LogDog: Fix archival Get/Tail implementations.

logdog/common/storage/entry.go

+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package storage
+
+import (
+        "github.com/luci/luci-go/common/errors"
+        "github.com/luci/luci-go/logdog/api/logpb"
+        "github.com/luci/luci-go/logdog/common/types"
+
+        "github.com/golang/protobuf/proto"
+)
+
+// Entry is a logpb.LogEntry wrapper that lazily evaluates / unmarshals the
+// underlying LogEntry data as needed.
+//
+// Entry is not goroutine-safe.
+type Entry struct {
+        D []byte
+
+        streamIndex types.MessageIndex
+        le          *logpb.LogEntry
+}
+
+// MakeEntry creates a new Entry.
+//
+// All Entry must be backed by data. The index, "idx", is optional. If <0, it
+// will be calculated by unmarshalling the data into a LogEntry and pulling the
+// value from there.
+func MakeEntry(d []byte, idx types.MessageIndex) *Entry {
+        return &Entry{
+                D:           d,
+                streamIndex: idx,
+        }
+}
+
+// GetStreamIndex returns the LogEntry's stream index.
+//
+// If this needs to be calculated by unmarshalling the LogEntry, this will be
+// done. If this fails, an error will be returned.
+//
+// If GetLogEntry has succeeded, subsequent GetStreamIndex calls will always
+// be fast and succeed.
+func (e *Entry) GetStreamIndex() (types.MessageIndex, error) {
+        if e.streamIndex < 0 {
+                le, err := e.GetLogEntry()
+                if err != nil {
+                        return -1, err
+                }
+
+                e.streamIndex = types.MessageIndex(le.StreamIndex)
+        }
+
+        return e.streamIndex, nil
+}
+
+// GetLogEntry returns the unmarshalled LogEntry data.
+//
+// The first time this is called, the LogEntry will be unmarshalled from its
+// underlying data.
+func (e *Entry) GetLogEntry() (*logpb.LogEntry, error) {
+        if e.le == nil {
+                if e.D == nil {
+                        // This can happen with keys-only results.
+                        return nil, errors.New("no log entry data")
+                }
+
+                var le logpb.LogEntry
+                if err := proto.Unmarshal(e.D, &le); err != nil {
+                        return nil, errors.Annotate(err).Reason("failed to unmarshal").Err()
+                }
+                e.le = &le
+        }
+
+        return e.le, nil
+}