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+# Proposal: Extended backwards compatibility for Go
+
+Russ Cox \
+December 2022
+
+Earlier discussion at https://go.dev/issue/55090.
+
+Proposal at https://golang.org/issue/NNNNN.
+
+## Abstract
+
+Go's emphasis on backwards compatibility is one of its key strengths.
+There are, however, times when we cannot maintain strict compatibility,
+such as when changing sort algorithms or fixing clear bugs,
+when existing code depends on the old algorithm or the buggy behavior.
+This proposal aims to address many such situations by keeping older Go programs
+executing the same way even when built with newer Go distributions.
+
+## Background
+
+This proposal is about backward compatibility, meaning
+**new versions of Go compiling older Go code**.
+Old versions of Go compiling newer Go code is a separate problem,
+with a different solution.
+There is not a proposal yet.
+For now, see
+[the discussion about forward compatibility](https://github.com/golang/go/discussions/55092).
+
+Go 1 introduced Go's [compatibility promise](https://go.dev/doc/go1compat),
+which says that old programs will by and large continue to run correctly in new versions of Go.
+There is an exception for security problems and certain other implementation overfitting.
+For example, code that depends on a given type _not_ implementing a particular interface
+may change behavior when the type adds a new method, which we are allowed to do.
+
+We now have about ten years of experience with Go 1 compatibility.
+In general it works very well for the Go team and for developers.
+However, there are also practices we've developed since then
+that it doesn't capture (specifically GODEBUG settings),
+and there are still times when developers' programs break.
+I think it is worth extending our approach to try to break programs even less often,
+as well as to explicitly codify GODEBUG settings
+and clarify when they are and are not appropriate.
+
+As background, I've been talking to the Kubernetes team
+about their experiences with Go.
+It turns out that Go's been averaging about one Kubernetes-breaking
+change per year for the past few years.
+I don't think Kubernetes is an outlier here:
+I expect most large projects have similar experiences.
+Once per year is not high, but it's not zero either,
+and our goal with Go 1 compatibility is zero.
+
+Here are some examples of Kubernetes-breaking changes that we've made:
+
+ - [Go 1.17 changed net.ParseIP](https://go.dev/doc/go1.17#net)
+   to reject addresses with leading zeros, like 0127.0000.0000.0001.
+   Go interpreted them as decimal, following some RFCs,
+   while all BSD-derived systems interpret them as octal.
+   Rejecting them avoids taking part in parser misalignment bugs.
+   (Here is an [arguably exaggerated security report](https://github.com/sickcodes/security/blob/master/advisories/SICK-2021-016.md).)
+
+   Kubernetes clusters may have stored configs using such addresses,
+   so this bug [required them to make a copy of the parsers](https://github.com/kubernetes/kubernetes/issues/100895)
+   in order to keep accessing old data.
+   In the interim, they were blocked from updating to Go 1.17.
+
+ - [Go 1.15 changed crypto/x509](https://go.dev/doc/go1.15#commonname)
+   not to fall back to a certificate's CN field to find a host name when the SAN field was omitted.
+   The old behavior was preserved when using `GODEBUG=x509ignoreCN=0`.
+   [Go 1.17 removed support for that setting](https://go.dev/doc/go1.17#crypto/x509).
+
+   The Go 1.15 change [broke a Kubernetes test](https://github.com/kubernetes/kubernetes/pull/93426)
+   and [required a warning to users in Kubernetes 1.19 release notes](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.19.md#api-change-4).
+
+    The [Kubernetes 1.23 release notes](https://github.com/kubernetes/kubernetes/blob/776cff391524478b61212dbb6ea48c58ab4359e1/CHANGELOG/CHANGELOG-1.23.md#no-really-you-must-read-this-before-you-upgrade)
+    warned users who were using the GODEBUG override that it was gone.
+
+ - [Go 1.18 dropped support for SHA1 certificates](https://go.dev/doc/go1.18#sha1),
+   with a `GODEBUG=x509sha1=1` override.
+   We announced removal of that setting for Go 1.19
+   but changed plans on request from Kubernetes.
