Serve a serve_stale member's previous index while it refreshes

Add a serve_stale remote flag. rpm, deb and alpine virtual merges use a
flagged member's last good fetch immediately and refresh it in the
background, single-flighted per member.

docs/common/caching.md

@@ -15,6 +15,7 @@ Every proxied request on a remote is classified as **denied**, **immutable** or
 | `mutable_ttl` | Seconds a mutable path stays fresh. `0` also means never expires. Omitted on create, it defaults to `3600`; omitted on update, the stored value is kept. |
 | `check_mutable` | When a mutable path's `mutable_ttl` expires, sends a conditional `HEAD` (`If-None-Match`) with the stored upstream ETag. A `304` re-arms the TTL and serves the stored copy; anything else refetches in full. The ETag is kept for `max(10 × mutable_ttl, 7 days)`, re-armed on each `304`, so idle paths age out. It is only stored when `check_mutable` is set and `mutable_ttl` is non-zero, and never for immutable paths. |
 | `stale_on_error` | On an upstream network error, serve the stored copy instead of failing. |
+| `serve_stale` | rpm/deb/alpine virtual members only. When a virtual re-merges, use this member's previous index at once and refresh it in the background; the next merge picks up the refresh. The first merge still waits. Default `false`. |
 | `quarantine_enabled` / `quarantine_days` | Stored on the remote but not enforced by the proxy yet. |
 | `mirrorlist` / `mirror_strategy` | rpm/deb/alpine only. Upstream pool is `[base_url] + mirrorlist`; `round_robin` (default) rotates the starting mirror, `least_conn` starts at the one with the fewest in-flight requests. Network errors and 5xx fail over to the next mirror; other statuses return as-is. |
 

internal/database/remotes.go

@@ -15,7 +15,7 @@ const remoteCols = `name, package_type, repo_type, base_url, mirrorlist, mirror_
 	quarantine_enabled, quarantine_days, stale_on_error,
 	releases_remote, managed_by,
 	upstream_dial_timeout, upstream_tls_timeout, upstream_response_header_timeout,
-	created_at, updated_at`
+	serve_stale, created_at, updated_at`
 
 // normalizeMirrorStrategy maps an empty strategy to the round_robin default so
 // the NOT NULL mirror_strategy column always stores a canonical value.
@@ -35,7 +35,7 @@ func scanRemote(scanner interface{ Scan(...any) error }, r *models.Remote) error
 		&r.QuarantineEnabled, &r.QuarantineDays, &r.StaleOnError,
 		&r.ReleasesRemote, &r.ManagedBy,
 		&r.UpstreamDialTimeout, &r.UpstreamTLSTimeout, &r.UpstreamResponseHeaderTimeout,
-		&r.CreatedAt, &r.UpdatedAt,
+		&r.ServeStale, &r.CreatedAt, &r.UpdatedAt,
 	)
 }
 
@@ -76,8 +76,8 @@ func (db *DB) CreateRemote(ctx context.Context, r *models.Remote) error {
 			quarantine_enabled, quarantine_days, stale_on_error,
 			releases_remote, managed_by,
 			upstream_dial_timeout, upstream_tls_timeout, upstream_response_header_timeout,
-			mirror_strategy
-		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25,$26)
+			mirror_strategy, serve_stale
+		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$20,$21,$22,$23,$24,$25,$26,$27)
 	`,
 		r.Name, r.PackageType, r.RepoType, r.BaseURL, r.Mirrorlist, r.Description, r.Username, r.Password,
 		r.ImmutableTTL, r.MutableTTL, r.CheckMutable,
@@ -86,7 +86,7 @@ func (db *DB) CreateRemote(ctx context.Context, r *models.Remote) error {
 		r.QuarantineEnabled, r.QuarantineDays, r.StaleOnError,
 		r.ReleasesRemote, r.ManagedBy,
 		r.UpstreamDialTimeout, r.UpstreamTLSTimeout, r.UpstreamResponseHeaderTimeout,
-		normalizeMirrorStrategy(r.MirrorStrategy),
+		normalizeMirrorStrategy(r.MirrorStrategy), r.ServeStale,
 	)
 	return err
 }
@@ -101,7 +101,7 @@ func (db *DB) UpdateRemote(ctx context.Context, r *models.Remote) error {
 			quarantine_enabled=$17, quarantine_days=$18, stale_on_error=$19,
 			releases_remote=$20, managed_by=$21,
 			upstream_dial_timeout=$22, upstream_tls_timeout=$23, upstream_response_header_timeout=$24,
-			mirror_strategy=$26,
+			mirror_strategy=$26, serve_stale=$27,
 			updated_at=NOW()
 		WHERE name=$1
 	`,
@@ -113,7 +113,7 @@ func (db *DB) UpdateRemote(ctx context.Context, r *models.Remote) error {
 		r.ReleasesRemote, r.ManagedBy,
 		r.UpstreamDialTimeout, r.UpstreamTLSTimeout, r.UpstreamResponseHeaderTimeout,
 		r.Mirrorlist,
-		normalizeMirrorStrategy(r.MirrorStrategy),
+		normalizeMirrorStrategy(r.MirrorStrategy), r.ServeStale,
 	)
 	return err
 }

internal/virtual/engine.go

@@ -55,6 +55,9 @@ type Engine struct {
 	sf   singleflight.Group
 	mu   sync.Mutex
 	gens map[string]*repoGen
+	// stale holds the last good fetch of each serve_stale member, keyed by
+	// merge key and member name.
+	stale map[string]any
 }
 
