Simplify `pkg/dockerfile` interface by ditching pointer

This means slightly more typing in "zero-value" cases (`nil` vs `dockerfile.Metadata{}`), but the tradeoff is that it's simpler to use and reason about (and all the struct members are pointer-type map/slice values anyhow, so copying the struct is still pretty cheap).

This also swaps the scanner error handling to return the partially parsed Metadata object alongside the scanner error -- the error already tells us the object isn't fully complete data, so it's fair/fine to return and will likely just be ignored by the caller instead.  This also allows us to get to 100% code coverage. 👀

This also updates our "treat `oci-import` just like `FROM scratch`" code to *actually* parse `FROM scratch` so we can't accidentally cause "missing data" bugs there in the future, and I implemented that using `sync.OnceValues` which requires upgrading to Go 1.21, but IMO that's a worthwhile tradeoff (because `sync.OnceValues` makes that code so clean/simple).

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20-bullseye AS build
+FROM golang:1.21-bookworm AS build
 
 SHELL ["bash", "-Eeuo", "pipefail", "-xc"]
 

Dockerfile.release

@@ -1,4 +1,4 @@
-FROM golang:1.20-bullseye
+FROM golang:1.21-bookworm
 
 SHELL ["bash", "-Eeuo", "pipefail", "-xc"]
 

Dockerfile.test

@@ -1,4 +1,4 @@
-FROM golang:1.20-bullseye
+FROM golang:1.21-bookworm
 
 SHELL ["bash", "-Eeuo", "pipefail", "-xc"]
 

bashbrew.sh

@@ -6,11 +6,11 @@ set -Eeuo pipefail
 dir="$(readlink -f "$BASH_SOURCE")"
 dir="$(dirname "$dir")"
 
-: "${CGO_ENABLED:=0}"
-export GO111MODULE=on CGO_ENABLED
+: "${CGO_ENABLED=0}" "${GOTOOLCHAIN=local}"
+export CGO_ENABLED GOTOOLCHAIN
 (
 	cd "$dir"
-	go build -o bin/bashbrew ./cmd/bashbrew > /dev/null
+	go build -trimpath -o bin/bashbrew ./cmd/bashbrew > /dev/null
 )
 
 exec "$dir/bin/bashbrew" "$@"

cmd/bashbrew/docker.go

@@ -10,6 +10,7 @@ import (
 	"os/exec"
 	"path"
 	"strings"
+	"sync"
 
 	"github.com/docker-library/bashbrew/manifest"
 	"github.com/docker-library/bashbrew/pkg/dockerfile"
@@ -37,27 +38,25 @@ func (r Repo) ArchDockerFroms(arch string, entry *manifest.Manifest2822Entry) ([
 	return dockerfileMeta.Froms, nil
 }
 
-func (r Repo) dockerfileMetadata(entry *manifest.Manifest2822Entry) (*dockerfile.Metadata, error) {
+func (r Repo) dockerfileMetadata(entry *manifest.Manifest2822Entry) (dockerfile.Metadata, error) {
 	return r.archDockerfileMetadata(arch, entry)
 }
 
-var dockerfileMetadataCache = map[string]*dockerfile.Metadata{}
+var (
+	dockerfileMetadataCache   = map[string]dockerfile.Metadata{}
+	scratchDockerfileMetadata = sync.OnceValues(func() (dockerfile.Metadata, error) {
+		return dockerfile.Parse(`FROM scratch`)
+	})
+)
 
-func (r Repo) archDockerfileMetadata(arch string, entry *manifest.Manifest2822Entry) (*dockerfile.Metadata, error) {
+func (r Repo) archDockerfileMetadata(arch string, entry *manifest.Manifest2822Entry) (dockerfile.Metadata, error) {
 	if builder := entry.ArchBuilder(arch); builder == "oci-import" {
-		return &dockerfile.Metadata{
-			StageFroms: []string{
-				"scratch",
-			},
-			Froms: []string{
-				"scratch",
-			},
-		}, nil
+		return scratchDockerfileMetadata()
 	}
 
 	commit, err := r.fetchGitRepo(arch, entry)
 	if err != nil {
-		return nil, cli.NewMultiError(fmt.Errorf("failed fetching Git repo for arch %q from entry %q", arch, entry.String()), err)
+		return dockerfile.Metadata{}, cli.NewMultiError(fmt.Errorf("failed fetching Git repo for arch %q from entry %q", arch, entry.String()), err)
 	}
 
