dashboard: omit the website repo on plan9

The plan9 builders have relatively slow filesystems, and the website
repo is relatively filesystem-intensive — many of its tests walk and
process large trees of website content.

Instead of trying to precisely tune the x/website tests to pass on the
plan9 builders, let's skip this repo on them.

For golang/go#49338.

Change-Id: Ieaf50fb6955a6b39012a23ff72e61c1c31e4ce7c
Reviewed-on: https://go-review.googlesource.com/c/build/+/362975
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2010,7 +2010,7 @@ func init() {
 			}
 			return run
 		},
-		buildsRepo: onlyMasterDefault,
+		buildsRepo: plan9Default,
 	})
 	addBuilder(BuildConfig{
 		Name:           "windows-amd64-2008",
@@ -2489,7 +2489,7 @@ func init() {
 		Name:           "plan9-arm",
 		HostType:       "host-plan9-arm-0intro",
 		distTestAdjust: noTestDirAndNoReboot,
-		buildsRepo:     onlyMasterDefault,
+		buildsRepo:     plan9Default,
 	})
 	addBuilder(BuildConfig{
 		Name:     "plan9-amd64-0intro",
@@ -2503,7 +2503,7 @@ func init() {
 			}
 			return run
 		},
-		buildsRepo: onlyMasterDefault,
+		buildsRepo: plan9Default,
 	})
 	addBuilder(BuildConfig{
 		Name:     "plan9-386-0intro",
@@ -2517,7 +2517,7 @@ func init() {
 			}
 			return run
 		},
-		buildsRepo: onlyMasterDefault,
+		buildsRepo: plan9Default,
 	})
 	addBuilder(BuildConfig{
 		Name:             "aix-ppc64",
@@ -2742,6 +2742,19 @@ func onlyMasterDefault(repo, branch, goBranch string) bool {
 	return branch == "master" && goBranch == "master" && buildRepoByDefault(repo)
 }
 
+// plan9Default is like onlyMasterDefault, but also omits repos that are
+// both filesystem-intensive and unlikely to be relevant to plan9 users.
+func plan9Default(repo, branch, goBranch string) bool {
+	switch repo {
+	case "website":
+		// The x/website tests read and check the website code snippets,
+		// which require many filesystem walk and read operations.
+		return false
+	default:
+		return onlyMasterDefault(repo, branch, goBranch)
+	}
+}
+
 // disabledBuilder is a buildsRepo policy function that always return false.
 func disabledBuilder(repo, branch, goBranch string) bool { return false }