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, 13 insertions, 1 deletions

diff --git a/internal/io/fs/tailfile.go b/internal/io/fs/tailfile.go
index 7a40ac4..a5f172d 100644
--- a/internal/io/fs/tailfile.go
+++ b/internal/io/fs/tailfile.go
@@ -6,7 +6,9 @@ type TailFile struct {
 }
 
 // NewTailFile returns a new file tailer.
-func NewTailFile(filePath string, globID string, serverMessages chan<- string) TailFile {
+func NewTailFile(filePath string, globID string, serverMessages chan<- string,
+	maxLineLength int) TailFile {
+
 	return TailFile{
 		readFile: readFile{
 			filePath:       filePath,
@@ -15,6 +17,16 @@ func NewTailFile(filePath string, globID string, serverMessages chan<- string) T
 			retry:          true,
 			canSkipLines:   true,
 			seekEOF:        true,
+			maxLineLength:  maxLineLength,
 		},
 	}
 }
+
+// NewValidatedTailFile returns a new file tailer backed by a rooted open target.
+func NewValidatedTailFile(filePath string, target ValidatedReadTarget, globID string,
+	serverMessages chan<- string, maxLineLength int) TailFile {
+
+	tail := NewTailFile(filePath, globID, serverMessages, maxLineLength)
+	tail.readFile.validatedTarget = &target
+	return tail
+}