Doris BE development notes

Compile-time hygiene gates

BE configure runs build-support/check-build-hygiene.sh (option ENABLE_BUILD_HYGIENE, default ON): a set of seconds-level text checks that keep the backend's compile-time invariants from regressing. They exist because include-graph regressions are silent -- the code still compiles, only every build afterwards is slower. The gates make them loud, in the first seconds of configure, with the mechanism and the fix in the message.

What each gate guards, and what to do when it fires:

• Header layering rules (check-header-deps.py). A hub header (exec_env.h, thread_context.h, ...) must not reach a named subsystem, because everything a hub includes is reparsed by the ~1000 TUs behind it. Fix: forward-declare the type in the header and include the real header in the .cpp; if only declarations are needed by many files, route them through a *_fwd.h (see below). A genuinely leaf-like header can be added to the rule's exception set -- in the same PR, with the reasoning in the commit message.
• Third-party bans. A few headers deliberately moved their fmt/boost/ <ranges>-using bodies out of line; the ban keeps the template machinery from coming back. Fix: put the code that needs the library into the matching .cpp.
• Closure / reach budgets. The safety net for edges no rule names: light hubs must stay light (--closure <header> lists what grew), heavy payloads must not spread (--reach <header> ranks the spreading edges). Fix: cut the new edge, or -- when the growth is intended -- bump the number in the budget table in the same PR and say why in the commit message. The diff of the budget table is the review signal; the gate is never a dead end.
• PCH whitelist. pch/pch.h's quoted includes are pinned exactly: every header on the PCH rebuilds the whole backend when touched. Adding one is almost never right; if it is, change the whitelist in the same PR.
• extern template pairing (check-extern-template-pairing.py). Every extern template declaration in a header must have its explicit instantiation definition in a .cpp, and -- within a family that uses externs -- every definition must have its declaration. The missing-extern direction is the one silent case: it compiles and links, every TU just quietly re-instantiates the specialization. Fix: keep both sides in sync, spelling the template arguments the same way.
• Unity-skip coverage (check-unity-skip-coverage.py). A test that #includes a be/src .cpp needs that file opted out of unity batching (doris_skip_unity_inclusion), otherwise the BE UT link fails with duplicate symbols an hour later. The error names the exact entry to add.

The gates are python3 sources and resolve their own interpreter (python3, then python, then /usr/libexec/platform-python, the RHEL8/AlmaLinux8 system Python that the build-env image ships in place of /usr/bin/python3). They deliberately ignore PYTHON, which env.sh exports as the interpreter the build uses for code generation and which is python2 by default. Set BUILD_HYGIENE_PYTHON to pin a specific interpreter.

Escape hatch for emergencies: configure with -DENABLE_BUILD_HYGIENE=OFF. Self-tests live in build-support/tests/ (run.sh).

The *_fwd.h convention

*_fwd.h headers are the sanctioned way through a layering barrier: they carry forward declarations (and lightweight aliases) only, so they cost nothing to include and are exempt from the layering rules by suffix. When a hub header needs a subsystem's type names but not its definitions, put the declarations in <subsystem>_fwd.h (precedent: exec/common/hash_table/phmap_fwd_decl.h) and include the real headers only in .cpp files. Do not put function bodies, class bodies, or includes of non-fwd project headers into a *_fwd.h.