 // mergedRepo is a merged index document plus the files it references.
@@ -353,7 +356,9 @@ func (e *Engine) merged(ctx context.Context, key string, merge func(context.Cont
 }
 
 func (e *Engine) mergeRPM(ctx context.Context, virt models.Virtual) (*RPMRepo, error) {
-	members, err := fetchMembers(virt, func(name string) (*RPMMember, error) { return e.rpmMember(ctx, name) })
+	members, err := fetchMembers(virt, func(name string) (*RPMMember, error) {
+		return staleMember(ctx, e, "rpm\x00"+virt.Name, name, func(ctx context.Context) (*RPMMember, error) { return e.rpmMember(ctx, name) })
+	})
 	if err != nil {
 		return nil, err
 	}
@@ -402,7 +407,9 @@ func (e *Engine) fetchDeb(ctx context.Context, virt models.Virtual, reqPath stri
 	}
 
 	repo, err := e.merged(ctx, key, func(ctx context.Context) (mergedRepo, error) {
-		members, err := fetchMembers(virt, func(name string) (*DebMember, error) { return e.debMember(ctx, name, base) })
+		members, err := fetchMembers(virt, func(name string) (*DebMember, error) {
+			return staleMember(ctx, e, key, name, func(ctx context.Context) (*DebMember, error) { return e.debMember(ctx, name, base) })
+		})
 		if err != nil {
 			return nil, err
 		}
@@ -453,8 +460,11 @@ func (e *Engine) fetchDebMember(ctx context.Context, name, base string) (*DebMem
 }
 
 func (e *Engine) mergedAlpine(ctx context.Context, virt models.Virtual, dir string) (mergedRepo, error) {
-	repo, err := e.merged(ctx, "alpine\x00"+virt.Name+"\x00"+dir, func(ctx context.Context) (mergedRepo, error) {
-		members, err := fetchMembers(virt, func(name string) (*AlpineMember, error) { return e.apkMember(ctx, name, dir) })
+	key := "alpine\x00" + virt.Name + "\x00" + dir
+	repo, err := e.merged(ctx, key, func(ctx context.Context) (mergedRepo, error) {
+		members, err := fetchMembers(virt, func(name string) (*AlpineMember, error) {
+			return staleMember(ctx, e, key, name, func(ctx context.Context) (*AlpineMember, error) { return e.apkMember(ctx, name, dir) })
+		})
 		if err != nil {
 			return nil, err
 		}
@@ -524,6 +534,43 @@ func (e *Engine) fetchAlpineMember(ctx context.Context, name, dir string) (*Alpi
 	return &AlpineMember{RemoteName: name, Index: idx}, nil
 }
 
+// staleMember fetches member name for the merge under key. A serve_stale member
+// with a previous good fetch returns it at once and refreshes in the
+// background, one refresh per member at a time; the next merge picks it up.
+func staleMember[T any](ctx context.Context, e *Engine, key, name string, fetch func(context.Context) (*T, error)) (*T, error) {
+	remote, err := e.getRemote(ctx, name)
+	if err != nil || !remote.ServeStale {
+		return fetch(ctx)
+	}
+	sk := key + "\x00" + name
+	refresh := func() (any, error) {
+		m, err := fetch(ctx)
+		if err == nil {
+			e.mu.Lock()
+			if e.stale == nil {
+				e.stale = map[string]any{}
+			}
+			e.stale[sk] = m
+			e.mu.Unlock()
+		}
+		return m, err
+	}
+	e.mu.Lock()
+	prev, ok := e.stale[sk].(*T)
+	e.mu.Unlock()
+	if !ok {
+		m, err := refresh()
+		return m.(*T), err
+	}
+	ch := e.sf.DoChan("stale\x00"+sk, refresh)
+	go func() {
+		if r := <-ch; r.Err != nil {
+			slog.Warn("virtual member background refresh failed", "member", name, "error", r.Err)
+		}
+	}()
+	return prev, nil
+}
+
 // evictLRU drops the least recently used merges under prefix beyond keep.
 func (e *Engine) evictLRU(prefix string, keep int) {
 	e.mu.Lock()
@@ -537,6 +584,11 @@ func (e *Engine) evictLRU(prefix string, keep int) {
 	slices.SortFunc(keys, func(a, b string) int { return e.gens[b].used.Compare(e.gens[a].used) })
 	for _, k := range keys[min(len(keys), keep):] {
 		delete(e.gens, k)
+		for sk := range e.stale {
+			if strings.HasPrefix(sk, k+"\x00") {
+				delete(e.stale, sk)
+			}
+		}
 	}
 }
 

