feat: DTail fork — server/client feature development

Squashed development of the snonux/dtail fork's product code (internal/, cmd/)
since diverging from mimecast/dtail. Major areas:

- Read/output path: the former "turbo" channel-less path is now the single,
  default server-side read/output path for cat/grep/tail and MapReduce; the old
  channel-based path and its config/env toggles were removed.
- MapReduce: single aggregate implementation (server + serverless) fed directly
  by a processor pipeline, with input-exhausted finalization via the shutdown
  coordinator; high-concurrency and data-race fixes.
- Journal source reads (journal:unit.service) via journalctl, Linux-gated behind
  a journal-v1 capability.
- Auth-key fast reconnect: in-memory per-user public-key cache with TTL/max-keys,
  registered over an authenticated session (AUTHKEY), checked before
  authorized_keys.
- Interactive query reload (--interactive-query) with SESSION START/UPDATE
  generation boundaries and capability negotiation.
- Client-side deadlines: --timeout / --shutdownAfter as context deadlines;
  follow shutdown handling.
- Client logging: diagnostics-only daily log by default, opt-in payload tee via
  --log-payload.
- Numerous correctness fixes (buffer-pool double-recycle races, EOF-sentinel
  leaks, glob-expansion cap, TOCTOU in CSV parsing) with accompanying unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 87 insertions, 0 deletions

diff --git a/internal/server/handlers/protocol_codec.go b/internal/server/handlers/protocol_codec.go
new file mode 100644
index 0000000..58c78c0
--- /dev/null
+++ b/internal/server/handlers/protocol_codec.go
@@ -0,0 +1,87 @@
+package handlers
+
+import (
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/mimecast/dtail/internal/io/dlog"
+	"github.com/mimecast/dtail/internal/protocol"
+	user "github.com/mimecast/dtail/internal/user/server"
+)
+
+type protocolCodec struct {
+	user *user.User
+}
+
+func newProtocolCodec(user *user.User) protocolCodec {
+	return protocolCodec{user: user}
+}
+
+func (c protocolCodec) handleProtocolVersion(args []string) ([]string, int, string, error) {
+	argc := len(args)
+	var add string
+
+	if argc <= 2 || args[0] != "protocol" {
+		return args, argc, add, errors.New("unable to determine protocol version")
+	}
+
+	if args[1] != protocol.ProtocolCompat {
+		clientMajor, clientMinor := parseProtocolCompat(args[1])
+		serverMajor, serverMinor := parseProtocolCompat(protocol.ProtocolCompat)
+		if clientMajor <= 3 {
+			// Protocol version 3 or lower expect a newline as message separator
+			// One day (after 2 major versions) this exception may be removed!
+			add = "\n"
+		}
+
+		toUpdate := "client"
+		if clientMajor > serverMajor || (clientMajor == serverMajor && clientMinor > serverMinor) {
+			toUpdate = "server"
+		}
+		err := fmt.Errorf("the DTail server protocol version '%s' does not match "+
+			"client protocol version '%s', please update DTail %s",
+			protocol.ProtocolCompat, args[1], toUpdate)
+		return args, argc, add, err
+	}
+
+	return args[2:], argc - 2, add, nil
+}
+
+func parseProtocolCompat(version string) (int, int) {
+	major := 0
+	minor := 0
+
+	parts := strings.SplitN(version, ".", 2)
+	if len(parts) > 0 {
+		major, _ = strconv.Atoi(parts[0])
+	}
+	if len(parts) == 2 {
+		minor, _ = strconv.Atoi(parts[1])
+	}
+
+	return major, minor
+}
+
+func (c protocolCodec) handleBase64(args []string, argc int) ([]string, int, error) {
+	err := errors.New("unable to decode client message, DTail server and client " +
+		"versions may not be compatible")
+	if argc != 2 || args[0] != "base64" {
+		return args, argc, err
+	}
+
+	decoded, err := base64.StdEncoding.DecodeString(args[1])
+	if err != nil {
+		return args, argc, err
+	}
+	decodedStr := string(decoded)
+
+	args = strings.Split(decodedStr, " ")
+	argc = len(args)
+	dlog.Server.Trace(c.user, "Base64 decoded received command",
+		decodedStr, argc, args)
+
+	return args, argc, nil
+}