Allow branches to be excluded in tide queries

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

@@ -186,6 +186,11 @@ func byRepoAndNumber(prs []PullRequest) map[string]PullRequest {
 // requirementDiff calculates the diff between a PR and a TideQuery.
 // This diff is defined with a string that describes some subset of the
 // differences and an integer counting the total number of differences.
+// The diff count should always reflect the total number of differences between
+// the current state of the PR and the query, but the message returned need not
+// attempt to convey all of that information if some differences are more severe.
+// For instance, we need to convey that a PR is open against a forbidden branch
+// more than we need to detail which status contexts are failed against the PR.
 // Note: an empty diff can be returned if the reason that the PR does not match
 // the TideQuery is unknown. This can happen happen if this function's logic
 // does not match GitHub's and does not indicate that the PR matches the query.
@@ -206,6 +211,13 @@ func requirementDiff(pr *PullRequest, q *config.TideQuery) (string, int) {
 		return labels[:i]
 	}
 
+	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
+		}
+	}
+
 	var missingLabels []string
 	for _, l1 := range q.Labels {
 		var found bool
@@ -220,7 +232,7 @@ func requirementDiff(pr *PullRequest, q *config.TideQuery) (string, int) {
 		}
 	}
 	diff += len(missingLabels)
-	if len(missingLabels) > 0 {
+	if desc == "" && len(missingLabels) > 0 {
 		sort.Strings(missingLabels)
 		trunced := truncate(missingLabels)
 		if len(trunced) == 1 {

prow/tide/tide_test.go

@@ -404,6 +404,7 @@ func TestExpectedStatus(t *testing.T) {
 	testcases := []struct {
 		name string
 
+		baseref  string
 		labels   []string
 		contexts []Context
 		inPool   bool
@@ -457,6 +458,15 @@ 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:     "only failed tide context",
 			labels:   neededLabels,
@@ -508,8 +518,9 @@ 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
@@ -520,6 +531,12 @@ 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,