Add simple dump command

cmd/waldump/.gitignore

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+waldump

cmd/waldump/README.md

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+# waldump
+
+A simple command for dumping the contents of WAL segment files to JSON for
+debugging.
+
+## Usage
+
+```
+$ waldump /path/to/wal/dir [-after INDEX] [-before INDEX]
+...
+{"Index":227281,"Term":4,"Type":0,"Data":"hpGEpUNvb3JkhKpBZGp1c3RtZW50yz7pEPrkTc4tpUVycm9yyz/B4NJg87MZpkhlaWdodMs/ABkEWHeDZqNWZWOYyz8FyF63P/XOyz8Fe2fyqYpayz7eXgvdsOWVyz7xX/ARy9MByz7XZq0fmx5eyz7x8ic7zxhJy78EgvusSgKUy77xVfw2sEr5pE5vZGWiczGpUGFydGl0aW9uoKdTZWdtZW50oA==","Extensions":null,"AppendedAt":"2023-03-23T12:24:05.440317Z"}
+...
+```
+
+Each `raft.Log` is written out as JSON followed by a newline. The `Data` and
+`Extensions` fields are opaque byte strings that will be base64 encoded.
+Decoding those requires knowledge of the encoding used by the writing
+application.
+
+## Limitations
+
+This tool is designed for debugging only. It does _not_ inspect the wal-meta
+database. This has the nice property that you can safely dump the contexts of
+WAL files even while the application is still writing to the WAL since we don't
+have to take a lock on the meta database.
+
+The downside is that this tool might in some edge cases output logs that have
+already been deleted from the WAL. It's possible although extremely unlikely
+that the WAL could be in the process of truncating the tail which could result
+in there being both pre-truncate and post-truncate segment files present. This
+tool might possibly output duplicate and out-of-order log indexes from before
+and after the truncation. Or if `before` and `after` are used, it's possible we
+might skip records entirely because an older file that has already been removed
+was read instead of the newer one. These are all very unlikely in practice and
+if the application that writes the WAL is still up and running are likely to be
+resolved by the time you run the tool again.

cmd/waldump/waldump.go

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+// Copyright (c) HashiCorp, Inc.
+
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"os"
+
+	"github.com/hashicorp/raft"
+	wal "github.com/hashicorp/raft-wal"
+	"github.com/hashicorp/raft-wal/fs"
+	"github.com/hashicorp/raft-wal/segment"
+	"github.com/hashicorp/raft-wal/types"
+)
+
+type opts struct {
+	Dir    string
+	After  uint64
+	Before uint64
+}
+
+func main() {
+	var o opts
+	flag.Uint64Var(&o.After, "after", 0, "specified a raft index to use as an exclusive lower bound when dumping log entries.")
+	flag.Uint64Var(&o.Before, "before", 0, "specified a raft index to use as an exclusive upper bound when dumping log entries.")
+
+	flag.Parse()
+
+	// Accept dir as positional arg
+	o.Dir = flag.Arg(0)
+	if o.Dir == "" {
+		fmt.Println("Usage: waldump <path to WAL dir> [-after INDEX] [-before INDEX]")
+		os.Exit(1)
+	}
+
+	vfs := fs.New()
+	f := segment.NewFiler(o.Dir, vfs)
+
+	codec := &wal.BinaryCodec{}
+	var log raft.Log
+	enc := json.NewEncoder(os.Stdout)
+
+	err := f.DumpLogs(o.After, o.Before, func(info types.SegmentInfo, e types.LogEntry) (bool, error) {
+		if info.Codec != wal.CodecBinaryV1 {
+			return false, fmt.Errorf("unsupported codec %d in file %s", info.Codec, segment.FileName(info))
+		}
+		if err := codec.Decode(e.Data, &log); err != nil {
+			return false, err
+		}
+		// Output the raft Log struct as JSON
+		if err := enc.Encode(log); err != nil {
+			return false, err
+		}
+		return true, nil
+	})
+	if err != nil {
+		fmt.Printf("ERROR: %s\n", err)
+		os.Exit(1)
+	}
+}