Fix known-hosts trust deadlock, host key stat, and optional nozstd build

- stdout logger: release mutex while waiting on pause resume so prompt
  callbacks can log (fixes hang after trusting new hosts; known_hosts
  was written but Resume never ran).
- known hosts callback: stop borrowing the SSH dial throttle channel
  (could block or interact badly with parallel handshakes).
- host key path: use errors.Is(..., fs.ErrNotExist) for RootedPath.Stat
  wrapped errors; stat errors now fail fast instead of mis-read.
- public key path: same ErrNotExist check for authorized_keys miss.
- Build: optional DTAIL_NO_ZSTD=yes / nozstd tag for CGO-free builds;
  split zstd readers into tagged files.
- Docs/examples: firewalld note for port 2222, log prune timer+script,
  SSHBindAddress note, dserver unit disabled-by-default comment;
  firewalld helper script example.
- Regression test for stdout pause/mutex behavior.

Made-with: Cursor

5 files changed, 79 insertions, 9 deletions

diff --git a/internal/io/dlog/loggers/stdout.go b/internal/io/dlog/loggers/stdout.go
index a2575c8..b4e695a 100644
--- a/internal/io/dlog/loggers/stdout.go
+++ b/internal/io/dlog/loggers/stdout.go
@@ -44,14 +44,17 @@ func (s *stdout) RawWithColors(now time.Time, message, coloredMessage string) {
 
 func (s *stdout) log(message string, nl bool) {
 	s.mutex.Lock()
-	defer s.mutex.Unlock()
-
 	select {
 	case <-s.pauseCh:
-		// Pause until resumed.
+		// Wait for Resume without holding the mutex: the prompt path calls
+		// dlog after the user answers while Pause is still active; holding the
+		// mutex here would deadlock (Info blocks on Lock, Resume never runs).
+		s.mutex.Unlock()
 		<-s.resumeCh
+		s.mutex.Lock()
 	default:
 	}
+	defer s.mutex.Unlock()
 
 	if nl {
 		fmt.Println(message)
diff --git a/internal/io/dlog/loggers/stdout_test.go b/internal/io/dlog/loggers/stdout_test.go
new file mode 100644
index 0000000..4f70efc
--- /dev/null
+++ b/internal/io/dlog/loggers/stdout_test.go
@@ -0,0 +1,36 @@
+package loggers
+
+import (
+	"testing"
+	"time"
+)
+
+// Regression: during an interactive prompt, dlog.Common.Pause() unblocks when some
+// goroutine hits stdout.log(); that goroutine must not hold the stdout mutex while
+// waiting on resume, or dlog.Client.Info from the prompt callback deadlocks forever.
+func TestStdoutSecondLogDuringPauseWaitDoesNotDeadlock(t *testing.T) {
+	s := newStdout()
+
+	go s.Pause()
+	time.Sleep(50 * time.Millisecond)
+
+	go func() {
+		s.Log(time.Now(), "first log consumes pause and waits on resume")
+	}()
+	time.Sleep(50 * time.Millisecond)
+
+	secondDone := make(chan struct{})
+	go func() {
+		s.Log(time.Now(), "second log must acquire mutex while first waits for Resume")
+		close(secondDone)
+	}()
+
+	select {
+	case <-secondDone:
+	case <-time.After(2 * time.Second):
+		t.Fatal("deadlock: second Log blocked on mutex while first waits for Resume")
+	}
+
+	s.Resume()
+	time.Sleep(50 * time.Millisecond)
+}
diff --git a/internal/io/fs/readfile.go b/internal/io/fs/readfile.go
index d305c4d..5241556 100644
--- a/internal/io/fs/readfile.go
+++ b/internal/io/fs/readfile.go
@@ -19,8 +19,6 @@ import (
 	"github.com/mimecast/dtail/internal/io/pool"
 	"github.com/mimecast/dtail/internal/lcontext"
 	"github.com/mimecast/dtail/internal/regex"
-
-	"github.com/DataDog/zstd"
 )
 
 type readStatus int
@@ -193,10 +191,7 @@ func (f *readFile) makeCompressedFileReader(fd *os.File) (reader *bufio.Reader,
 		decompressor = gzipReader
 		reader = bufio.NewReader(gzipReader)
 	case strings.HasSuffix(f.FilePath(), ".zst"):
-		dlog.Common.Info(f.FilePath(), "Detected zstd compression format")
-		zstdReader := zstd.NewReader(fd)
-		decompressor = zstdReader
-		reader = bufio.NewReader(zstdReader)
+		return f.makeZstdReader(fd)
 	default:
 		reader = bufio.NewReader(fd)
 	}
diff --git a/internal/io/fs/readfile_nozstd.go b/internal/io/fs/readfile_nozstd.go
new file mode 100644
index 0000000..afd4523
--- /dev/null
+++ b/internal/io/fs/readfile_nozstd.go
@@ -0,0 +1,16 @@
+//go:build nozstd
+
+package fs
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+)
+
+func (f *readFile) makeZstdReader(fd *os.File) (reader *bufio.Reader, decompressor io.Closer, err error) {
+	_ = fd
+	err = fmt.Errorf("%s: zstd is not supported in this build (built with -tags nozstd)", f.FilePath())
+	return
+}
diff --git a/internal/io/fs/readfile_zstd.go b/internal/io/fs/readfile_zstd.go
new file mode 100644
index 0000000..a7e479b
--- /dev/null
+++ b/internal/io/fs/readfile_zstd.go
@@ -0,0 +1,20 @@
+//go:build !nozstd
+
+package fs
+
+import (
+	"bufio"
+	"io"
+	"os"
+
+	"github.com/DataDog/zstd"
+	"github.com/mimecast/dtail/internal/io/dlog"
+)
+
+func (f *readFile) makeZstdReader(fd *os.File) (reader *bufio.Reader, decompressor io.Closer, err error) {
+	dlog.Common.Info(f.FilePath(), "Detected zstd compression format")
+	zstdReader := zstd.NewReader(fd)
+	decompressor = zstdReader
+	reader = bufio.NewReader(zstdReader)
+	return
+}