Add cache_repodata toggle for github_rpm remotes (#168)

github_rpm remotes regenerate repomd and all three gz files from every metadata row on each request, which is expensive for large repos under repeated dnf traffic.

- add a per-remote `cache_repodata` toggle (column, model, v2 API, docs)
- cache built repodata per replica, rebuilt via singleflight when the `count(*)`+`max(id)` row fingerprint moves
- store a build only if no newer build has landed; evict idle entries hourly
- map cached-path DB errors like the uncached path: 503 + Retry-After on cancel/deadline, generic 500 otherwise
- check `cacheEmpty` via the fingerprint for all github_rpm remotes, not only cached ones

Reviewed-on: #168
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/api/v2/remotes_cache_repodata_test.go

@@ -0,0 +1,57 @@
+package v2
+
+import (
+	"encoding/json"
+	"net/http/httptest"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+)
+
+func TestCacheRepodataRoundTrip(t *testing.T) {
+	if testDSN == "" {
+		t.Skip("Docker unavailable")
+	}
+	db, err := database.New(testDSN)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	h := NewRemotesHandler(db, nil, nil).Routes()
+	get := func() bool {
+		t.Helper()
+		w := httptest.NewRecorder()
+		h.ServeHTTP(w, httptest.NewRequest("GET", "/cache-rd", nil))
+		var got struct {
+			CacheRepodata *bool `json:"cache_repodata"`
+		}
+		if err := json.Unmarshal(w.Body.Bytes(), &got); err != nil || got.CacheRepodata == nil {
+			t.Fatalf("get = %d %s", w.Code, w.Body.String())
+		}
+		return *got.CacheRepodata
+	}
+	const body = `"package_type":"github_rpm","base_url":"https://api.github.com"`
+
+	steps := []struct {
+		method, path, json string
+		want               bool
+	}{
+		{"POST", "/", `{"name":"cache-rd",` + body + `}`, false},
+		{"PUT", "/cache-rd", `{` + body + `,"cache_repodata":true}`, true},
+		{"PUT", "/cache-rd", `{` + body + `,"cache_repodata":false}`, false},
+	}
+	for i, s := range steps {
+		if c := do(t, h, s.method, s.path, s.json); c/100 != 2 {
+			t.Fatalf("step %d %s %s = %d", i, s.method, s.path, c)
+		}
+		if got := get(); got != s.want {
+			t.Errorf("step %d: cache_repodata = %v, want %v", i, got, s.want)
+		}
+	}
+	if c := do(t, h, "POST", "/", `{"name":"cache-rd-on",`+body+`,"cache_repodata":true}`); c != 201 {
+		t.Fatalf("create with cache_repodata = %d", c)
+	}
+	if r, err := db.GetRemote(t.Context(), "cache-rd-on"); err != nil || !r.CacheRepodata {
+		t.Fatalf("create did not store cache_repodata: %v %v", r, err)
+	}
+}