+   SHA1 certificates are apparently still used by some enterprise CAs
+   for on-prem Kubernetes installations.
+
+ - [Go 1.19 changed LookPath behavior](https://go.dev/doc/go1.19#os-exec-path)
+   to remove an important class of security bugs,
+   but the change may also break existing programs,
+   so we included a `GODEBUG=execerrdot=0` override.
+
+   The impact of this change on Kubernetes is still uncertain:
+   the Kubernetes developers flagged it as risky enough to warrant further investigation.
+
+These kinds of behavioral changes don't only cause pain for Kubernetes developers and users.
+They also make it impossible to update older, long-term-supported versions
+of Kubernetes to a newer version of Go.
+Those older versions don't have the same access to performance improvements and bug fixes.
+Again, this is not specific to Kubernetes.
+I am sure lots of projects are in similar situations.
+
+As the examples show, over time we've adopted a practice
+of being able to opt out of these risky changes using `GODEBUG` settings.
+The examples also show that we have probably been too aggressive
+about removing those settings.
+But the settings themselves have clearly become an important part of Go's compatibility story.
+
+Other important compatibility-related GODEBUG settings include:
+
+ - `GODEBUG=asyncpreemptoff=1` disables signal-based goroutine preemption, which occasionally uncovers operating system bugs.
+ - `GODEBUG=cgocheck=0` disables the runtime's cgo pointer checks.
+ - `GODEBUG=cpu.<extension>=off` disables use of a particular CPU extension at run time.
+ - `GODEBUG=http2client=0` disables client-side HTTP/2.
+ - `GODEBUG=http2server=0` disables server-side HTTP/2.
+ - `GODEBUG=netdns=cgo` forces use of the cgo resolver.
+ - `GODEBUG=netdns=go` forces use of the Go DNS resolver
+
+Programs that need one to use these can usually set
+the GODEBUG variable in `func init` of package main,
+but for runtime variables, that's too late:
+the runtime reads the variable early in Go program startup,
+before any of the user program has run yet.
+For those programs, the environment variable must be set in the execution environment.
+It cannot be “carried with” the program.
+
+Another problem with the GODEBUGs is that you have to know they exist.
+If you have a large system written for Go 1.17 and want to update to Go 1.18's toolchain,
+you need to know which settings to flip to keep as close to Go 1.17 semantics as possible.
+
+I believe that we should make it even easier and safer
+for large projects like Kubernetes to update to new Go releases.
+
+See also my [talk on this topic at GopherCon](https://www.youtube.com/watch?v=v24wrd3RwGo).
+
+## Proposal
+
+I propose that we formalize and expand our use of GODEBUG to provide
+compatibility beyond what is guaranteed by the current
+[compatibility guidelines](https://go.dev/doc/go1compat).
+
+Specifically, I propose that we:
+
+1. Commit to always adding a GODEBUG setting for changes
+   allowed by the compatibility guidelines but that
+   nonetheless are likely to break a significant number of real programs.
+
+2. Guarantee that GODEBUG settings last for at least 2 years (4 releases).
+   That is only a minimum; some, like `http2server`, will likely last forever.
+
+3. Provide a runtime/metrics counter `/godebug/non-default-behavior/<name>:events`
+   to observe non-default-behavior due to GODEBUG settings.
+
+4. Set the default GODEBUG settings based on the `go` line the main module's go.mod,
+   so that updating to a new Go toolchain with an unmodified go.mod
+   mimics the older release.
+
+5. Allow overriding specific default GODEBUG settings in the source code for package main
+   using one or more lines of the form
+
+       //go:debug <name>=<value>
+
+   The GODEBUG environment variable set when a programs runs
+   would continue to override both these lines
+   and the default inferred from the go.mod `go` line.
+   An unrecognized //go:debug setting is a build error.
+   It is also an error
+
+6. Adjust the `go/build` API to report these new `//go:debug` lines. Specifically, add this type:
+
+       type Comment struct {
+           Pos token.Position
+           Text string
+       }
+
+   and then in type `Package` we would add a new field
+
+       Directives []Comment
+
+   This field would collect all `//go:*` directives before the package line, not just `//go:debug`,
+   in the hopes of supporting any future need for directives.
