Fix unnecessary recompilation when dbg_callback is modified at runtime #15007
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When :elixir app starts, store the dbg_callback value in dbg_callback_initial. Mix compiler now compares against dbg_callback_initial instead of dbg_callback. This prevents unnecessary recompilation when tools like Kino modify dbg_callback at runtime. Previously, such runtime modifications would trigger a full recompilation of all files using dbg/2, even though the config hadn't actually changed. The key insight is that dbg/2 is a compile-time macro, so runtime modifications to dbg_callback don't affect already-compiled code. Only actual config changes (reflected in dbg_callback_initial) should trigger recompilation. This is a more general solution than detecting specific wrapping patterns, as it works with any tool that modifies dbg_callback at runtime.
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josevalim
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Dec 10, 2025
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Great find @jechol! Really nicely done change too. I have added a comment on where to best store it and we can ship this! |
Per José's feedback, store initial_dbg_callback in elixir_config instead of using application:set_env. This is more consistent with how other Elixir internal configs are managed.
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Done! Updated to use |
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💚 💙 💜 💛 ❤️ |
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#15007) When :elixir app starts, store the dbg_callback value in dbg_callback_initial. Mix compiler now compares against dbg_callback_initial instead of dbg_callback. This prevents unnecessary recompilation when tools like Kino modify dbg_callback at runtime. Previously, such runtime modifications would trigger a full recompilation of all files using dbg/2, even though the config hadn't actually changed. The key insight is that dbg/2 is a compile-time macro, so runtime modifications to dbg_callback don't affect already-compiled code. Only actual config changes (reflected in dbg_callback_initial) should trigger recompilation. This is a more general solution than detecting specific wrapping patterns, as it works with any tool that modifies dbg_callback at runtime.
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Backported it too! |
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Problem
Tools like Kino modify
dbg_callbackat runtime to customizedbg/2behavior. Previously, this runtime modification would trigger unnecessary recompilation on server start, even though the config hadn't actually changed.This is problematic for Phoenix projects with
code_reloader: true.Solution
Store the initial
dbg_callbackvalue ininitial_dbg_callbackwhen the:elixirapp starts. Mix compiler now compares againstinitial_dbg_callbackinstead ofdbg_callback.The key insight is that
dbg/2is a compile-time macro, so runtime modifications todbg_callbackdon't affect already-compiled code. Only actual config changes (reflected ininitial_dbg_callback) should trigger recompilation.Changes
lib/elixir/src/elixir.erl: Storeinitial_dbg_callbackon app startlib/mix/lib/mix/compilers/elixir.ex: Compare againstinitial_dbg_callbackI'm currently using Elixir 1.19.4 with Kino in a Phoenix project and experiencing this issue. A backport to v1.19 would be appreciated.