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, 117 insertions, 0 deletions

diff --git a/internal/session/spec_test.go b/internal/session/spec_test.go
new file mode 100644
index 0000000..9d4a9c9
--- /dev/null
+++ b/internal/session/spec_test.go
@@ -0,0 +1,117 @@
+package session
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/mimecast/dtail/internal/omode"
+)
+
+func TestSpecStartCommandEncodesPayload(t *testing.T) {
+	t.Parallel()
+
+	spec := Spec{
+		Mode:    omode.TailClient,
+		Files:   []string{"/var/log/app.log"},
+		Options: "plain=true",
+		Regex:   "ERROR",
+		Timeout: 15,
+	}
+
+	command, err := spec.StartCommand()
+	if err != nil {
+		t.Fatalf("StartCommand() error = %v", err)
+	}
+	if !strings.HasPrefix(command, "SESSION START ") {
+		t.Fatalf("unexpected start command prefix: %q", command)
+	}
+
+	var decoded Spec
+	if err := decodeSpecPayload(strings.TrimPrefix(command, "SESSION START "), &decoded); err != nil {
+		t.Fatalf("decode start payload: %v", err)
+	}
+	if !reflect.DeepEqual(decoded, spec) {
+		t.Fatalf("unexpected decoded spec: got %#v want %#v", decoded, spec)
+	}
+}
+
+func TestSpecUpdateCommandIncludesGeneration(t *testing.T) {
+	t.Parallel()
+
+	spec := Spec{
+		Mode:  omode.MapClient,
+		Files: []string{"/var/log/app.log"},
+		Query: "from STATS select count(*)",
+	}
+
+	command, err := spec.UpdateCommand(7)
+	if err != nil {
+		t.Fatalf("UpdateCommand() error = %v", err)
+	}
+	if !strings.HasPrefix(command, "SESSION UPDATE 7 ") {
+		t.Fatalf("unexpected update command prefix: %q", command)
+	}
+
+	var decoded Spec
+	if err := decodeSpecPayload(strings.TrimPrefix(command, "SESSION UPDATE 7 "), &decoded); err != nil {
+		t.Fatalf("decode update payload: %v", err)
+	}
+	if !reflect.DeepEqual(decoded, spec) {
+		t.Fatalf("unexpected decoded spec: got %#v want %#v", decoded, spec)
+	}
+}
+
+func TestSpecHasJournalFiles(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		files []string
+		want  bool
+	}{
+		{
+			name:  "journal file",
+			files: []string{"journal:ssh.service"},
+			want:  true,
+		},
+		{
+			name:  "journal file with surrounding spaces",
+			files: []string{" /var/log/app.log ", " journal:nginx.service "},
+			want:  true,
+		},
+		{
+			name:  "regular file",
+			files: []string{"/var/log/app.log"},
+			want:  false,
+		},
+		{
+			name:  "journal substring is not prefix",
+			files: []string{"/var/log/journal:ssh.service.log"},
+			want:  false,
+		},
+		{
+			name: "empty files",
+			want: false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			spec := Spec{Files: tc.files}
+			if got := spec.HasJournalFiles(); got != tc.want {
+				t.Fatalf("HasJournalFiles() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
+func decodeSpecPayload(payload string, out *Spec) error {
+	raw, err := base64.StdEncoding.DecodeString(payload)
+	if err != nil {
+		return err
+	}
+	return json.Unmarshal(raw, out)
+}