gopls: clarify policy with respect to support for older Go versions

Clarify that we have best-effort support for the last 4 major Go
versions, and that support for Go releases 3 and 4 versions ago may not
block releases and may be dropped if necessary.

Also explain that Kokoro CI is not automatically re-run when the result
is removed in Gerrit.

For golang/go#45919

Change-Id: Ic1480c9276dad9502aaf885b98bb9445deeed0c5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/316349
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: findleyr

gopls/README.md

@@ -72,11 +72,18 @@ If you are having issues with `gopls`, please follow the steps described in the
 
 ## Supported Go versions and build systems
 
-`gopls` follows the
-[Go Release Policy](https://golang.org/doc/devel/release.html#policy),
-meaning that it officially supports the last 2 major Go releases. Though we
-try not to break older versions, we do not prioritize issues only affecting
-legacy Go releases.
+`gopls` follows the [Go Release
+Policy](https://golang.org/doc/devel/release.html#policy), meaning that it
+officially supports the last 2 major Go releases. Per
+[issue #39146](golang.org/issues/39146), we attempt to maintain best-effort
+support for the last 4 major Go releases, but this support extends only to not
+breaking the build and avoiding easily fixable regressions.
+
+Our extended support is enforced via [continuous integration with older Go
+versions](doc/contributing.md#ci). This legacy Go CI may not block releases:
+test failures may be skipped rather than fixed. Furthermore, if a regression in
+an older Go version causes irreconcilable CI failures, we may drop support for
+that Go version in CI if it is 3 or 4 Go versions old.
 
 `gopls` currently only supports the `go` command, so if you are using a
 different build system, `gopls` will not work well. Bazel support is currently

gopls/doc/contributing.md

@@ -97,7 +97,15 @@ Jenkins-like Google infrastructure for running Dockerized tests. This allows us
 to run gopls tests in various environments that would be difficult to add to
 the TryBots. Notably, Kokoro runs tests on
 [older Go versions](../README.md#supported-go-versions) that are no longer supported
-by the TryBots.
+by the TryBots. Per that that policy, support for these older Go versions is
+best-effort, and test failures may be skipped rather than fixed.
+
+Kokoro runs are triggered by the `Run-TryBot=1` label, just like TryBots, but
+unlike TryBots they do not automatically re-run if the "gopls-CI" result is
+removed in Gerrit. In order to force a new run, you must upload a new patch
+set. (Technically, Googlers can force a new run on an existing patch-set via an
+internal Kokoro dashboard, but unfortunately this ability can't be made more
+generally available).
 
 ## Debugging