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, 95 insertions, 0 deletions

diff --git a/internal/mapr/globalgroupset_test.go b/internal/mapr/globalgroupset_test.go
new file mode 100644
index 0000000..3e2a9e5
--- /dev/null
+++ b/internal/mapr/globalgroupset_test.go
@@ -0,0 +1,95 @@
+package mapr
+
+import (
+	"testing"
+	"time"
+)
+
+// TestMergeNoblockSemaphoreReleasedOnPanic verifies that MergeNoblock releases
+// the semaphore even when g.merge panics (e.g. due to a nil GroupSet).
+// Without the fix (using defer), the semaphore would be leaked and subsequent
+// calls like NumSets would deadlock forever.
+func TestMergeNoblockSemaphoreReleasedOnPanic(t *testing.T) {
+	g := NewGlobalGroupSet()
+
+	// Calling MergeNoblock with a nil *GroupSet causes a nil-pointer dereference
+	// inside g.merge when it iterates over group.sets. We catch the panic in a
+	// goroutine and verify that the GlobalGroupSet is still usable afterwards.
+	done := make(chan struct{})
+	go func() {
+		defer func() {
+			// Recover the expected panic so the goroutine exits cleanly.
+			if r := recover(); r == nil {
+				t.Errorf("expected a panic from MergeNoblock with nil GroupSet, got none")
+			}
+			close(done)
+		}()
+		// This must panic internally; with the bug the semaphore is never released.
+		//nolint:staticcheck // intentional nil dereference to exercise the panic path
+		g.MergeNoblock(nil, nil) //nolint:errcheck
+	}()
+
+	// Wait for the goroutine to finish (panic recovered).
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal("timed out waiting for MergeNoblock panic to be recovered")
+	}
+
+	// After the panic the semaphore must have been released by the deferred
+	// release in MergeNoblock. If the bug is present NumSets acquires the same
+	// 1-slot semaphore and blocks forever, causing the test to time out.
+	result := make(chan int, 1)
+	go func() {
+		result <- g.NumSets()
+	}()
+
+	select {
+	case n := <-result:
+		if n != 0 {
+			t.Errorf("expected 0 sets in empty GlobalGroupSet, got %d", n)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("NumSets deadlocked: semaphore was not released after MergeNoblock panic (bug reproduced)")
+	}
+}
+
+// TestMergeNoblockNormalOperation verifies the non-panic happy path still works
+// correctly: a successful merge returns (true, nil) and NumSets reflects the
+// merged data.
+func TestMergeNoblockNormalOperation(t *testing.T) {
+	g := NewGlobalGroupSet()
+	group := NewGroupSet()
+
+	// Populate the group set with one entry so there is something to merge.
+	set := NewAggregateSet()
+	set.FValues["count"] = 1
+	group.sets["key1"] = set
+
+	// A minimal query is enough; the merge loop only needs query.Select which
+	// can be empty for this structural test (no select conditions to iterate).
+	query := &Query{}
+
+	merged, err := g.MergeNoblock(query, group)
+	if err != nil {
+		t.Errorf("unexpected error from MergeNoblock: %v", err)
+	}
+	if !merged {
+		t.Error("expected MergeNoblock to return merged=true when semaphore is free")
+	}
+
+	// After merging, NumSets must return 1 and must not deadlock.
+	result := make(chan int, 1)
+	go func() {
+		result <- g.NumSets()
+	}()
+
+	select {
+	case n := <-result:
+		if n != 1 {
+			t.Errorf("expected 1 set after merge, got %d", n)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("NumSets deadlocked after normal MergeNoblock (unexpected)")
+	}
+}