Fix Clang -Wconditional-uninitialized warning

[hebasto]

Steps to reproduce the warning on Ubuntu 25.10 using Clang 22.0:

1. Apply a diff to workaround a build issue:

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,7 +12,7 @@ set(CMAKE_C_STANDARD_REQUIRED OFF)
 set(CMAKE_C_EXTENSIONS OFF)
 
 # This project requires C++11.
-set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 

2. Configure:

$ cmake -B build -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_FLAGS=-Wconditional-uninitialized -DLEVELDB_BUILD_BENCHMARKS=ON

3. Build:

$ cmake --build build
[23/161] Building CXX object CMakeFiles/leveldb.dir/db/version_set.cc.o
FAILED: CMakeFiles/leveldb.dir/db/version_set.cc.o 
/usr/bin/clang++ -DLEVELDB_COMPILE_LIBRARY -DLEVELDB_PLATFORM_POSIX=1 -I/home/hebasto/dev/leveldb/build/include -I/home/hebasto/dev/leveldb/. -I/home/hebasto/dev/leveldb/include -Wconditional-uninitialized -fno-exceptions -fno-rtti -std=c++20 -Werror -Wthread-safety -MD -MT CMakeFiles/leveldb.dir/db/version_set.cc.o -MF CMakeFiles/leveldb.dir/db/version_set.cc.o.d -o CMakeFiles/leveldb.dir/db/version_set.cc.o -c /home/hebasto/dev/leveldb/db/version_set.cc
/home/hebasto/dev/leveldb/db/version_set.cc:1016:55: error: variable 'manifest_size' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
 1016 |   descriptor_log_ = new log::Writer(descriptor_file_, manifest_size);
      |                                                       ^~~~~~~~~~~~~
/home/hebasto/dev/leveldb/db/version_set.cc:997:25: note: initialize the variable 'manifest_size' to silence this warning
  997 |   uint64_t manifest_size;
      |                         ^
      |                          = 0
1 error generated.
[40/161] Building CXX object CMakeFiles/leveldb.dir/util/env_posix.cc.o
ninja: build stopped: subcommand failed.
hebasto@TS-P340:~/dev/leveldb

This eliminates the need for the following code:

else()
  try_append_cxx_flags("-Wconditional-uninitialized" TARGET nowarn_leveldb_interface SKIP_LINK
    IF_CHECK_PASSED "-Wno-conditional-uninitialized"
  )

[maflcko]

lgtm

[maflcko]

Unrelated: Google is using clang to build their projects, so I wonder why they haven't fixed it yet upstream or if they are also using a workaround 🤔

reuse_logs isn't set by Bitcoin Core, so this is just refactoring dead code?

Seems fine, if the motivation is to remove build code.

[hebasto]

reuse_logs isn't set by Bitcoin Core, so this is just refactoring dead code?

From the build log of the Bitcoin Core patched to enable the warning:

[55/626] Building CXX object src/CMakeFiles/leveldb.dir/leveldb/db/version_set.cc.o
../src/leveldb/db/version_set.cc:1016:55: warning: variable 'manifest_size' may be uninitialized when used here [-Wconditional-uninitialized]
 1016 |   descriptor_log_ = new log::Writer(descriptor_file_, manifest_size);
      |                                                       ^~~~~~~~~~~~~
../src/leveldb/db/version_set.cc:997:25: note: initialize the variable 'manifest_size' to silence this warning
  997 |   uint64_t manifest_size;
      |                         ^
      |                          = 0
1 warning generated.

[fanquake]

Yes the compiler warns, but Core doesn't set reuse_logs (which defaults to false), so the code you are changing here, can never be hit, because we always return early out of ReuseManifest.

[maflcko]

lgtm ACK 0711e6d