cmd/coordinator: support skipping redundant tests on try runs

Also, don't start obtaining test sharding buildlets early if they're
reverse buildlets. Reverse buildlets are either immediately available,
or they're not. No point monopolizing them earlier than needed.

Updates golang/go#17104

Change-Id: If5a0bbd0c59b55750adfeeaa8d0f81cdbcc8ad48
Reviewed-on: https://go-review.googlesource.com/29473
Reviewed-by: Matthew Dempsky <[email protected]>

Author: bradfitz

cmd/coordinator/coordinator.go

@@ -1341,7 +1341,7 @@ func (st *buildStatus) expectedBuildletStartDuration() time.Duration {
 // ready, such that they're ready when make.bash is done. But we don't
 // want to start too early, lest we waste idle resources during make.bash.
 func (st *buildStatus) getHelpersReadySoon() {
-	if st.isSubrepo() || st.conf.NumTestHelpers(st.isTry()) == 0 {
+	if st.isSubrepo() || st.conf.NumTestHelpers(st.isTry()) == 0 || st.conf.IsReverse {
 		return
 	}
 	time.AfterFunc(st.expectedMakeBashDuration()-st.expectedBuildletStartDuration(),
@@ -1819,11 +1819,25 @@ func (st *buildStatus) distTestList() (names []string, remoteErr, err error) {
 		return
 	}
 	for _, test := range strings.Fields(buf.String()) {
+		if st.shouldSkipTest(test) {
+			continue
+		}
 		names = append(names, test)
 	}
 	return names, nil, nil
 }
 
+// shouldSkipTest reports whether this test should be skipped.  We
+// only do this for slow builders running redundant tests. (That is,
+// tests which have identical behavior across different ports)
+func (st *buildStatus) shouldSkipTest(testName string) bool {
+	switch testName {
+	case "api":
+		return st.isTry() && st.name != "linux-amd64"
+	}
+	return false
+}
+
 func (st *buildStatus) newTestSet(names []string) *testSet {
 	set := &testSet{
 		st: st,