100cdf128a" />

2. tracing: fix pointer argument handling in mempool_monitor.py

   The BPF code was incorrectly passing pointer variables by value to
   bpf_usdt_readarg(), causing the function to fail silently and resulting
   in transaction hashes and reason strings displaying as zeros or garbage.
   
   This fix adds the missing reference operator (&) when passing pointer
   variables to bpf_usdt_readarg(), allowing the function to properly
   write the pointer values and enabling correct display of transaction
   hashes and removal/rejection reasons.
   
   Fixes the regression introduced in ec47ba349d where bpf_usdt_readarg_p
   was replaced with bpf_usdt_readarg but the calling convention wasn't
   properly updated for pointer arguments.

   0ce041ea88
3. DrahtBot added the label Scripts and tools on Jul 29, 2025
4. 
   DrahtBot commented at 8:39 AM on July 29, 2025: contributor

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   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33086.

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5. fanquake requested review from 0xB10C on Jul 29, 2025
6. 
   0xB10C commented at 9:26 AM on July 29, 2025: contributor

   tested ACK 0ce041ea88dbea2bdfaf21fd0e60a86012498f0a

   Thanks! I can reproduce the problem and confirm that this fixes it.

7. fanquake added the label Needs backport (29.x) on Jul 29, 2025
8. fanquake merged this on Jul 29, 2025
9. fanquake closed this on Jul 29, 2025

   ---

10. fanquake referenced this in commit 9437415024 on Jul 29, 2025
11. 
    fanquake commented at 9:56 AM on July 29, 2025: member

    Backported to 29.x in #33074.

12. deadmanoz deleted the branch on Jul 29, 2025
13. fanquake removed the label Needs backport (29.x) on Jul 29, 2025

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