diff --git a/src/structs.h b/src/structs.h
index 2a4284a..7a4d7fb 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1499,19 +1499,21 @@
 
 /* Ordering matters, it is used in for loops: IN is last, only SOCK/OUT/ERR
  * are polled. */
-#define PART_SOCK   0
+typedef enum {
+    PART_SOCK = 0,
 #define CH_SOCK_FD	ch_part[PART_SOCK].ch_fd
-
 #ifdef FEAT_JOB_CHANNEL
-# define INVALID_FD  (-1)
-
-# define PART_OUT   1
-# define PART_ERR   2
-# define PART_IN    3
+    PART_OUT,
 # define CH_OUT_FD	ch_part[PART_OUT].ch_fd
+    PART_ERR,
 # define CH_ERR_FD	ch_part[PART_ERR].ch_fd
+    PART_IN,
 # define CH_IN_FD	ch_part[PART_IN].ch_fd
 #endif
+    PART_COUNT
+} ch_part_T;
+
+#define INVALID_FD	(-1)
 
 /* The per-fd info for a channel. */
 typedef struct {
@@ -1566,14 +1568,14 @@
     int		ch_id;		/* ID of the channel */
     int		ch_last_msg_id;	/* ID of the last message */
 
-    chanpart_T	ch_part[4];	/* info for socket, out, err and in */
+    chanpart_T	ch_part[PART_COUNT]; /* info for socket, out, err and in */
 
     char	*ch_hostname;	/* only for socket, allocated */
     int		ch_port;	/* only for socket */
 
-    int		ch_to_be_closed; /* When TRUE reading or writing failed and
-				  * the channel must be closed when it's safe
-				  * to invoke callbacks. */
+    int		ch_to_be_closed; /* bitset of readable fds to be closed.
+				  * When all readable fds have been closed,
+				  * set to (1 << PART_COUNT). */
     int		ch_to_be_freed; /* When TRUE channel must be freed when it's
 				 * safe to invoke callbacks. */
     int		ch_error;	/* When TRUE an error was reported.  Avoids