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, 43 insertions, 0 deletions

diff --git a/internal/cli/authkeyflags.go b/internal/cli/authkeyflags.go
new file mode 100644
index 0000000..4c945d3
--- /dev/null
+++ b/internal/cli/authkeyflags.go
@@ -0,0 +1,43 @@
+package cli
+
+import (
+	"flag"
+
+	"github.com/mimecast/dtail/internal/config"
+)
+
+const authKeyPathHelpText = "Path to auth key/private key (defaults to ~/.ssh/id_rsa via config)"
+
+// BindAuthKeyFlags registers the legacy and current auth-key flags.
+func BindAuthKeyFlags(fs *flag.FlagSet, legacyKey *string, args *config.Args) {
+	fs.StringVar(legacyKey, "key", "", "Deprecated alias for -auth-key-path")
+	fs.StringVar(&args.SSHPrivateKeyFilePath, "auth-key-path", "", authKeyPathHelpText)
+}
+
+// FlagWasSet reports whether the named flag was explicitly set on the command line.
+func FlagWasSet(name string) bool {
+	var wasSet bool
+	flag.Visit(func(f *flag.Flag) {
+		if f.Name == name {
+			wasSet = true
+		}
+	})
+	return wasSet
+}
+
+// ApplyAuthKeyPathCompatibility copies the deprecated legacy key into args when
+// the new flag was not explicitly set. If both were set, the new flag wins and
+// a warning message is returned.
+func ApplyAuthKeyPathCompatibility(args *config.Args, legacyKey string, authKeyPathSet bool) string {
+	if authKeyPathSet {
+		if legacyKey != "" {
+			return "WARN: -key is deprecated; ignoring it because -auth-key-path was also set"
+		}
+		return ""
+	}
+
+	if legacyKey != "" {
+		args.SSHPrivateKeyFilePath = legacyKey
+	}
+	return ""
+}