log: check fclose() results and report safely in logging.cpp #33646

1. 
   cedwies commented at 7:06 pm on October 17, 2025: none

   fclose() can report write errors (for example if the disk is full or the filesystem has a problem). Right now, some fclose() calls in src/logging.cpp ignore the return value. This means errors might go unnoticed and log lines could be lost without warning.

   What this PR does:

   • Add a small helper that prints fclose() errors to stderr (with path and errno).
   • In shutdown: close m_fileout safely and report errors.
   • In reopen: open the new file, swap it in, close the old one, and report errors if closing fails.
   • In shrink/rotate: check all fclose(file) calls and report failures.

   No other behavior changes. Normal logging, rotation, and console output remain unchanged.

2. DrahtBot added the label Utils/log/libs on Oct 17, 2025
3. 
   DrahtBot commented at 7:06 pm on October 17, 2025: contributor

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

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33646.

   Reviews

   See the guideline for information on the review process. A summary of reviews will appear here.

   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #29641 (scripted-diff: Use LogInfo over LogPrintf [WIP, NOMERGE, DRAFT] by maflcko)

   If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

4. cedwies closed this on Oct 17, 2025

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5. cedwies reopened this on Oct 17, 2025

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6. cedwies force-pushed on Oct 17, 2025
7. log: check fclose() results and report safely

   Some buffered I/O errors surface only on fclose(). Several fclose() calls in
   src/logging.cpp ignored the return value which can risk silent loss of log lines
   or misleading success during rotate/truncate.
   
   Check return of fclose() and include errno in diagnostics. For the active log sink,
   detach under lock and close outside the lock. On failure, report directly
   to stderr (which avoids recursive logging) and flush. No behavior change on success.

   ddfef829f7
8. cedwies force-pushed on Oct 17, 2025

Contributors
cedwies DrahtBot

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Utils/log/libs