feat(create): implement --compatibility to fetch supported image list

Ports show_compatibility from bash distrobox-create. Downloads
docs/compatibility.md from upstream and caches the parsed list
in the user cache dir.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: balanza

internal/cli/compatibility.go

@@ -0,0 +1,350 @@
+package cli
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+	"time"
+	"unicode"
+
+	"github.com/89luca89/distrobox/pkg/version"
+)
+
+const (
+	// compatibilityURLTemplate is the upstream URL for docs/compatibility.md.
+	// %s is the git ref (tag or branch) to fetch from.
+	compatibilityURLTemplate = "https://raw.githubusercontent.com/89luca89/distrobox/%s/docs/compatibility.md"
+
+	// compatibilityFetchTimeout caps the time we wait for the HTTP request.
+	compatibilityFetchTimeout = 15 * time.Second
+
+	// devFallbackRef is used in place of the "dev" version (set on local
+	// builds) to fetch the latest upstream compatibility list.
+	devFallbackRef = "main"
+
+	// compatibilityTableStartMarker matches the first row of the
+	// "Containers Distros" table in docs/compatibility.md.
+	compatibilityTableStartMarker = "| Alma"
+
+	// compatibilityTableEndMarker matches the last row of the
+	// "Containers Distros" table in docs/compatibility.md.
+	compatibilityTableEndMarker = "| Void"
+)
+
+// showCompatibility prints the list of container images known to work with
+// distrobox. The list is fetched from the upstream docs/compatibility.md (for
+// the current version) and cached on disk so subsequent invocations are
+// offline-friendly. The provided context is used to cancel the upstream HTTP
+// fetch (e.g., via SIGINT); local cache reads/writes are intentionally
+// uncancellable since they are bounded and very fast.
+//
+// This is the Go port of the bash show_compatibility helper:
+// https://github.com/89luca89/distrobox/blob/main/distrobox-create#L254
+func showCompatibility(ctx context.Context) error {
+	ref := compatibilityRef(version.Version)
+
+	cacheDir, err := compatibilityCacheDir()
+	if err != nil {
+		return fmt.Errorf("resolve cache directory: %w", err)
+	}
+	// Use the sanitized ref in the cache filename so that an unusual
+	// build-time version string (e.g. one containing "/" or "..") can
+	// never escape the cache directory.
+	cachePath := filepath.Join(cacheDir, "distrobox-compatibility-"+sanitizeRefForFilename(ref))
+
+	content, err := readCompatibilityCache(cachePath)
+	if err == nil {
+		fmt.Print(content) //nolint:forbidigo // CLI output by design
+		return nil
+	}
+	if !errors.Is(err, os.ErrNotExist) && !errors.Is(err, errEmptyCache) {
+		return fmt.Errorf("read compatibility cache: %w", err)
+	}
+
+	fetchCtx, cancel := context.WithTimeout(ctx, compatibilityFetchTimeout)
+	defer cancel()
+
+	markdown, err := fetchCompatibilityMarkdown(fetchCtx, http.DefaultClient, ref)
+	if err != nil {
+		return fmt.Errorf("fetch compatibility list: %w", err)
+	}
+
+	images := parseCompatibilityImages(markdown)
+	if len(images) == 0 {
+		return errors.New("parse compatibility list: no images found")
+	}
+
+	rendered := strings.Join(images, "\n") + "\n"
+
+	if err := writeCompatibilityCache(cachePath, rendered); err != nil {
+		// Cache write failure is not fatal: we still got the data and
+		// the user is entitled to see it.
+		fmt.Fprintf(os.Stderr, "warning: could not write compatibility cache to %s: %v\n", cachePath, err)
+	}
+
+	fmt.Print(rendered) //nolint:forbidigo // CLI output by design
+	return nil
+}
+
+// gitDescribeSuffixRE matches the trailing `-N-gHEX` segment that
+// `git describe --tags --always` appends to a tag when HEAD is N commits
+// past the tag (e.g. `v1.8.1-3-g1bc3554`). Stripping it gives us the tag
+// itself, which is a valid GitHub ref upstream.
+var gitDescribeSuffixRE = regexp.MustCompile(`-\d+-g[0-9a-f]{4,40}$`)
+
+// compatibilityRef returns the git ref to fetch docs/compatibility.md from.