internal/virtual/engine_stale_test.go

@@ -0,0 +1,113 @@
+package virtual
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func rpmPkgMember(name, ver string) *RPMMember {
+	return &RPMMember{RemoteName: name, Data: map[string][]byte{"primary": primaryXML(primaryPkgXML("foo", ver, "h"+ver, "foo-"+ver+".rpm"))}}
+}
+
+// staleEngine re-merges on every request; member "a" is fetched through fetch
+// and has serve_stale set to stale.
+func staleEngine(stale bool, fetch func() *RPMMember) (*Engine, *atomic.Int32) {
+	var calls atomic.Int32
+	e := fakeEngine(nil)
+	e.getRemote = func(_ context.Context, name string) (*models.Remote, error) {
+		return &models.Remote{Name: name, RepoType: models.RepoTypeRemote, ServeStale: stale}, nil
+	}
+	e.rpmMember = func(_ context.Context, name string) (*RPMMember, error) {
+		calls.Add(1)
+		return fetch(), nil
+	}
+	return e, &calls
+}
+
+func fetchRepomd(t *testing.T, e *Engine) string {
+	t.Helper()
+	b, _, err := e.Fetch(context.Background(), rpmVirt("a"), "repodata/repomd.xml", "")
+	if err != nil {
+		t.Error(err)
+	}
+	return string(b)
+}
+
+func TestServeStaleMemberDoesNotBlockMerge(t *testing.T) {
+	release := make(chan struct{})
+	var slow atomic.Bool
+	e, calls := staleEngine(true, func() *RPMMember {
+		if slow.Load() {
+			<-release
+			return rpmPkgMember("a", "2")
+		}
+		return rpmPkgMember("a", "1")
+	})
+	v1 := fetchRepomd(t, e)
+	slow.Store(true)
+
+	for range 3 {
+		done := make(chan string)
+		go func() { done <- fetchRepomd(t, e) }()
+		select {
+		case got := <-done:
+			if got != v1 {
+				t.Fatal("stale merge should serve the previous member copy")
+			}
+		case <-time.After(time.Second):
+			t.Fatal("merge blocked on a serve_stale member refresh")
+		}
+	}
+	if n := calls.Load(); n != 2 {
+		t.Fatalf("member fetched %d times, want 2 (cold + one single-flighted refresh)", n)
+	}
+
+	close(release)
+	deadline := time.Now().Add(time.Second)
+	for fetchRepomd(t, e) == v1 {
+		if time.Now().After(deadline) {
+			t.Fatal("background refresh never reached a merge")
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+}
+
+func assertMergeBlocks(t *testing.T, e *Engine, release chan struct{}) {
+	t.Helper()
+	done := make(chan struct{})
+	go func() { fetchRepomd(t, e); close(done) }()
+	select {
+	case <-done:
+		t.Fatal("merge returned before the member fetch finished")
+	case <-time.After(50 * time.Millisecond):
+	}
+	close(release)
+	<-done
+}
+
+func TestNonServeStaleMemberBlocksMerge(t *testing.T) {
+	release := make(chan struct{})
+	var slow atomic.Bool
+	e, _ := staleEngine(false, func() *RPMMember {
+		if slow.Load() {
+			<-release
+		}
+		return rpmPkgMember("a", "1")
+	})
+	fetchRepomd(t, e)
+	slow.Store(true)
+	assertMergeBlocks(t, e, release)
+}
+
+func TestServeStaleColdStartBlocksMerge(t *testing.T) {
+	release := make(chan struct{})
+	e, _ := staleEngine(true, func() *RPMMember {
+		<-release
+		return rpmPkgMember("a", "1")
+	})
+	assertMergeBlocks(t, e, release)
+}

migrations/0008_remote_serve_stale.sql

@@ -0,0 +1,3 @@
+-- serve_stale lets a virtual merge a member's previous index while that member
+-- refreshes in the background.
+ALTER TABLE remotes ADD COLUMN IF NOT EXISTS serve_stale BOOLEAN NOT NULL DEFAULT FALSE;

pkg/models/remote.go

@@ -73,6 +73,9 @@ type Remote struct {
 	QuarantineDays    int  `json:"quarantine_days,omitempty"`
 
 	StaleOnError bool `json:"stale_on_error"`
+	// ServeStale lets an rpm/deb/alpine virtual merge this member's previous
+	// index while it refreshes in the background.
+	ServeStale bool `json:"serve_stale"`
 
 	ReleasesRemote string `json:"releases_remote,omitempty"`
 	ManagedBy      string `json:"managed_by,omitempty"`