Make absl::InitializeLog() safe to call more than once (#1656)

[Othmane-Ch]

Fixes #1656.

Previously, calling absl::InitializeLog() more than once aborted the program: the underlying absl::log_internal::SetTimeZone() fires ABSL_RAW_LOG(FATAL, "...has already been called") on its second invocation. The header documented this as "an error to call this function twice", but in real-world systems multiple independent libraries each want to bring up the logging library and have no way to coordinate among themselves — Google's XLA project being the example cited in #1656.

Change

Wrap the body of InitializeLog() in absl::call_once. Exactly the first invocation runs the underlying initialization; all subsequent calls (including concurrent ones from other threads) become no-ops. The first caller's absl::LocalTimeZone() value wins, matching the documented "only the first call has effect" intent — just made safe instead of UB.

Net diff: +106 / −2 across 5 files.

• absl/log/initialize.cc — absl::call_once guard around InitializeLogImpl(absl::LocalTimeZone()).
• absl/log/initialize.h — docstring updated to document the new contract (safe to call repeatedly; concurrent callers block until the first call completes).
• absl/log/initialize_test.cc — new gtest covering: first call initializes, second call is a no-op, concurrent calls from N threads are safe.
• absl/log/BUILD.bazel, absl/log/CMakeLists.txt — wire the new test target and add the //absl/base (CMake: absl::base) dep to the log_initialize library for call_once.

Testing

Built and ran the new test locally with CMake/Ninja + MSVC 19.36 on Windows:

[==========] 3 tests from InitializeLogTest
[ RUN      ] InitializeLogTest.FirstCallInitializes      OK (0 ms)
[ RUN      ] InitializeLogTest.SecondCallIsNoOp          OK (0 ms)
[ RUN      ] InitializeLogTest.ConcurrentCallsAreSafe    OK (1 ms)
[==========] 3 tests from 1 test suite ran. (2 ms total)
[  PASSED  ] 3 tests.

Sibling regression check: absl_log_globals_test also passes after the BUILD/CMake edits.

Without the call_once guard, ConcurrentCallsAreSafe either races on the initialization globals or aborts via the FATAL raw log inside SetTimeZone(), so it doubles as a regression marker.

Notes

• No public API surface change — same function name, same signature.
• The cost on every InitializeLog() call is one absl::call_once flag check, which after the first call is a single relaxed atomic load on the fast path.
• Out of scope: adding a ShutdownLog() / re-init capability (the issue does not request it)

[google-cla]

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[rmjoona27-create]

- Othmane envió una solicitud de pull para hacer que absl::InitializeLog() se pueda llamar de forma segura más de una vez. - Google-cla[bot] indicó que Othmane debe firmar un Contributor License Agreement (CLA) antes de revisar la solicitud del Ministerio . El El mié, may 6, 2026 a la(s) 15:11, google-cla[bot] < ***@***.***> escribió:

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