+//
+//   - "" and "dev" (local builds without ldflags overrides) fall back to "main".
+//   - A `git describe` output like "v1.8.1-3-g1bc3554" or
+//     "v1.8.1-3-g1bc3554-dirty" is normalized to the nearest tag ("v1.8.1"),
+//     which is a real ref upstream.
+//   - A bare commit-only output like "g1bc3554" (no tag prefix) also falls
+//     back to "main".
+//   - Anything else is used as-is.
+func compatibilityRef(buildVersion string) string {
+	if buildVersion == "" || buildVersion == "dev" {
+		return devFallbackRef
+	}
+
+	// `git describe --dirty` appends "-dirty"; drop it first so the
+	// suffix regex sees the canonical `-N-gHEX` shape.
+	ref := strings.TrimSuffix(buildVersion, "-dirty")
+	ref = gitDescribeSuffixRE.ReplaceAllString(ref, "")
+
+	// `git describe --always` falls back to a bare hash for untagged
+	// repositories. That is not a useful ref for upstream, so fall back
+	// to main.
+	if isGitHashOnly(ref) {
+		return devFallbackRef
+	}
+
+	return ref
+}
+
+// sanitizeRefForFilename returns a version of ref safe to embed in a
+// filename: every character that is not [A-Za-z0-9._-] is replaced with
+// `_`. This prevents a ref containing path separators (e.g.
+// "feature/foo") or dot-segments (e.g. "../etc/passwd") from creating
+// nested paths or escaping the cache directory when concatenated into
+// the cache file name.
+func sanitizeRefForFilename(ref string) string {
+	if ref == "" {
+		return "_"
+	}
+	mapped := strings.Map(func(r rune) rune {
+		switch {
+		case r >= 'a' && r <= 'z',
+			r >= 'A' && r <= 'Z',
+			r >= '0' && r <= '9',
+			r == '.', r == '_', r == '-':
+			return r
+		default:
+			return '_'
+		}
+	}, ref)
+	// A ref of "." or ".." would still be problematic after the per-char
+	// mapping (it stays as-is), so explicitly neutralize it.
+	if mapped == "." || mapped == ".." {
+		return "_"
+	}
+	return mapped
+}
+
+// isGitHashOnly reports whether ref looks like a bare git hash with no
+// surrounding tag information (matches the "g<hex>" or plain "<hex>"
+// shapes that `git describe --always` produces when no tag is reachable).
+func isGitHashOnly(ref string) bool {
+	candidate := strings.TrimPrefix(ref, "g")
+	if len(candidate) < 4 {
+		return false
+	}
+	for _, r := range candidate {
+		if (r < '0' || r > '9') && (r < 'a' || r > 'f') {
+			return false
+		}
+	}
+	return true
+}
+
+// compatibilityCacheDir returns the per-user cache directory used to store
+// the parsed compatibility list. It honours XDG_CACHE_HOME and falls back to
+// $HOME/.cache.
+func compatibilityCacheDir() (string, error) {
+	base := os.Getenv("XDG_CACHE_HOME")
+	if base == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", fmt.Errorf("user home directory: %w", err)
+		}
+		base = filepath.Join(home, ".cache")
+	}
+	return filepath.Join(base, "distrobox"), nil
+}
+
+// errEmptyCache is returned by readCompatibilityCache when the cache file
+// exists but contains no data. The caller treats this the same as a miss.
+var errEmptyCache = errors.New("compatibility cache file is empty")
+
+// readCompatibilityCache returns the cached compatibility list if a non-empty
+// cache file exists. It returns os.ErrNotExist when the cache is absent and
+// errEmptyCache when the file is present but empty.
+func readCompatibilityCache(path string) (string, error) {
+	info, err := os.Stat(path)
+	if err != nil {
+		return "", err //nolint:wrapcheck // sentinel errors are propagated as-is for the caller
+	}
+	if info.Size() == 0 {
+		return "", errEmptyCache
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("read cache file: %w", err)
+	}
+	return string(data), nil
+}
+
+// writeCompatibilityCache atomically writes the parsed compatibility list
+// to the on-disk cache, creating the parent directory if needed. The write
+// goes to a temp file in the same directory and is then renamed into place
+// so that a crashed or concurrent writer can never leave a partially
+// populated cache file visible to readCompatibilityCache.
