Make pulse-sensor-proxy resilient to read-only filesystems

Related to #637

The sensor-proxy was failing to start on systems with read-only filesystems
because audit logging required a writable /var/log/pulse/sensor-proxy directory.

Changes:
- Modified newAuditLogger() to automatically fall back to stderr (systemd journal)
  if the audit log file cannot be opened
- Removed error return from newAuditLogger() since it now always succeeds
- Added warning logs when fallback mode is used to alert operators
- Updated tests to handle the new signature
- Added better debugging to audit log tests

This allows the sensor-proxy to run on:
- Immutable/read-only root filesystems
- Hardened systems with restricted /var mounts
- Containerized environments with limited write access

Audit events are still captured via systemd journal when file logging is
unavailable, maintaining the security audit trail.

cmd/pulse-sensor-proxy/audit.go

@@ -47,18 +47,46 @@ type AuditEvent struct {
 }
 
 // newAuditLogger opens the audit log file and prepares hash chaining.
-func newAuditLogger(path string) (*auditLogger, error) {
+// If the file cannot be opened (e.g., read-only filesystem), it automatically
+// falls back to stderr which integrates with systemd journal.
+// This function always succeeds and returns a valid audit logger.
+func newAuditLogger(path string) *auditLogger {
+	// Try to open the audit log file
 	file, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o640)
+	var writer zerolog.Logger
+	var usedFallback bool
+
 	if err != nil {
-		return nil, err
+		// Fallback to stderr if file cannot be opened
+		log.Warn().
+			Err(err).
+			Str("path", path).
+			Msg("Cannot open audit log file, falling back to stderr (systemd journal)")
+
+		writer = zerolog.New(os.Stderr).With().Timestamp().Logger()
+		usedFallback = true
+		file = nil
+	} else {
+		writer = zerolog.New(file).With().Timestamp().Logger()
 	}
 
-	writer := zerolog.New(file).With().Timestamp().Logger()
+	// Log initialization event to standard logger (not to audit log itself)
+	if usedFallback {
+		log.Warn().
+			Str("path", path).
+			Str("mode", "stderr").
+			Msg("Audit logger initialized with stderr fallback due to filesystem constraints")
+	} else {
+		log.Info().
+			Str("path", path).
+			Str("mode", "file").
+			Msg("Audit logger initialized with file backend")
+	}
 
 	return &auditLogger{
 		file:   file,
 		logger: writer,
-	}, nil
+	}
 }
 
 // Close flushes and closes the audit log file.

cmd/pulse-sensor-proxy/audit_test.go

@@ -18,10 +18,7 @@ func TestAuditLogValidationFailure(t *testing.T) {
     tmp.Close()
     defer os.Remove(path)
 
-    logger, err := newAuditLogger(path)
-    if err != nil {
-        t.Fatalf("newAuditLogger: %v", err)
-    }
+    logger := newAuditLogger(path)
 
     cred := &peerCredentials{uid: 1000, gid: 1000, pid: 4242}
     logger.LogValidationFailure("corr-123", cred, "remote", "get_temperature", []string{"node"}, "invalid_node")
@@ -35,16 +32,25 @@ func TestAuditLogValidationFailure(t *testing.T) {
 
     scanner := bufio.NewScanner(file)
     if !scanner.Scan() {
-        t.Fatalf("expected at least one audit entry")
+        t.Fatalf("expected at least one audit entry (file may be empty)")
     }
 
+    line := scanner.Bytes()
+    if len(line) == 0 {
+        t.Fatalf("empty line in audit log")
+    }
+
+    t.Logf("Audit log line: %s", string(line))
+
     var record auditRecord
-    if err := json.Unmarshal(scanner.Bytes(), &record); err != nil {
-        t.Fatalf("unmarshal: %v", err)
+    if err := json.Unmarshal(line, &record); err != nil {
+        t.Fatalf("unmarshal (line=%s): %v", string(line), err)
     }
 
+    t.Logf("Parsed record: %+v", record)
+
     if record["event_type"] != "command.validation_failed" {
-        t.Fatalf("unexpected event_type: %v", record["event_type"])
+        t.Fatalf("unexpected event_type: %v (full record: %+v)", record["event_type"], record)
     }
     if record["correlation_id"] != "corr-123" {
         t.Fatalf("unexpected correlation id: %v", record["correlation_id"])

cmd/pulse-sensor-proxy/main.go

@@ -354,10 +354,8 @@ func runProxy() {
 		auditPath = defaultAuditLogPath
 	}
 
-	auditLogger, err := newAuditLogger(auditPath)
-	if err != nil {
-		log.Fatal().Err(err).Str("path", auditPath).Msg("Failed to initialize audit logger")
-	}
+	// Initialize audit logger with automatic fallback to stderr if file is unavailable
+	auditLogger := newAuditLogger(auditPath)
 	defer auditLogger.Close()
 
 	// Initialize metrics