kola: support relative indices for --build arg

I often want to target the previous build when e.g. doing `cosa run` or
`kola testiso` to compare between an older and newer build.

Instead of having to copy/paste the previous build ID, let's support
passing negative indices like `-1` for the previous build, `-2` for the
one before that, etc...

This is similar to the `journalctl --boot N` syntax.

Author: jlebon

mantle/cmd/kola/options.go

@@ -67,7 +67,7 @@ func init() {
 	bv(&kola.Options.SSHOnTestFailure, "ssh-on-test-failure", false, "SSH into a machine when tests fail")
 	sv(&kola.Options.Stream, "stream", "", "CoreOS stream ID (e.g. for Fedora CoreOS: stable, testing, next)")
 	sv(&kola.Options.CosaWorkdir, "workdir", "", "coreos-assembler working directory")
-	sv(&kola.Options.CosaBuildId, "build", "", "coreos-assembler build ID")
+	sv(&kola.Options.CosaBuildId, "build", "", "coreos-assembler build ID (or e.g. -1, -2, for previous builds)")
 	sv(&kola.Options.CosaBuildArch, "arch", coreosarch.CurrentRpmArch(), "The target architecture of the build")
 	sv(&kola.Options.AppendButane, "append-butane", "", "Path to Butane config which is merged with test code")
 	sv(&kola.Options.AppendIgnition, "append-ignition", "", "Path to Ignition config which is merged with test code")
@@ -273,6 +273,14 @@ func syncOptionsImpl(useCosa bool) error {
 			kola.Options.CosaWorkdir = "."
 		}
 
+		if strings.HasPrefix(kola.Options.CosaBuildId, "-") {
+			var err error
+			kola.Options.CosaBuildId, err = util.GetRelativeLocalBuildId(kola.Options.CosaWorkdir, kola.Options.CosaBuildId)
+			if err != nil {
+				return err
+			}
+		}
+
 		localbuild, err := util.GetLocalBuild(kola.Options.CosaWorkdir,
 			kola.Options.CosaBuildId,
 			kola.Options.CosaBuildArch)

mantle/util/repo.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 
 	"github.com/pkg/errors"
@@ -82,6 +83,27 @@ func GetLatestLocalBuild(root, arch string) (*LocalBuild, error) {
 	return GetLocalBuild(root, "latest", arch)
 }
 
+func GetRelativeLocalBuildId(root, relid string) (string, error) {
+	// resolve relative builds, e.g. -1, -2
+	relBuild, err := strconv.ParseInt(relid, 10, 0)
+	if err != nil {
+		return "", fmt.Errorf("invalid relative build index %s: %w", relid, err)
+	}
+	if relBuild >= 0 {
+		return "", fmt.Errorf("invalid relative build index %s: not negative", relid)
+	}
+	// we know it'll fit in an int since we used bitSize: 0 above
+	relBuildInt := int(-relBuild)
+	builds, err := cosa.GetBuilds(filepath.Join(root, "builds"))
+	if err != nil {
+		return "", err
+	}
+	if relBuildInt >= len(builds.Builds) {
+		return "", fmt.Errorf("relative build index %d does not exist", -relBuild)
+	}
+	return builds.Builds[relBuildInt].ID, nil
+}
+
 func GetLocalBuild(root, buildid, arch string) (*LocalBuild, error) {
 	if err := RequireCosaRoot(root); err != nil {
 		return nil, err

pkg/builds/build.go

@@ -96,7 +96,7 @@ func ReadBuild(dir, buildID, arch string) (*Build, string, error) {
 	}
 
 	if buildID == "" {
-		b, err := getBuilds(dir)
+		b, err := GetBuilds(dir)
 		if err != nil {
 			return nil, "", err
 		}

pkg/builds/builds.go

@@ -26,15 +26,15 @@ type build struct {
 	Arches []string `json:"arches"`
 }
 
-// buildsJSON represents the JSON that records the builds
+// BuildsJSON represents the JSON that records the builds
 // TODO: this should be generated by a schema
-type buildsJSON struct {
+type BuildsJSON struct {
 	SchemaVersion string  `json:"schema-version"`
 	Builds        []build `json:"builds"`
 	TimeStamp     string  `json:"timestamp"`
 }
 
-func getBuilds(dir string) (*buildsJSON, error) {
+func GetBuilds(dir string) (*BuildsJSON, error) {
 	path := filepath.Join(dir, CosaBuildsJSON)
 	f, err := os.Open(path)
 	if err != nil {
@@ -45,15 +45,15 @@ func getBuilds(dir string) (*buildsJSON, error) {
 	if _, err := io.Copy(bufD, f); err != nil {
 		return nil, err
 	}
-	b := &buildsJSON{}
+	b := &BuildsJSON{}
 	if err := json.Unmarshal(bufD.Bytes(), b); err != nil {
 		return nil, err
 	}
 	return b, nil
 }
 
 // getLatest returns the latest build for the arch.
-func (b *buildsJSON) getLatest(arch string) (string, bool) {
+func (b *BuildsJSON) getLatest(arch string) (string, bool) {
 	for _, b := range b.Builds {
 		for _, a := range b.Arches {
 			if a == arch {

pkg/builds/builds_test.go

@@ -30,7 +30,7 @@ func TestBuildsMeta(t *testing.T) {
 		t.Fatalf("failed to write the test data %v", err)
 	}
 
-	b, err := getBuilds(tmpd)
+	b, err := GetBuilds(tmpd)
 	if err != nil {
 		t.Fatalf("failed to find the builds")
 	}

pkg/builds/schema_test.go

@@ -151,7 +151,7 @@ func TestMergeMeta(t *testing.T) {
 
 	// Create a fake build dir
 	fakeBuildID := "999.1"
-	bjson, _ := json.Marshal(buildsJSON{
+	bjson, _ := json.Marshal(BuildsJSON{
 		SchemaVersion: "0.1.0",
 		Builds: []build{
 			{