build: detect Chainguard base Python version from apko.json

When the base doesn't set PYTHON_VERSION (Chainguard/Wolfi images), fall back to
reading /etc/apko.json from the top-most layer and parsing the python-X.Y
package, so --python validation works for those bases too. Env still takes
precedence. Adds tests for the apko fallback and env precedence.

Co-authored-by: Jason Hall <imjasonh@users.noreply.github.com>

pymage/README.md

@@ -73,10 +73,12 @@ A floating tag such as `cgr.dev/chainguard/python:latest` works, but be aware:
   keep working (they're matched by `py3` and found via `PYTHONPATH`), but
   version-specific compiled wheels (`cp312`…) break when the interpreter moves.
 
-To prevent silent breakage, pymage reads the base's advertised `PYTHON_VERSION`
-and **fails the build** if it doesn't match `--python`, telling you which
-version to target. (Bases that don't advertise a version can't be validated —
-another reason to pin.)
+To prevent silent breakage, pymage detects the base's Python version and
+**fails the build** if it doesn't match `--python`, telling you which version to
+target. It looks at the `PYTHON_VERSION` env var (official python images) and,
+when that's absent, the `python-X.Y` package in `/etc/apko.json` from the top
+layer (Chainguard/Wolfi images). Bases that expose neither can't be validated —
+another reason to pin.
 
 ### Useful flags
 

pymage/internal/build/build_test.go

@@ -13,11 +13,17 @@ import (
 	"github.com/google/go-containerregistry/pkg/v1/mutate"
 
 	"github.com/imjasonh/terraform-playground/pymage/internal/cache"
+	"github.com/imjasonh/terraform-playground/pymage/internal/ptar"
 	"github.com/imjasonh/terraform-playground/pymage/internal/testwheel"
 	"github.com/imjasonh/terraform-playground/pymage/internal/wheel"
 	"github.com/imjasonh/terraform-playground/pymage/internal/wheelhouse"
 )
 
+func ptarLayer(t *testing.T, name, content string) (v1.Layer, error) {
+	t.Helper()
+	return ptar.Layer([]ptar.File{{Path: name, Data: []byte(content)}})
+}
+
 func layerPaths(t *testing.T, l v1.Layer) map[string]bool {
 	t.Helper()
 	rc, err := l.Uncompressed()
@@ -262,6 +268,39 @@ func TestInterpreterVersion(t *testing.T) {
 	}
 }
 
+func TestInterpreterVersionFromAPKO(t *testing.T) {
+	// A Chainguard-style apko.json (no PYTHON_VERSION env) in the top layer.
+	apko := `{"contents":{"packages":["ca-certificates-bundle=20260413-r0","py3-pip-wheel=26.1.2-r0","python-3.14-base=3.14.5-r2","python-3.14=3.14.5-r2","zlib=1.3.2-r3"]},"entrypoint":{"command":"/usr/bin/python"}}`
+	layer, err := ptarLayer(t, "etc/apko.json", apko)
+	if err != nil {
+		t.Fatal(err)
+	}
+	img, err := mutate.AppendLayers(empty.Image, layer)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	maj, min, ok := InterpreterVersion(img)
+	if !ok || maj != 3 || min != 14 {
+		t.Fatalf("apko fallback: got %d.%d ok=%v, want 3.14 ok=true", maj, min, ok)
+	}
+
+	// The PYTHON_VERSION env, when present, takes precedence over apko.json.
+	cf, err := img.ConfigFile()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cf = cf.DeepCopy()
+	cf.Config.Env = []string{"PYTHON_VERSION=3.12.7"}
+	imgEnv, err := mutate.ConfigFile(img, cf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if maj, min, ok := InterpreterVersion(imgEnv); !ok || maj != 3 || min != 12 {
+		t.Fatalf("env precedence: got %d.%d ok=%v, want 3.12", maj, min, ok)
+	}
+}
+
 func TestSourceLayerIgnore(t *testing.T) {
 	src := t.TempDir()
 	must := func(p, content string) {

pymage/internal/build/resolve.go

@@ -1,8 +1,12 @@
 package build
 
 import (
+	"archive/tar"
 	"context"
+	"encoding/json"
 	"fmt"
+	"io"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -37,12 +41,29 @@ func Base(ctx context.Context, ref string, platform *v1.Platform, kc authn.Keych
 	return img, nil
 }
 
