1 files changed, 275 insertions, 0 deletions
diff --git a/objectstore/mix/mix.go b/objectstore/mix/mix.go
new file mode 100644
index 00000000..00b2a4af
--- /dev/null
+++ b/objectstore/mix/mix.go
@@ -0,0 +1,275 @@
+// Package mix provides an adaptive wrapper over multiple object storage
+// backends.
+package mix
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"sync"
+
+	"codeberg.org/lindenii/furgit/objectid"
+	"codeberg.org/lindenii/furgit/objectstore"
+	"codeberg.org/lindenii/furgit/objecttype"
+)
+
+// Mix queries multiple object databases with an MRU backend preference.
+type Mix struct {
+	mu sync.RWMutex
+
+	backendHead        *backendNode
+	backendTail        *backendNode
+	backendNodeByStore map[objectstore.Store]*backendNode
+}
+
+// New creates a Mix from backends.
+func New(backends ...objectstore.Store) *Mix {
+	nodeByStore := make(map[objectstore.Store]*backendNode, len(backends))
+
+	var (
+		head *backendNode
+		tail *backendNode
+	)
+
+	for _, backend := range backends {
+		if backend == nil {
+			continue
+		}
+
+		node := &backendNode{
+			backend: backend,
+			prev:    tail,
+		}
+		if tail != nil {
+			tail.next = node
+		}
+
+		if head == nil {
+			head = node
+		}
+
+		tail = node
+		nodeByStore[backend] = node
+	}
+
+	return &Mix{
+		backendHead:        head,
+		backendTail:        tail,
+		backendNodeByStore: nodeByStore,
+	}
+}
+
+// ReadBytesFull reads a full serialized object from one backend that has it.
+func (mix *Mix) ReadBytesFull(id objectid.ObjectID) ([]byte, error) {
+	for i, backend := 0, mix.firstBackend(); backend != nil; i, backend = i+1, mix.nextBackend(backend) {
+		full, err := backend.ReadBytesFull(id)
+		if err == nil {
+			mix.touchBackend(backend)
+
+			return full, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return nil, fmt.Errorf("objectstore: backend %d read bytes full: %w", i, err)
+	}
+
+	return nil, objectstore.ErrObjectNotFound
+}
+
+// ReadBytesContent reads an object's type and content bytes from one backend
+// that has it.
+func (mix *Mix) ReadBytesContent(id objectid.ObjectID) (objecttype.Type, []byte, error) {
+	for i, backend := 0, mix.firstBackend(); backend != nil; i, backend = i+1, mix.nextBackend(backend) {
+		ty, content, err := backend.ReadBytesContent(id)
+		if err == nil {
+			mix.touchBackend(backend)
+
+			return ty, content, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return objecttype.TypeInvalid, nil, fmt.Errorf("objectstore: backend %d read bytes content: %w", i, err)
+	}
+
+	return objecttype.TypeInvalid, nil, objectstore.ErrObjectNotFound
+}
+
+// ReadReaderFull reads a full serialized object stream from one backend that
+// has it.
+func (mix *Mix) ReadReaderFull(id objectid.ObjectID) (io.ReadCloser, error) {
+	for i, backend := 0, mix.firstBackend(); backend != nil; i, backend = i+1, mix.nextBackend(backend) {
+		reader, err := backend.ReadReaderFull(id)
+		if err == nil {
+			mix.touchBackend(backend)
+
+			return reader, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return nil, fmt.Errorf("objectstore: backend %d read reader full: %w", i, err)
+	}
+
+	return nil, objectstore.ErrObjectNotFound
+}
+
+// ReadReaderContent reads an object's type, declared content length, and
+// content stream from one backend that has it.