 	dockerfileFile := path.Join(entry.ArchDirectory(arch), entry.ArchFile(arch))
@@ -72,12 +71,12 @@ func (r Repo) archDockerfileMetadata(arch string, entry *manifest.Manifest2822En
 
 	df, err := gitShow(commit, dockerfileFile)
 	if err != nil {
-		return nil, cli.NewMultiError(fmt.Errorf(`failed "git show" for %q from commit %q`, dockerfileFile, commit), err)
+		return dockerfile.Metadata{}, cli.NewMultiError(fmt.Errorf(`failed "git show" for %q from commit %q`, dockerfileFile, commit), err)
 	}
 
 	meta, err := dockerfile.Parse(df)
 	if err != nil {
-		return nil, cli.NewMultiError(fmt.Errorf(`failed parsing Dockerfile metadata for %q from commit %q`, dockerfileFile, commit), err)
+		return dockerfile.Metadata{}, cli.NewMultiError(fmt.Errorf(`failed parsing Dockerfile metadata for %q from commit %q`, dockerfileFile, commit), err)
 	}
 
 	dockerfileMetadataCache[cacheKey] = meta

go.mod

@@ -1,6 +1,6 @@
 module github.com/docker-library/bashbrew
 
-go 1.20
+go 1.21
 
 require (
 	github.com/containerd/containerd v1.6.19

go.sum

@@ -617,6 +617,7 @@ github.com/prometheus/procfs v0.1.3/go.mod h1:lV6e/gmhEcM9IjHGsFOCxxuZ+z1YqCvr4O
 github.com/prometheus/procfs v0.2.0/go.mod h1:lV6e/gmhEcM9IjHGsFOCxxuZ+z1YqCvr4OA4YeYWdaU=
 github.com/prometheus/procfs v0.6.0/go.mod h1:cz+aTbrPOrUb4q7XlbU9ygM+/jj0fzG6c1xBZuNvfVA=
 github.com/prometheus/procfs v0.7.3 h1:4jVXhlkAyzOScmCkXBTOLRLTz8EeU+eyjrwB/EPq0VU=
+github.com/prometheus/procfs v0.7.3/go.mod h1:cz+aTbrPOrUb4q7XlbU9ygM+/jj0fzG6c1xBZuNvfVA=
 github.com/prometheus/tsdb v0.7.1/go.mod h1:qhTCs0VvXwvX/y3TZrWD7rabWM+ijKTux40TwIPHuXU=
 github.com/rogpeppe/fastuuid v0.0.0-20150106093220-6724a57986af/go.mod h1:XWv6SoW27p1b0cqNHllgS5HIMJraePCO15w5zCzIWYg=
 github.com/rogpeppe/fastuuid v1.2.0/go.mod h1:jVj6XXZzXRy/MSR5jhDC/2q6DgLz+nrA6LYCDYWNEvQ=

pkg/dockerfile/parse.go

@@ -16,12 +16,12 @@ type Metadata struct {
 	Froms []string // every "FROM" or "COPY --from=xxx" value (minus named and/or numbered stages in the case of "--from=")
 }
 
-func Parse(dockerfile string) (*Metadata, error) {
+func Parse(dockerfile string) (Metadata, error) {
 	return ParseReader(strings.NewReader(dockerfile))
 }
 
-func ParseReader(dockerfile io.Reader) (*Metadata, error) {
-	meta := &Metadata{
+func ParseReader(dockerfile io.Reader) (Metadata, error) {
+	meta := Metadata{
 		// panic: assignment to entry in nil map
 		StageNameFroms: map[string]string{},
 		// (nil slices work fine)
@@ -171,10 +171,7 @@ func ParseReader(dockerfile io.Reader) (*Metadata, error) {
 			}
 		}
 	}
-	if err := scanner.Err(); err != nil {
-		return nil, err
-	}
-	return meta, nil
+	return meta, scanner.Err()
 }
 
 func latestizeRepoTag(repoTag string) string {

pkg/dockerfile/parse_test.go

@@ -175,14 +175,9 @@ func TestParse(t *testing.T) {
 			if err != nil {
 				t.Fatal(err)
 			}
-
-			if parsed == nil {
+			if !reflect.DeepEqual(parsed, td.metadata) {
 				t.Fatalf("expected:\n%#v\ngot:\n%#v", td.metadata, parsed)
 			}
-
-			if !reflect.DeepEqual(*parsed, td.metadata) {
-				t.Fatalf("expected:\n%#v\ngot:\n%#v", td.metadata, *parsed)
-			}
 		})
 	}
 }