+func writeCompatibilityCache(path, content string) error {
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o750); err != nil {
+		return fmt.Errorf("create cache directory: %w", err)
+	}
+
+	tmp, err := os.CreateTemp(dir, filepath.Base(path)+".tmp-*")
+	if err != nil {
+		return fmt.Errorf("create temp cache file: %w", err)
+	}
+	tmpPath := tmp.Name()
+	// If anything below fails after the temp file is opened, do our best
+	// to leave no garbage behind.
+	cleanup := func() { _ = os.Remove(tmpPath) }
+
+	if _, err := tmp.WriteString(content); err != nil {
+		_ = tmp.Close()
+		cleanup()
+		return fmt.Errorf("write temp cache file: %w", err)
+	}
+	if err := tmp.Chmod(0o644); err != nil {
+		_ = tmp.Close()
+		cleanup()
+		return fmt.Errorf("chmod temp cache file: %w", err)
+	}
+	if err := tmp.Close(); err != nil {
+		cleanup()
+		return fmt.Errorf("close temp cache file: %w", err)
+	}
+	if err := os.Rename(tmpPath, path); err != nil {
+		cleanup()
+		return fmt.Errorf("rename temp cache file into place: %w", err)
+	}
+	return nil
+}
+
+// fetchCompatibilityMarkdown downloads docs/compatibility.md for the given
+// git ref using the provided HTTP client.
+func fetchCompatibilityMarkdown(ctx context.Context, client *http.Client, ref string) (string, error) {
+	url := fmt.Sprintf(compatibilityURLTemplate, ref)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return "", fmt.Errorf("build request: %w", err)
+	}
+	resp, err := client.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("perform request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("unexpected HTTP status %d fetching %s", resp.StatusCode, url)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", fmt.Errorf("read response body: %w", err)
+	}
+	return string(body), nil
+}
+
+// parseCompatibilityImages extracts the container image references from the
+// "Containers Distros" table in compatibility.md. The third column of each
+// row (image names, optionally separated by "<br>") is harvested between the
+// first line that starts with "| Alma" and the next line that starts with
+// "| Void". Returned slice is sorted and deduplicated, mirroring the
+// `sort -u` behaviour of the original bash implementation.
+func parseCompatibilityImages(markdown string) []string {
+	seen := make(map[string]struct{})
+
+	inTable := false
+	for _, line := range strings.Split(markdown, "\n") {
+		if !inTable {
+			if strings.HasPrefix(line, compatibilityTableStartMarker) {
+				inTable = true
+			} else {
+				continue
+			}
+		}
+
+		for _, image := range extractImagesFromRow(line) {
+			seen[image] = struct{}{}
+		}
+
+		if strings.HasPrefix(line, compatibilityTableEndMarker) {
+			break
+		}
+	}
+
+	images := make([]string, 0, len(seen))
+	for image := range seen {
+		images = append(images, image)
+	}
+	sort.Strings(images)
+	return images
+}
+
+// extractImagesFromRow returns the image references from a single markdown
+// table row. The original bash pipeline cuts on `|`, takes the fourth field
+// (column 3 of the table when counting from 1), splits on `<br>`, strips
+// whitespace and drops empty entries.
+func extractImagesFromRow(line string) []string {
+	fields := strings.Split(line, "|")
+	// `| a | b | c |` splits into ["", " a ", " b ", " c ", ""], so the
+	// fourth field (index 4 with `cut -f 4`, index 3 here) holds the
+	// images column. Anything shorter is not a table row we care about.
+	if len(fields) < 5 {
+		return nil
+	}
+
+	cell := fields[3]
+	var images []string
+	for _, part := range strings.Split(cell, "<br>") {
+		image := strings.TrimSpace(part)
+		// Mirror the bash `tr -d ' '` and the later `sort -u`: also drop
+		// any stray whitespace within the entry (some rows have stray
+		// spaces inside the image name).
+		image = strings.Map(stripWhitespace, image)
+		if image == "" {
+			continue
+		}
+		images = append(images, image)
+	}
+	return images
+}
+
+// stripWhitespace is a strings.Map helper that drops every whitespace rune.
+func stripWhitespace(r rune) rune {
+	if unicode.IsSpace(r) {
+		return -1
+	}
+	return r
+}