+func (mix *Mix) ReadReaderContent(id objectid.ObjectID) (objecttype.Type, int64, io.ReadCloser, error) {
+	for i, backend := 0, mix.firstBackend(); backend != nil; i, backend = i+1, mix.nextBackend(backend) {
+		ty, size, reader, err := backend.ReadReaderContent(id)
+		if err == nil {
+			mix.touchBackend(backend)
+
+			return ty, size, reader, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return objecttype.TypeInvalid, 0, nil, fmt.Errorf("objectstore: backend %d read reader content: %w", i, err)
+	}
+
+	return objecttype.TypeInvalid, 0, nil, objectstore.ErrObjectNotFound
+}
+
+// ReadSize reads object content length from one backend that has it.
+func (mix *Mix) ReadSize(id objectid.ObjectID) (int64, error) {
+	for i, backend := 0, mix.firstBackend(); backend != nil; i, backend = i+1, mix.nextBackend(backend) {
+		size, err := backend.ReadSize(id)
+		if err == nil {
+			mix.touchBackend(backend)
+
+			return size, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return 0, fmt.Errorf("objectstore: backend %d read size: %w", i, err)
+	}
+
+	return 0, objectstore.ErrObjectNotFound
+}
+
+// ReadHeader reads object header data from one backend that has it.
+func (mix *Mix) ReadHeader(id objectid.ObjectID) (objecttype.Type, int64, error) {
+	for i, backend := 0, mix.firstBackend(); backend != nil; i, backend = i+1, mix.nextBackend(backend) {
+		ty, size, err := backend.ReadHeader(id)
+		if err == nil {
+			mix.touchBackend(backend)
+
+			return ty, size, nil
+		}
+
+		if errors.Is(err, objectstore.ErrObjectNotFound) {
+			continue
+		}
+
+		return objecttype.TypeInvalid, 0, fmt.Errorf("objectstore: backend %d read header: %w", i, err)
+	}
+
+	return objecttype.TypeInvalid, 0, objectstore.ErrObjectNotFound
+}
+
+// Close closes all backends and joins close errors.
+func (mix *Mix) Close() error {
+	mix.mu.RLock()
+
+	backends := make([]objectstore.Store, 0, len(mix.backendNodeByStore))
+	for node := mix.backendHead; node != nil; node = node.next {
+		backends = append(backends, node.backend)
+	}
+
+	mix.mu.RUnlock()
+
+	var errs []error
+
+	for _, backend := range backends {
+		err := backend.Close()
+		if err != nil {
+			errs = append(errs, err)
+		}
+	}
+
+	return errors.Join(errs...)
+}
+
+type backendNode struct {
+	backend objectstore.Store
+	prev    *backendNode
+	next    *backendNode
+}
+
+func (mix *Mix) firstBackend() objectstore.Store {
+	mix.mu.RLock()
+	defer mix.mu.RUnlock()
+
+	if mix.backendHead == nil {
+		return nil
+	}
+
+	return mix.backendHead.backend
+}
+
+func (mix *Mix) nextBackend(current objectstore.Store) objectstore.Store {
+	mix.mu.RLock()
+	defer mix.mu.RUnlock()
+
+	node := mix.backendNodeByStore[current]
+	if node == nil || node.next == nil {
+		return nil
+	}
+
+	return node.next.backend
+}
+
+func (mix *Mix) touchBackend(backend objectstore.Store) {
+	if backend == nil {
+		return
+	}
+
+	if !mix.mu.TryLock() {
+		return
+	}
+	defer mix.mu.Unlock()
+
+	node := mix.backendNodeByStore[backend]
+	if node == nil || node == mix.backendHead {
+		return
+	}
+
+	if node.prev != nil {
+		node.prev.next = node.next
+	}
+
+	if node.next != nil {
+		node.next.prev = node.prev
+	}
+
+	if mix.backendTail == node {
+		mix.backendTail = node.prev
+	}
+
+	node.prev = nil
+
+	node.next = mix.backendHead
+	if mix.backendHead != nil {
+		mix.backendHead.prev = node
+	}
+
+	mix.backendHead = node
+	if mix.backendTail == nil {
+		mix.backendTail = node
+	}
+}