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, 54 insertions, 0 deletions

diff --git a/internal/io/dlog/dlog_test.go b/internal/io/dlog/dlog_test.go
new file mode 100644
index 0000000..87e834a
--- /dev/null
+++ b/internal/io/dlog/dlog_test.go
@@ -0,0 +1,54 @@
+package dlog
+
+import "testing"
+
+// TestTraceEnabled verifies that TraceEnabled mirrors Trace's internal
+// maxLevel < Trace early-return: it must report true exactly when the
+// configured level is Trace or higher (Devel/All disable trace? no — the level
+// ladder is monotonically increasing, so any level >= Trace enables trace),
+// and false for every level below Trace.
+func TestTraceEnabled(t *testing.T) {
+	tests := []struct {
+		name  string
+		level level
+		want  bool
+	}{
+		{"none", None, false},
+		{"fatal", Fatal, false},
+		{"error", Error, false},
+		{"warn", Warn, false},
+		{"info", Info, false},
+		{"default", Default, false},
+		{"verbose", Verbose, false},
+		{"debug", Debug, false},
+		{"devel", Devel, false},
+		{"trace", Trace, true},
+		{"all", All, true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			d := &DLog{maxLevel: tc.level}
+			if got := d.TraceEnabled(); got != tc.want {
+				t.Fatalf("TraceEnabled() at level %v = %v, want %v",
+					tc.level, got, tc.want)
+			}
+			// TraceEnabled must agree with what Trace itself would do: Trace
+			// returns "" (no work) exactly when trace is disabled.
+			traceDidWork := d.maxLevel >= Trace
+			if traceDidWork != tc.want {
+				t.Fatalf("TraceEnabled() disagrees with Trace's own gate at level %v", tc.level)
+			}
+		})
+	}
+}
+
+// TestTraceEnabledNilSafe guards the footgun that call sites rely on:
+// TraceEnabled is invoked on the package-level loggers (Server/Client/Common),
+// which are nil until Start runs. A nil receiver must report false, not panic.
+func TestTraceEnabledNilSafe(t *testing.T) {
+	var d *DLog
+	if d.TraceEnabled() {
+		t.Fatal("nil *DLog.TraceEnabled() must be false")
+	}
+}