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, 148 insertions, 0 deletions

diff --git a/internal/io/fs/validatedreadtarget.go b/internal/io/fs/validatedreadtarget.go
new file mode 100644
index 0000000..f5a840c
--- /dev/null
+++ b/internal/io/fs/validatedreadtarget.go
@@ -0,0 +1,148 @@
+package fs
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"unicode"
+)
+
+// JournalSpecPrefix marks a read target as a systemd journal source.
+const JournalSpecPrefix = "journal:"
+
+// ReadTargetKind identifies the backing source for a validated read target.
+type ReadTargetKind int
+
+// Valid read target kinds.
+const (
+	FileKind ReadTargetKind = iota
+	JournalKind
+)
+
+// ValidatedReadTarget stores a resolved regular file path for rooted re-opens.
+type ValidatedReadTarget struct {
+	Kind         ReadTargetKind
+	resolvedPath string
+	rootedPath   RootedPath
+}
+
+// IsJournalSpec reports whether spec names a journal-backed read source.
+func IsJournalSpec(spec string) bool {
+	return strings.HasPrefix(spec, JournalSpecPrefix)
+}
+
+// NewValidatedReadTarget returns a rooted target for a resolved regular file.
+func NewValidatedReadTarget(resolvedPath string) (ValidatedReadTarget, error) {
+	cleanedPath := filepath.Clean(resolvedPath)
+	if !filepath.IsAbs(cleanedPath) {
+		return ValidatedReadTarget{}, fmt.Errorf("validated read target requires absolute path: %s", cleanedPath)
+	}
+
+	info, err := os.Lstat(cleanedPath)
+	if err != nil {
+		return ValidatedReadTarget{}, fmt.Errorf("lstat validated read target %s: %w", cleanedPath, err)
+	}
+	if !info.Mode().IsRegular() {
+		return ValidatedReadTarget{}, fmt.Errorf("validated read target must be a regular file: %s", cleanedPath)
+	}
+
+	rootedPath, err := NewRootedPath(cleanedPath)
+	if err != nil {
+		return ValidatedReadTarget{}, err
+	}
+
+	return ValidatedReadTarget{
+		Kind:         FileKind,
+		resolvedPath: cleanedPath,
+		rootedPath:   rootedPath,
+	}, nil
+}
+
+// NewValidatedJournalTarget returns a validated journal-backed read target.
+func NewValidatedJournalTarget(spec string) (ValidatedReadTarget, error) {
+	if !IsJournalSpec(spec) {
+		return ValidatedReadTarget{}, fmt.Errorf("journal read target requires %q prefix: %s", JournalSpecPrefix, spec)
+	}
+	if err := validateJournalSpec(spec); err != nil {
+		return ValidatedReadTarget{}, err
+	}
+	return ValidatedReadTarget{
+		Kind:         JournalKind,
+		resolvedPath: spec,
+	}, nil
+}
+
+func validateJournalSpec(spec string) error {
+	source := strings.TrimPrefix(spec, JournalSpecPrefix)
+	if source == "" {
+		return fmt.Errorf("journal read target requires a unit name after %q", JournalSpecPrefix)
+	}
+	if strings.HasPrefix(source, "-") {
+		return fmt.Errorf("journal read target unit must not start with '-'")
+	}
+	for _, r := range source {
+		if unicode.IsControl(r) || unicode.IsSpace(r) {
+			return fmt.Errorf("journal read target unit contains invalid whitespace or control character")
+		}
+	}
+	return nil
+}
+
+// Open re-opens the validated file beneath its resolved parent directory.
+func (t ValidatedReadTarget) Open() (*os.File, error) {
+	if t.Kind != FileKind {
+		return nil, fmt.Errorf("read target kind %d cannot be opened as a file", t.Kind)
+	}
+
+	root, err := t.rootedPath.OpenRoot()
+	if err != nil {
+		return nil, fmt.Errorf("open root for %s: %w", t.resolvedPath, err)
+	}
+	defer root.Close()
+
+	if err := t.validateEntry(root); err != nil {
+		return nil, err
+	}
+
+	fd, err := root.Open(t.rootedPath.Name())
+	if err != nil {
+		return nil, fmt.Errorf("open rooted file %s: %w", t.resolvedPath, err)
+	}
+
+	if err := validateOpenedFile(fd, t.resolvedPath); err != nil {
+		fd.Close()
+		return nil, err
+	}
+	if err := t.validateEntry(root); err != nil {
+		fd.Close()
+		return nil, err
+	}
+
+	return fd, nil
+}
+
+func (t ValidatedReadTarget) validateEntry(root *os.Root) error {
+	info, err := root.Lstat(t.rootedPath.Name())
+	if err != nil {
+		return fmt.Errorf("lstat rooted file %s: %w", t.resolvedPath, err)
+	}
+	if info.Mode()&os.ModeSymlink != 0 {
+		return fmt.Errorf("rooted file changed to symlink: %s", t.resolvedPath)
+	}
+	if !info.Mode().IsRegular() {
+		return fmt.Errorf("rooted file changed to non-regular file: %s", t.resolvedPath)
+	}
+	return nil
+}
+
+func validateOpenedFile(fd *os.File, resolvedPath string) error {
+	info, err := fd.Stat()
+	if err != nil {
+		return fmt.Errorf("stat opened file %s: %w", resolvedPath, err)
+	}
+	if !info.Mode().IsRegular() {
+		return fmt.Errorf("opened file is not regular: %s", resolvedPath)
+	}
+	return nil
+}