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, 50 insertions, 0 deletions

diff --git a/internal/server/auth_test.go b/internal/server/auth_test.go
new file mode 100644
index 0000000..ecd3c08
--- /dev/null
+++ b/internal/server/auth_test.go
@@ -0,0 +1,50 @@
+package server
+
+import (
+	"crypto/subtle"
+	"testing"
+)
+
+// TestConstantTimePasswordCompare verifies that password comparisons use
+// constant-time comparison to prevent timing side-channel attacks.
+// This is a regression test for the bug where `!=` was used directly,
+// leaking timing information to an attacker who can measure response latency.
+func TestConstantTimePasswordCompare(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		a        string
+		b        string
+		wantSame bool
+	}{
+		{"equal passwords", "secret123", "secret123", true},
+		{"different passwords", "secret123", "wrongpass", false},
+		{"empty vs non-empty", "", "secret", false},
+		{"both empty", "", "", true},
+		{"prefix match only", "secret", "secretXYZ", false},
+		{"suffix match only", "XYZsecret", "secret", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			// secretsEqual is the function under test — it must use
+			// crypto/subtle.ConstantTimeCompare, not plain ==.
+			got := secretsEqual(tt.a, tt.b)
+			if got != tt.wantSame {
+				t.Errorf("secretsEqual(%q, %q) = %v, want %v",
+					tt.a, tt.b, got, tt.wantSame)
+			}
+
+			// Cross-check with the reference implementation to ensure our
+			// helper is semantically correct, not just timing-safe.
+			reference := subtle.ConstantTimeCompare([]byte(tt.a), []byte(tt.b)) == 1
+			if got != reference {
+				t.Errorf("secretsEqual(%q, %q) = %v but crypto/subtle gives %v",
+					tt.a, tt.b, got, reference)
+			}
+		})
+	}
+}