fix: compilation error with modern gcc for leveldb subtree

[knst]

Issue being fixed or feature implemented

Dash Core's build fails with gcc-16:

In copy constructor 'constexpr leveldb::DBImpl::CompactionState::Output::Output(const leveldb::DBImpl::CompactionState::Output&)',
inlined from 'constexpr _Tp* std::construct_at(_Tp*, _Args&& ...) [with _Tp = leveldb::DBImpl::CompactionState::Output; _Args = {const leveldb::DBImpl::CompactionState::Output&}]' at /usr/include/c++/16/bits/stl_construct.h:110:9,
inlined from 'static constexpr void std::allocator_traits<std::allocator<_Up> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = leveldb::DBImpl::CompactionState::Output; _Args = {const leveldb::DBImpl::CompactionState::Output&}; _Tp = leveldb::DBImpl::CompactionState::Output]' at /usr/include/c++/16/bits/alloc_traits.h:716:21,
inlined from 'constexpr void std::vector<_Tp, _Alloc>::_M_realloc_append(_Args&& ...) [with _Args = {const leveldb::DBImpl::CompactionState::Output&}; _Tp = leveldb::DBImpl::CompactionState::Output; _Alloc = std::allocator<leveldb::DBImpl::CompactionState::Output>]' at /usr/include/c++/16/bits/vector.tcc:594:26,
inlined from 'constexpr void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = leveldb::DBImpl::CompactionState::Output; _Alloc = std::allocator<leveldb::DBImpl::CompactionState::Output>]' at /usr/include/c++/16/bits/stl_vector.h:1417:21,
inlined from 'leveldb::Status leveldb::DBImpl::OpenCompactionOutputFile(CompactionState*)' at leveldb/db/db_impl.cc:808:31:
leveldb/db/db_impl.cc:57:10: error: 'out.leveldb::DBImpl::CompactionState::Output::file_size' may be used uninitialized [-Werror=maybe-uninitialized]
57 | struct Output {
| ^~~~~~
leveldb/db/db_impl.cc: In member function 'leveldb::Status leveldb::DBImpl::OpenCompactionOutputFile(CompactionState*)':
leveldb/db/db_impl.cc:804:29: note: 'out' declared here
804 | CompactionState::Output out;

What was done?

Original leveldb's commit is ad9b1c989380538bfb19c5c65df0d3f72d8ed62b Backported to Bitcoin Core by bitcoin#34470 Though, that too many unrelated changes meanwhile. That's temporary fix until leveldb's version won't be properly updated.

How Has This Been Tested?

Together with #7392 I finally compile dash core with gcc 16 again.

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: f8f0c79c-96e4-4069-85fd-96e8e7fe6512

📥 Commits

Reviewing files that changed from the base of the PR and between cdcf00a and 75b19b3.

📒 Files selected for processing (1)

• src/leveldb/db/db_impl.cc

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Walkthrough

In DBImpl::OpenCompactionOutputFile, a single line out.file_size = 0 is added when constructing a new CompactionState::Output entry, explicitly initializing the file_size field to zero instead of leaving it with an indeterminate value.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: a GCC compatibility fix for the LevelDB subtree.
Description check	✅ Passed	The description directly describes the GCC compilation error and the LevelDB fix being applied.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[thepastaclaw]

✅ Review complete (commit 75b19b3)

[thepastaclaw]

Code Review

Verified PR #7393 against the checked-out source. The change is a single initialization of CompactionState::Output::file_size before the Output object is copied into compact->outputs; successful compaction overwrites it before install, and failed compaction paths do not call InstallCompactionResults, so the change fixes the GCC warning without changing persisted LevelDB behavior.

[UdjinM6]

utACK 75b19b3