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, 96 insertions, 0 deletions

diff --git a/internal/io/fs/rootedpath.go b/internal/io/fs/rootedpath.go
new file mode 100644
index 0000000..1369687
--- /dev/null
+++ b/internal/io/fs/rootedpath.go
@@ -0,0 +1,96 @@
+package fs
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// RootedPath scopes file operations to the path's parent directory via os.Root.
+type RootedPath struct {
+	path     string
+	rootDir  string
+	rootName string
+}
+
+// NewRootedPath returns a file path split into a trusted parent directory root
+// and a relative leaf name for os.Root operations.
+func NewRootedPath(path string) (RootedPath, error) {
+	if path == "" {
+		return RootedPath{}, fmt.Errorf("rooted path requires non-empty path")
+	}
+
+	cleanedPath := filepath.Clean(path)
+	if cleanedPath == "." || cleanedPath == string(filepath.Separator) {
+		return RootedPath{}, fmt.Errorf("rooted path requires a file path: %s", path)
+	}
+
+	return RootedPath{
+		path:     path,
+		rootDir:  filepath.Dir(cleanedPath),
+		rootName: filepath.Base(cleanedPath),
+	}, nil
+}
+
+// Name returns the rooted leaf name.
+func (p RootedPath) Name() string {
+	return p.rootName
+}
+
+// Path returns the original file path.
+func (p RootedPath) Path() string {
+	return p.path
+}
+
+// OpenRoot opens the rooted parent directory for this file path.
+func (p RootedPath) OpenRoot() (*os.Root, error) {
+	root, err := os.OpenRoot(p.rootDir)
+	if err != nil {
+		return nil, fmt.Errorf("open root for %s: %w", p.path, err)
+	}
+	return root, nil
+}
+
+// ReadFile reads the rooted file contents.
+func (p RootedPath) ReadFile() ([]byte, error) {
+	root, err := p.OpenRoot()
+	if err != nil {
+		return nil, err
+	}
+	defer root.Close()
+
+	data, err := root.ReadFile(p.rootName)
+	if err != nil {
+		return nil, fmt.Errorf("read rooted file %s: %w", p.path, err)
+	}
+	return data, nil
+}
+
+// Stat stats the rooted file.
+func (p RootedPath) Stat() (os.FileInfo, error) {
+	root, err := p.OpenRoot()
+	if err != nil {
+		return nil, err
+	}
+	defer root.Close()
+
+	info, err := root.Stat(p.rootName)
+	if err != nil {
+		return nil, fmt.Errorf("stat rooted file %s: %w", p.path, err)
+	}
+	return info, nil
+}
+
+// WriteFile writes the rooted file contents.
+func (p RootedPath) WriteFile(data []byte, perm os.FileMode) error {
+	root, err := p.OpenRoot()
+	if err != nil {
+		return err
+	}
+	defer root.Close()
+
+	if err := root.WriteFile(p.rootName, data, perm); err != nil {
+		return fmt.Errorf("write rooted file %s: %w", p.path, err)
+	}
+	return nil
+}