deb/apk: make local repodata deterministic (#119)

Part of #117. Local generated repodata must be byte-identical across the two no-affinity replicas and across every regeneration, so apt/apk never hit a checksum mismatch between an index's advertised hash and the bytes actually served. This does the deb+apk half (the rpm half landed in #118).

How:
- Derive the deb `Release` `Date:` from the newest persisted `created_at` (RFC1123Z, UTC) instead of `time.Now()`; carry `created_at` through the deb metadata SELECT and `DebMetadata`. An empty repo falls back to the Unix epoch. This also stops `Date:` running ahead of wall clock.
- Pin the apk `APKINDEX` tar header `ModTime` to the Unix epoch instead of the zero-value `time.Time`, so it is never wall-clock derived.
- Give both list queries a genuine total order by adding a `file_path` tiebreak (name/version/arch is not unique).
- Add guard tests: deb generators byte-identical across generations; the `Release` checksum/size matches the served `Packages`/`Packages.gz` bytes (the exact apt invariant); `Date:` pinned to `created_at`; apk index byte-identical and tar `ModTime` pinned to epoch.

Reviewed-on: #119
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/provider/alpine/alpine.go

@@ -15,6 +15,7 @@ import (
 	"path"
 	"strconv"
 	"strings"
+	"time"
 
 	"archive/tar"
 
@@ -376,7 +377,10 @@ func generateAPKIndex(metas []provider.AlpineMetadata) []byte {
 	var tarBuf bytes.Buffer
 	tw := tar.NewWriter(&tarBuf)
 	body := idx.Bytes()
-	tw.WriteHeader(&tar.Header{Name: "APKINDEX", Mode: 0o644, Size: int64(len(body)), Typeflag: tar.TypeReg})
+	// ModTime is pinned to the Unix epoch (never wall clock) so APKINDEX.tar.gz
+	// is byte-identical across replicas and regenerations (issue #117); apk
+	// clients ignore the tar mtime.
+	tw.WriteHeader(&tar.Header{Name: "APKINDEX", Mode: 0o644, Size: int64(len(body)), Typeflag: tar.TypeReg, ModTime: time.Unix(0, 0)})
 	tw.Write(body)
 	tw.Close()
 

internal/provider/alpine/alpine_deterministic_test.go

@@ -0,0 +1,72 @@
+package alpine
+
+import (
+	"archive/tar"
+	"bytes"
+	"compress/gzip"
+	"io"
+	"testing"
+	"time"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+)
+
+func apkFixture() []provider.AlpineMetadata {
+	return []provider.AlpineMetadata{
+		{
+			RepoName: "r", FilePath: "x86_64/aaa-1.0-r0.apk", Checksum: "Q1aaa",
+			Name: "aaa", Version: "1.0-r0", Arch: "x86_64", DownloadSize: 100, InstalledSize: 10,
+			Description: "pkg aaa", URL: "https://a", License: "MIT",
+			Depends: []string{"so:libc"}, Provides: []string{"cmd:aaa"}, BuildTime: 1710000000,
+		},
+		{
+			RepoName: "r", FilePath: "x86_64/bbb-2.0-r0.apk", Checksum: "Q1bbb",
+			Name: "bbb", Version: "2.0-r0", Arch: "x86_64", DownloadSize: 200, InstalledSize: 20,
+		},
+	}
+}
+
+// TestAPKIndexDeterministic asserts APKINDEX.tar.gz is byte-identical across two
+// generations separated by wall-clock time, so the two no-affinity replicas and
+// every regeneration serve the same bytes (issue #117).
+func TestAPKIndexDeterministic(t *testing.T) {
+	metas := apkFixture()
+
+	first := generateAPKIndex(metas)
+	time.Sleep(10 * time.Millisecond)
+	second := generateAPKIndex(metas)
+
+	if !bytes.Equal(first, second) {
+		t.Error("APKINDEX.tar.gz differs across generations")
+	}
+}
+
+// TestAPKIndexTarModTimePinned guards the tar header: its ModTime must be the
+// pinned Unix epoch, never wall clock. Fails if a future edit stamps time.Now().
+func TestAPKIndexTarModTimePinned(t *testing.T) {
+	metas := apkFixture()
+
+	zr, err := gzip.NewReader(bytes.NewReader(generateAPKIndex(metas)))
+	if err != nil {
+		t.Fatalf("gzip: %v", err)
+	}
+	if !zr.ModTime.IsZero() && zr.ModTime.Unix() != 0 {
+		t.Errorf("gzip header ModTime = %v, want zero/epoch", zr.ModTime)
+	}
+
+	tarBytes, err := io.ReadAll(zr)
+	if err != nil {
+		t.Fatalf("gunzip: %v", err)
+	}
+	tr := tar.NewReader(bytes.NewReader(tarBytes))
+	hdr, err := tr.Next()
+	if err != nil {
+		t.Fatalf("tar: %v", err)
+	}
+	if hdr.Name != "APKINDEX" {
+		t.Fatalf("tar entry = %q, want APKINDEX", hdr.Name)
+	}
+	if hdr.ModTime.Unix() != 0 {
+		t.Errorf("APKINDEX tar ModTime = %v (unix %d), want epoch (0)", hdr.ModTime, hdr.ModTime.Unix())
+	}
+}