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, 79 insertions, 0 deletions

diff --git a/internal/mapr/groupset_avg_nan_test.go b/internal/mapr/groupset_avg_nan_test.go
new file mode 100644
index 0000000..c3d48a4
--- /dev/null
+++ b/internal/mapr/groupset_avg_nan_test.go
@@ -0,0 +1,79 @@
+package mapr
+
+import (
+	"math"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+// TestGroupSetAvgZeroSamplesDoesNotProduceNaN is a negative test that reproduces
+// the bug where an empty aggregate set (Samples==0) causes 0/0 = NaN in the Avg
+// case of resultSelect. This happens when the server creates a group-set entry
+// via GetSet before confirming that any select fields matched, then serialises and
+// sends the empty set to the client. The client-side resultSelect must guard the
+// Avg division so that Samples==0 yields 0 instead of NaN.
+func TestGroupSetAvgZeroSamplesDoesNotProduceNaN(t *testing.T) {
+	t.Parallel()
+
+	query, err := NewQuery("select avg(latency) from stats group by host")
+	if err != nil {
+		t.Fatalf("Unable to parse query: %v", err)
+	}
+
+	groupSet := NewGroupSet()
+
+	// Simulate what the server does when no log line fields match the select
+	// clause: GetSet creates the entry, but Samples stays 0 and FValues is
+	// never populated. This is the bug trigger — previously 0/0 = NaN.
+	_ = groupSet.GetSet("host-a")
+
+	rows, _, err := groupSet.result(query, false)
+	if err != nil {
+		t.Fatalf("result() returned unexpected error: %v", err)
+	}
+	if len(rows) != 1 {
+		t.Fatalf("Expected 1 result row (even for empty set), got %d", len(rows))
+	}
+
+	// Before the fix each floating-point value in the row was the string "NaN".
+	for _, row := range rows {
+		for _, v := range row.values {
+			trimmed := strings.TrimSpace(v)
+			f, parseErr := strconv.ParseFloat(trimmed, 64)
+			if parseErr != nil {
+				// Non-numeric values (e.g. integer count or last-string fields)
+				// are fine; only floating-point results can be NaN.
+				continue
+			}
+			if math.IsNaN(f) {
+				t.Errorf("avg on empty set produced NaN in output %q; expected 0", v)
+			}
+		}
+	}
+}
+
+// TestGroupSetAvgZeroSamplesResultOutputContainsNoNaN verifies that the
+// higher-level Result method (which drives terminal output) also never emits
+// "NaN" strings when aggregate sets have zero samples.
+func TestGroupSetAvgZeroSamplesResultOutputContainsNoNaN(t *testing.T) {
+	t.Parallel()
+
+	query, err := NewQuery("select avg(latency) from stats group by host")
+	if err != nil {
+		t.Fatalf("Unable to parse query: %v", err)
+	}
+
+	groupSet := NewGroupSet()
+	// Empty set — Samples==0, no FValues populated.
+	_ = groupSet.GetSet("host-a")
+
+	output, _, err := groupSet.Result(query, 100, nil)
+	if err != nil {
+		t.Fatalf("Result() returned unexpected error: %v", err)
+	}
+
+	if strings.Contains(output, "NaN") {
+		t.Errorf("Result output must not contain 'NaN', got:\n%s", output)
+	}
+}