Allow branches to be excluded in tide queries

In order to support code freeze on a branch, the queries used to gather
pull requests for the tide pool need too allow negative base ref
searches. The status posted by `tide` also expose this behavior on PRs
against those branches that are otherwise good to merge.

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

prow/config/tide.go

@@ -85,6 +85,8 @@ func (t *Tide) MergeMethod(org, repo string) github.PullRequestMergeType {
 type TideQuery struct {
 	Repos []string `json:"repos,omitempty"`
 
+	ExcludedBranches []string `json:"excludedBranches,omitempty"`
+
 	Labels        []string `json:"labels,omitempty"`
 	MissingLabels []string `json:"missingLabels,omitempty"`
 
@@ -96,6 +98,9 @@ func (tq *TideQuery) Query() string {
 	for _, r := range tq.Repos {
 		toks = append(toks, fmt.Sprintf("repo:\"%s\"", r))
 	}
+	for _, b := range tq.ExcludedBranches {
+		toks = append(toks, fmt.Sprintf("-base:\"%s\"", b))
+	}
 	for _, l := range tq.Labels {
 		toks = append(toks, fmt.Sprintf("label:\"%s\"", l))
 	}

prow/tide/tide.go

@@ -192,6 +192,7 @@ func byRepoAndNumber(prs []PullRequest) map[string]PullRequest {
 func requirementDiff(pr *PullRequest, q *config.TideQuery) (string, int) {
 	const maxLabelChars = 50
 	var desc string
+	var diff int
 	// Drops labels if needed to fit the description text area, but keep at least 1.
 	truncate := func(labels []string) []string {
 		i := 1
@@ -246,11 +247,21 @@ func requirementDiff(pr *PullRequest, q *config.TideQuery) (string, int) {
 			desc = fmt.Sprintf(" Should not have %s labels.", strings.Join(trunced, ", "))
 		}
 	}
+	diff = len(missingLabels) + len(presentLabels)
+
+	if desc == "" {
+		for _, excludedBranch := range q.ExcludedBranches {
+			if string(pr.BaseRef.Name) == excludedBranch {
+				desc = fmt.Sprintf(" Merging to branch %s is forbidden.", pr.BaseRef.Name)
+			}
+		}
+		diff = 1
+	}
 
 	// TODO(cjwagner): List reviews (states:[APPROVED], first: 1) as part of open
 	// PR query and include status context description.
 
-	return desc, len(missingLabels) + len(presentLabels)
+	return desc, diff
 }
 
 // Returns expected status state and description.

prow/tide/tide_test.go

@@ -386,8 +386,9 @@ func TestExpectedStatus(t *testing.T) {
 	testcases := []struct {
 		name string
 
-		labels []string
-		inPool bool
+		baseref string
+		labels  []string
+		inPool  bool
 
 		state string
 		desc  string
@@ -438,6 +439,15 @@ func TestExpectedStatus(t *testing.T) {
 			state: github.StatusPending,
 			desc:  fmt.Sprintf(statusNotInPool, " Needs need-1 label."),
 		},
+		{
+			name:    "against excluded branch",
+			baseref: "bad",
+			labels:  neededLabels,
+			inPool:  false,
+
+			state: github.StatusPending,
+			desc:  fmt.Sprintf(statusNotInPool, " Merging to branch bad is forbidden."),
+		},
 		{
 			name:   "unknown requirement",
 			labels: neededLabels,
@@ -458,8 +468,9 @@ func TestExpectedStatus(t *testing.T) {
 	queriesByRepo := map[string]config.TideQueries{
 		"": {
 			config.TideQuery{
-				Labels:        neededLabels,
-				MissingLabels: forbiddenLabels,
+				ExcludedBranches: []string{"bad"},
+				Labels:           neededLabels,
+				MissingLabels:    forbiddenLabels,
 			},
 			config.TideQuery{
 				Labels: []string{"1", "2", "3", "4", "5", "6", "7"}, // lots of requirements
@@ -470,6 +481,12 @@ func TestExpectedStatus(t *testing.T) {
 	for _, tc := range testcases {
 		t.Logf("Test Case: %q\n", tc.name)
 		var pr PullRequest
+		pr.BaseRef = struct {
+			Name   githubql.String
+			Prefix githubql.String
+		}{
+			Name: githubql.String(tc.baseref),
+		}
 		for _, label := range tc.labels {
 			pr.Labels.Nodes = append(
 				pr.Labels.Nodes,
@@ -1126,7 +1143,8 @@ func TestServeHTTP(t *testing.T) {
 }
 
 func TestHeadContexts(t *testing.T) {
-	type commitContext struct { // one context per commit for testing
+	type commitContext struct {
+		// one context per commit for testing
 		context string
 		sha     string
 	}