-// InterpreterVersion reports the Python X.Y advertised by the base image via a
-// PYTHON_VERSION env var (set by the official python images and others). When a
-// base does not advertise one, ok is false and the caller cannot validate the
-// interpreter — a reason to pin a known base rather than rely on a floating
-// tag. It reads only the (already-fetched) config, never layer bytes.
+// apkoMaxBytes caps how much of the apko.json file we read.
+const apkoMaxBytes = 4 << 20
+
+// pythonPkgRE matches an apko "python-X.Y" package name (with or without the
+// "-base" suffix), as found in a Chainguard/Wolfi image's apko.json.
+var pythonPkgRE = regexp.MustCompile(`^python-(\d+)\.(\d+)(?:-base)?$`)
+
+// InterpreterVersion reports the Python X.Y the base image provides.
+//
+// It first checks a PYTHON_VERSION env var (set by the official python images),
+// which is free since the config is already fetched. If that is absent it falls
+// back to reading /etc/apko.json from the top-most layer and parsing the
+// "python-X.Y" package (Chainguard/Wolfi images, which don't set the env var);
+// this fetches only that one layer. When neither is present, ok is false and
+// the interpreter cannot be validated — a reason to pin a known base.
 func InterpreterVersion(img v1.Image) (major, minor int, ok bool) {
+	if maj, min, ok := interpreterFromEnv(img); ok {
+		return maj, min, ok
+	}
+	return interpreterFromAPKO(img)
+}
+
+func interpreterFromEnv(img v1.Image) (major, minor int, ok bool) {
 	cf, err := img.ConfigFile()
 	if err != nil || cf == nil {
 		return 0, 0, false
@@ -52,6 +73,74 @@ func InterpreterVersion(img v1.Image) (major, minor int, ok bool) {
 		if !found || k != "PYTHON_VERSION" {
 			continue
 		}
+		return parseMajorMinor(v)
+	}
+	return 0, 0, false
+}
+
+// interpreterFromAPKO reads /etc/apko.json from the top-most layer and extracts
+// the Python version from its package list.
+func interpreterFromAPKO(img v1.Image) (major, minor int, ok bool) {
+	layers, err := img.Layers()
+	if err != nil || len(layers) == 0 {
+		return 0, 0, false
+	}
+	data, found := readFileFromLayer(layers[len(layers)-1], "etc/apko.json")
+	if !found {
+		return 0, 0, false
+	}
+	var doc struct {
+		Contents struct {
+			Packages []string `json:"packages"`
+		} `json:"contents"`
+	}
+	if err := json.Unmarshal(data, &doc); err != nil {
+		return 0, 0, false
+	}
+	for _, pkg := range doc.Contents.Packages {
+		// Entries look like "python-3.14=3.14.5-r2"; take the package name.
+		name := pkg
+		if i := strings.IndexAny(pkg, "=<>~ "); i >= 0 {
+			name = pkg[:i]
+		}
+		if m := pythonPkgRE.FindStringSubmatch(name); m != nil {
+			maj, err1 := strconv.Atoi(m[1])
+			min, err2 := strconv.Atoi(m[2])
+			if err1 == nil && err2 == nil {
+				return maj, min, true
+			}
+		}
+	}
+	return 0, 0, false
+}
+
+// readFileFromLayer returns the contents of name (a slash path without a
+// leading slash) from a layer's uncompressed tar, if present.
+func readFileFromLayer(layer v1.Layer, name string) ([]byte, bool) {
+	rc, err := layer.Uncompressed()
+	if err != nil {
+		return nil, false
+	}
+	defer func() { _ = rc.Close() }()
+	tr := tar.NewReader(rc)
+	for {
+		h, err := tr.Next()
+		if err != nil {
+			return nil, false
+		}
+		entry := strings.TrimPrefix(strings.TrimPrefix(h.Name, "./"), "/")
+		if entry != name || h.Typeflag != tar.TypeReg {
+			continue
+		}
+		data, err := io.ReadAll(io.LimitReader(tr, apkoMaxBytes))
+		if err != nil {
+			return nil, false
+		}
+		return data, true
+	}
+}
+
+func parseMajorMinor(v string) (major, minor int, ok bool) {
 	parts := strings.SplitN(v, ".", 3)
 	if len(parts) < 2 {
 		return 0, 0, false
@@ -63,5 +152,3 @@ func InterpreterVersion(img v1.Image) (major, minor int, ok bool) {
 	}
 	return maj, min, true
 }
-	return 0, 0, false
-}