Retry failed GitHub release scans with backoff (#133)

Releasing a GitHub sync lease always advanced `last_synced_at`, even after a failed scan (e.g. a rate-limit 403). A remote that failed once waited a full `mutable_ttl` before retrying, and a cold remote kept returning 503 until then.

- record scan outcomes in one shared lease helper for github_rpm/deb/alpine
- keep `last_synced_at` and the ETag on failure; retry from 60s with exponential backoff, capped at min(10m, ttl/4)
- honour `Retry-After` / `X-RateLimit-Reset`, clamped to `mutable_ttl`
- add `sync_failures` / `next_retry_at` columns (migration 0002)

Reviewed-on: #133
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/database/github_sync_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 	"time"
 
+	"git.unkin.net/unkin/artifactapi/internal/provider"
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
@@ -47,7 +48,7 @@ func TestGitHubSyncLease(t *testing.T) {
 	}
 
 	// Replica 1 finishes: record the sync and persist an etag.
-	if err := testDB.ReleaseGitHubSyncLease(ctx(), name, "replica-1", `"etag-1"`, time.Now()); err != nil {
+	if err := testDB.ReleaseGitHubSyncLease(ctx(), name, "replica-1", provider.SyncResult{Etag: `"etag-1"`}); err != nil {
 		t.Fatalf("release: %v", err)
 	}
 
@@ -93,3 +94,116 @@ func TestListGitHubRPMRemotes(t *testing.T) {
 		t.Fatalf("seeded remote %q not returned", name)
 	}
 }
+
+// A failed scan must not count as a sync: the etag and last_synced_at stay put,
+// every claim (prime included) is held off until the backoff elapses, the delay
+// doubles per consecutive failure up to the cap, an upstream retry hint pushes
+// it later, and a success clears the retry state.
+func TestGitHubSyncLeaseFailureBackoff(t *testing.T) {
+	requireDB(t)
+	name := "gh-retry-" + time.Now().Format("150405.000000")
+	seedGitHubRPMRemote(t, name)
+
+	const lease = 15 * time.Minute
+	freshness := time.Hour
+	claim := func(f time.Duration) (bool, string) {
+		t.Helper()
+		ok, etag, err := testDB.ClaimGitHubSyncLease(ctx(), name, "r1", f, lease)
+		if err != nil {
+			t.Fatalf("claim: %v", err)
+		}
+		return ok, etag
+	}
+	release := func(res provider.SyncResult) {
+		t.Helper()
+		if err := testDB.ReleaseGitHubSyncLease(ctx(), name, "r1", res); err != nil {
+			t.Fatalf("release: %v", err)
+		}
+	}
+	state := func() (failures int, retryIn time.Duration, synced *time.Time, etag string) {
+		t.Helper()
+		var next *time.Time
+		var dbNow time.Time
+		if err := testDB.Pool.QueryRow(ctx(), `SELECT sync_failures, next_retry_at, last_synced_at, etag, now() FROM github_rpm_sync_state WHERE remote_name = $1`, name).
+			Scan(&failures, &next, &synced, &etag, &dbNow); err != nil {
+			t.Fatalf("read state: %v", err)
+		}
+		if next != nil {
+			retryIn = next.Sub(dbNow)
+		}
+		return
+	}
+	elapse := func() {
+		t.Helper()
+		if _, err := testDB.Pool.Exec(ctx(), `UPDATE github_rpm_sync_state SET next_retry_at = now() - interval '1 second' WHERE remote_name = $1`, name); err != nil {
+			t.Fatalf("elapse backoff: %v", err)
+		}
+	}
+	failed := provider.SyncResult{Failed: true, Etag: `"ignored"`, Backoff: time.Minute, MaxBackoff: 3 * time.Minute}
+
+	// Last good sync.
+	if ok, _ := claim(freshness); !ok {
+		t.Fatal("first claim failed")
+	}
+	release(provider.SyncResult{Etag: `"good"`})
+	_, _, goodSynced, _ := state()
+
+	// First failure: 60s backoff, sync time and etag untouched.
+	if ok, _ := claim(0); !ok {
+		t.Fatal("prime claim failed")
+	}
+	release(failed)
+	n, in, synced, etag := state()
+	if n != 1 || in < 55*time.Second || in > 65*time.Second {
+		t.Fatalf("after 1st failure: failures=%d retry in %v, want 1 and ~60s", n, in)
+	}
+	if etag != `"good"` || synced == nil || !synced.Equal(*goodSynced) {
+		t.Fatalf("failed scan overwrote sync state: etag=%q synced=%v", etag, synced)
+	}
+	if ok, _ := claim(0); ok {
+		t.Fatal("prime claimed inside the retry backoff")
+	}
+
+	// Backoff elapsed: claimable despite being inside the freshness window,
+	// and the second failure doubles the delay.
+	elapse()
+	if ok, _ := claim(freshness); !ok {
+		t.Fatal("periodic claim refused after backoff elapsed")
+	}
+	release(failed)
+	if n, in, _, _ := state(); n != 2 || in < 115*time.Second || in > 125*time.Second {
+		t.Fatalf("after 2nd failure: failures=%d retry in %v, want 2 and ~120s", n, in)
+	}
+
+	// Third failure caps at MaxBackoff (3m, not 4m).
+	elapse()
+	claim(freshness)
+	release(failed)
+	if n, in, _, _ := state(); n != 3 || in < 175*time.Second || in > 185*time.Second {
+		t.Fatalf("after 3rd failure: failures=%d retry in %v, want 3 and ~180s", n, in)
+	}
+
+	// An upstream rate-limit reset later than the backoff wins.
+	elapse()
+	claim(freshness)
+	hinted := failed
+	hinted.RetryAt = time.Now().Add(20 * time.Minute)
+	release(hinted)
+	if _, in, _, _ := state(); in < 19*time.Minute || in > 21*time.Minute {
+		t.Fatalf("retry hint ignored: retry in %v, want ~20m", in)
+	}
+
+	// Recovery: success persists the new etag and clears the retry, so the
+	// normal freshness window applies again.
+	elapse()
+	if ok, etag := claim(freshness); !ok || etag != `"good"` {
+		t.Fatalf("recovery claim ok=%v etag=%q, want last good etag", ok, etag)
+	}
+	release(provider.SyncResult{Etag: `"new"`})
+	if n, in, _, etag := state(); n != 0 || in != 0 || etag != `"new"` {
+		t.Fatalf("after success: failures=%d retry in %v etag=%q", n, in, etag)
+	}
+	if ok, _ := claim(freshness); ok {
+		t.Fatal("claimed inside freshness window after a successful sync")
+	}
+}