fix string corruption in string_impl::shrink_to_fit

[Ramya-9353]

Repro: build a heap-allocated string (size past the SBO), shrink its size to at most the SBO capacity, then call shrink_to_fit(); the contents come back as unrelated bytes. Second case: shrink a heap string that stays above the SBO capacity, then read c_str(), and strlen exceeds size().

Cause: in the heap-to-SBO branch s_.k is set to short_string_ before data() is read, so once the kind has flipped data() returns the inline SBO buffer and the memcpy copies that buffer onto itself rather than the heap characters, leaving stale union bytes (the old table pointer among them). In the heap-to-smaller-heap branch only size() bytes are copied, so the terminator is dropped and c_str() reads into uninitialised storage.

Fix: read the source pointer before switching the kind, and copy size() + 1 bytes on the reallocating branch so the terminator is carried across.

[cppalliance-bot]

An automated preview of the documentation is available at https://1176.json.prtest2.cppalliance.org/libs/json/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-07-21 16:39:00 UTC

[cppalliance-bot]

GCOVR code coverage report https://1176.json.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://1176.json.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff Report https://1176.json.prtest2.cppalliance.org/diff-report/index.html

Build time: 2026-07-21 16:54:17 UTC

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/1176/pullrequest.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (ea90668) to head (dca5076).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1176   +/-   ##
========================================
  Coverage    93.91%   93.91%           
========================================
  Files           91       91           
  Lines         9265     9267    +2     
========================================
+ Hits          8701     8703    +2     
  Misses         564      564           

Files with missing lines	Coverage Δ
include/boost/json/detail/impl/string_impl.ipp	99.11% <100.00%> (+<0.01%)	⬆️

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[grisumbras]

Thank you for catching this serious issue. Merged via #1177.