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go/doc/next/4-runtime.md
Michael Pratt c40a3731f4 internal/godebugs: add decoratemappings as an opaque godebug setting 
This adds a new godebug to control whether the runtime applies the
anonymous memory mapping annotations added in https://go.dev/cl/646095.
It is enabled by default.

This has several effects:

* The feature is only enabled by default when the main go.mod has go >=
  1.25.
* This feature can be disabled with GODEBUG=decoratemappings=0, or the
  equivalents in go.mod or package main. See https://go.dev/doc/godebug.
* As an opaque setting, this option will not appear in runtime/metrics.
* This setting is non-atomic, so it cannot be changed after startup.

I am not 100% sure about my decision for the last two points.

I've made this an opaque setting because it affects every memory mapping
the runtime performs. Thus every mapping would report "non-default
behavior", which doesn't seem useful.

This setting could trivially be atomic and allow changes at run time,
but those changes would only affect future mappings. That seems
confusing and not helpful. On the other hand, going back to annotate or
unannotate every previous mapping when the setting changes is
unwarranted complexity.

For #71546.

Change-Id: I6a6a636c5ad551d76691cba2a6f668d5cff0e352
Reviewed-on: https://go-review.googlesource.com/c/go/+/655895
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
2025-03-10 08:29:59 -07:00

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Runtime

The message printed when a program exits due to an unhandled panic that was recovered and re-raised no longer repeats the text of the panic value.

Previously, a program which panicked with panic("PANIC"), recovered the panic, and then re-panicked with the original value would print:

panic: PANIC [recovered]
  panic: PANIC

This program will now print:

panic: PANIC [recovered, reraised]

On Linux systems with kernel support for anonymous VMA names (CONFIG_ANON_VMA_NAME), the Go runtime will annotate anonymous memory mappings with context about their purpose. e.g., [anon: Go: heap] for heap memory. This can be disabled with the GODEBUG setting decoratemappings=0.