+
+7. Adjust `go list` output to have a new field `DefaultGODEBUG string` set for main packages,
+   reporting the combination of the go.mod-based defaults and the source code overrides,
+   as well as adding to `Package` new fields `Directives`, `TestDirectives,` and `XTestDirectives`, all of type `[]string`.
+
+8. Document these commitments as well as how to use GODEBUG in
+   the [compatibility guidelines](https://golang.org/doc/go1compat).
+
+## Rationale
+
+The main alternate approach is to keep on doing what we are doing,
+without these additions.
+That makes it difficult for Kubernetes and other large projects
+to update in a timely fashion, which cuts them off from performance improvements
+and eventually security fixes.
+An alternative way to provide these improvements and fixes would be to
+extend Go's release support window to two or more years,
+but that would require significantly more work
+and would be a serious drag on the Go project overall.
+It is better to focus our energy as well as the energy of Go developers
+on the latest release.
+Making it safer to update to the latest release does just that.
+
+The rest of this section gives the affirmative case for each of the enumerated items
+in the previous section.
+
+1. Building on the rest of the compatibility guidelines, this commitment will
+   give developers added confidence that they can update to a new Go toolchain
+   safely with minimal disruption to their programs.
+
+2. In the past we have planned to remove a GODEBUG after only a single release.
+   A single release cycle - six months - may well be too short for some developers,
+   especially where the GODEBUGs are adjusting settings that affect external
+   systems, like which protocols are used. For example, Go 1.14 (Feb 2020) removed
+   NPN support in crypto/tls,
+   but we patched it back into Google's internal Go toolchain
+   for almost three years while we waited for updates to
+   network devices that used NPN.
+   Today that would probably be a GODEBUG setting, and it would be
+   an example of something that takes a large company more than
+   six months to resolve.
+
+3. When a developer is using a GODEBUG override, they need to be able to find out
+   whether it is safe to remove the override. Obviously testing is a good first step,
+   but production metrics can confirm what testing seems to show.
+   If the production systems are reporting zeros for `/godebug/non-default-behavior/<name>`,
+   that is strong evidence for the safety of removing that override.
+
+4. Having the GODEBUG settings is not enough. Developers need to be able to determine
+   which ones to use when updating to a new Go toolchain.
+   Instead of forcing developers to look up what is new from one toolchain to the next,
+   setting the default to match the `go` line in `go.mod` keeps the program behavior
+   as close to the old toolchain as possible.
+
+5. When developers do update the `go` line to a new Go version, they may still need to
+   keep a specific GODEBUG set to mimic an older toolchain.
+   There needs to be some way to bake that into the build:
+   it's not okay to make end users set an environment variable to run a program,
+   and setting the variable in main.main or even main's init can be too late.
+   The `//go:debug` lines provide a clear way to set those specific GODEBUGs,
+   presumably alongside comments explaining why they are needed and
+   when they can be removed.
+
+6. This API is needed for the go command and other tools to scan source files
+   and find the new `//go:debug` lines.
+
+7. This provides an easy way for developers to understand which default GODEBUG
+   their programs are compiled with. It will be particularly useful when switching
+   from one `go` line to another.
+
+8. The compatibility documentation should explain all this so developers know about it.
+
+## Compatibility
+
+This entire proposal is about compatibility.
+It does not violate any existing compatibility requirements.
+
+## Implementation
+
+Overall the implementation is fairly short and straightforward.
+Documentation probably outweighs new code.
+Russ Cox, Michael Matloob, and Bryan Millls will do the work.
+
+A complete sketch of the implementation is in
+[CL 453618](https://go.dev/cl/453618),
+[CL 453619](https://go.dev/cl/453619),
+[CL 453603](https://go.dev/cl/453603),
+[CL 453604](https://go.dev/cl/453604), and
+[CL 453605](https://go.dev/cl/453605).
+The sketch does not include tests and documentation.