The engine debug log is game-written and truncated per launch, not an editor+game shared append

The engine debug log (user://logs/godot.log, path from the debug/file_logging/log_path project setting) is written only by the game/play process — the editor process never writes it. Godot gates the RotatedFileLogger on a !editor term ((!log_file.is_empty() || (!project_manager && !editor && GLOBAL_GET("debug/file_logging/enable_file_logging"))), main.cpp, source-verified on 4.2–4.5), so an editor --editor session adds no file logger; only an explicit --log-file on the command line would force one, which normal usage never does. Each game launch truncates godot.log fresh: RotatedFileLogger’s constructor calls rotate_file() unconditionally (core/io/logger.cpp), copying the prior godot.log to a timestamped backup and reopening the base path with FileAccess::WRITE (length 0). Prior sessions live in timestamped siblings; godot.log is always the current session.

An earlier mental model — captured in a now-corrected playtest_log_reader.gd comment — treated the file as “written by both editor + game processes,” an editor+game shared append target. That was factually wrong on every supported version and led two of the four log readers to hardcode the default user://logs/godot.log rather than resolve the configurable setting.

Decision

Model the engine debug log as a game-written, single-session, truncate-on-launch file whose path comes from debug/file_logging/log_path. Concretely:

• Resolve the path in one place. Every log reader resolves debug/file_logging/log_path through the export-clean LogHelpers (configured_log_path() raw / resolve_log_path() globalized) rather than hardcoding the default, so a relocated log path is honored uniformly.
• Treat a shrunken file as rotation — but distinguish the two reader kinds. A tail reader whose byte offset is a genuine high-water read cursor (log_buffer.gd) recognizes a fresh-truncated file when its offset exceeds the file length and resets that cursor to 0. The Mode-A debugger.get_log reader has no such cursor: after truncation the whole godot.log is the current session, so it reads unconditionally from byte 0 and keeps only a boolean “a session has started” flag (the prior snapshotted byte offset was a vestige of the discredited shared-append model, removed in 41o-undecies-bis).
• Do not re-introduce the editor+game shared-append assumption. The editor never writes the file; the buffer/file readers populate only from a running or already-run game (a core reason the 4.5 Logger API — real-time in-memory capture of editor output — matters on 4.5+, where 4.2–4.4 have no such capture).

Full engine analysis (writer guard per version, rotation call sites, verification greps): Insights/engine-log-file-writer-and-rotation.md in the plan repo.

Consequences

• The console source="file" reader and the runtime debugger.get_log file source now honor a relocated log_path; in the default case every reader’s output — including the runtime path metadata and the console hints — stays byte-identical.
• A future engineer reading the readers sees the corrected model at the source and in this record, and will not re-hardcode the default or re-assume a shared append.
• Because godot.log is truncated per launch, a tail reader that carries a high-water offset across a game launch must guard the rotation case, while the Mode-A session reader drops its byte offset entirely and reads from byte 0 (implemented in 41o-undecies-bis).