Bound remote control object reads

internal/adapters/storage/bounded_read.go

@@ -0,0 +1,90 @@
+package storage
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"math"
+	"strings"
+)
+
+// ReadLimitError reports that a remote object exceeded its caller-owned read
+// limit. The limit is enforced against both available object metadata and the
+// bytes returned by the opened object body.
+type ReadLimitError struct {
+	Key      string
+	Limit    int64
+	Observed int64
+}
+
+func (e *ReadLimitError) Error() string {
+	return fmt.Sprintf("object %q exceeds %d-byte read limit (observed at least %d bytes)", e.Key, e.Limit, e.Observed)
+}
+
+// ReadObjectBounded opens one object version and retains at most maxBytes of
+// its content. Object metadata may reject an oversized body early, but a
+// limit-plus-one read always enforces the boundary when transfer begins.
+func ReadObjectBounded(ctx context.Context, store ObjectStore, key string, maxBytes int64) (info ObjectInfo, data []byte, err error) {
+	key = strings.TrimSpace(key)
+	if store == nil {
+		return ObjectInfo{}, nil, fmt.Errorf("read bounded object: store is required")
+	}
+	if key == "" {
+		return ObjectInfo{}, nil, fmt.Errorf("read bounded object: key is required")
+	}
+	if maxBytes <= 0 || maxBytes == math.MaxInt64 {
+		return ObjectInfo{}, nil, fmt.Errorf("read bounded object %q: limit must be between 1 and %d bytes", key, int64(math.MaxInt64-1))
+	}
+	if err := ctx.Err(); err != nil {
+		return ObjectInfo{}, nil, err
+	}
+
+	info, body, err := store.Read(ctx, key)
+	if err != nil {
+		return ObjectInfo{}, nil, err
+	}
+	if body == nil {
+		return ObjectInfo{}, nil, fmt.Errorf("read bounded object %q: store returned no body", key)
+	}
+	defer func() {
+		if closeErr := body.Close(); closeErr != nil {
+			data = nil
+			err = errors.Join(err, fmt.Errorf("close object %q: %w", key, closeErr))
+		}
+	}()
+
+	if info.Size > maxBytes {
+		return info, nil, &ReadLimitError{Key: key, Limit: maxBytes, Observed: info.Size}
+	}
+
+	data, err = io.ReadAll(io.LimitReader(contextReader{ctx: ctx, reader: body}, maxBytes+1))
+	if err != nil {
+		return info, nil, err
+	}
+	if err := ctx.Err(); err != nil {
+		return info, nil, err
+	}
+	if int64(len(data)) > maxBytes {
+		return info, nil, &ReadLimitError{Key: key, Limit: maxBytes, Observed: int64(len(data))}
+	}
+	return info, data, nil
+}
+
+type contextReader struct {
+	ctx    context.Context
+	reader io.Reader
+}
+
+func (r contextReader) Read(p []byte) (int, error) {
+	if err := r.ctx.Err(); err != nil {
+		return 0, err
+	}
+	n, err := r.reader.Read(p)
+	if err == nil {
+		if contextErr := r.ctx.Err(); contextErr != nil {
+			return n, contextErr
+		}
+	}
+	return n, err
+}