move Backend interface to backend package

internal/backend/backend.go

@@ -0,0 +1,117 @@
+package backend
+
+import (
+	"context"
+	"hash"
+	"io"
+)
+
+// Backend is used to store and access data.
+//
+// Backend operations that return an error will be retried when a Backend is
+// wrapped in a RetryBackend. To prevent that from happening, the operations
+// should return a github.com/cenkalti/backoff/v4.PermanentError. Errors from
+// the context package need not be wrapped, as context cancellation is checked
+// separately by the retrying logic.
+type Backend interface {
+	// Location returns a string that describes the type and location of the
+	// repository.
+	Location() string
+
+	// Connections returns the maxmimum number of concurrent backend operations.
+	Connections() uint
+
+	// Hasher may return a hash function for calculating a content hash for the backend
+	Hasher() hash.Hash
+
+	// HasAtomicReplace returns whether Save() can atomically replace files
+	HasAtomicReplace() bool
+
+	// Remove removes a File described  by h.
+	Remove(ctx context.Context, h Handle) error
+
+	// Close the backend
+	Close() error
+
+	// Save stores the data from rd under the given handle.
+	Save(ctx context.Context, h Handle, rd RewindReader) error
+
+	// Load runs fn with a reader that yields the contents of the file at h at the
+	// given offset. If length is larger than zero, only a portion of the file
+	// is read.
+	//
+	// The function fn may be called multiple times during the same Load invocation
+	// and therefore must be idempotent.
+	//
+	// Implementations are encouraged to use util.DefaultLoad
+	Load(ctx context.Context, h Handle, length int, offset int64, fn func(rd io.Reader) error) error
+
+	// Stat returns information about the File identified by h.
+	Stat(ctx context.Context, h Handle) (FileInfo, error)
+
+	// List runs fn for each file in the backend which has the type t. When an
+	// error occurs (or fn returns an error), List stops and returns it.
+	//
+	// The function fn is called exactly once for each file during successful
+	// execution and at most once in case of an error.
+	//
+	// The function fn is called in the same Goroutine that List() is called
+	// from.
+	List(ctx context.Context, t FileType, fn func(FileInfo) error) error
+
+	// IsNotExist returns true if the error was caused by a non-existing file
+	// in the backend.
+	//
+	// The argument may be a wrapped error. The implementation is responsible
+	// for unwrapping it.
+	IsNotExist(err error) bool
+
+	// Delete removes all data in the backend.
+	Delete(ctx context.Context) error
+}
+
+type Unwrapper interface {
+	// Unwrap returns the underlying backend or nil if there is none.
+	Unwrap() Backend
+}
+
+func AsBackend[B Backend](b Backend) B {
+	for b != nil {
+		if be, ok := b.(B); ok {
+			return be
+		}
+
+		if be, ok := b.(Unwrapper); ok {
+			b = be.Unwrap()
+		} else {
+			// not the backend we're looking for
+			break
+		}
+	}
+	var be B
+	return be
+}
+
+type FreezeBackend interface {
+	Backend
+	// Freeze blocks all backend operations except those on lock files
+	Freeze()
+	// Unfreeze allows all backend operations to continue
+	Unfreeze()
+}
+
+// FileInfo is contains information about a file in the backend.
+type FileInfo struct {
+	Size int64
+	Name string
+}
+
+// ApplyEnvironmenter fills in a backend configuration from the environment
+type ApplyEnvironmenter interface {
+	ApplyEnvironment(prefix string)
+}
+
+// Lister allows listing files in a backend.
+type Lister interface {
+	List(context.Context, FileType, func(FileInfo) error) error
+}