patch 8.0.1170: using termdebug results in 100% CPU time

Problem:    Using termdebug results in 100% CPU time. (tomleb)
Solution:   Use polling instead of select().

src/channel.c

@@ -3960,6 +3960,8 @@ ch_raw_common(typval_T *argvars, typval_T *rettv, int eval)
     free_job_options(&opt);
 }
 
+# define KEEP_OPEN_TIME 20  /* msec */
+
 # if (defined(UNIX) && !defined(HAVE_SELECT)) || defined(PROTO)
 /*
  * Add open channels to the poll struct.
@@ -3967,7 +3969,7 @@ ch_raw_common(typval_T *argvars, typval_T *rettv, int eval)
  * The type of "fds" is hidden to avoid problems with the function proto.
  */
     int
-channel_poll_setup(int nfd_in, void *fds_in)
+channel_poll_setup(int nfd_in, void *fds_in, int *towait)
 {
     int		nfd = nfd_in;
     channel_T	*channel;
@@ -3982,10 +3984,21 @@ channel_poll_setup(int nfd_in, void *fds_in)
 
 	    if (ch_part->ch_fd != INVALID_FD)
 	    {
-		ch_part->ch_poll_idx = nfd;
+		if (channel->ch_keep_open)
-		fds[nfd].fd = ch_part->ch_fd;
+		{
-		fds[nfd].events = POLLIN;
+		    /* For unknown reason poll() returns immediately for a
-		nfd++;
+		     * keep-open channel.  Instead of adding it to the fds add
+		     * a short timeout and check, like polling. */
+		    if (*towait < 0 || *towait > KEEP_OPEN_TIME)
+			*towait = KEEP_OPEN_TIME;
+		}
+		else
+		{
+		    ch_part->ch_poll_idx = nfd;
+		    fds[nfd].fd = ch_part->ch_fd;
+		    fds[nfd].events = POLLIN;
+		    nfd++;
+		}
 	    }
 	    else
 		channel->ch_part[part].ch_poll_idx = -1;
@@ -4021,6 +4034,12 @@ channel_poll_check(int ret_in, void *fds_in)
 		channel_read(channel, part, "channel_poll_check");
 		--ret;
 	    }
+	    else if (channel->ch_part[part].ch_fd != INVALID_FD
+						      && channel->ch_keep_open)
+	    {
+		/* polling a keep-open channel */
+		channel_read(channel, part, "channel_poll_check_keep_open");
+	    }
 	}
 
 	in_part = &channel->ch_part[PART_IN];
@@ -4037,11 +4056,17 @@ channel_poll_check(int ret_in, void *fds_in)
 # endif /* UNIX && !HAVE_SELECT */
 
 # if (!defined(WIN32) && defined(HAVE_SELECT)) || defined(PROTO)
+
 /*
  * The "fd_set" type is hidden to avoid problems with the function proto.
  */
     int
-channel_select_setup(int maxfd_in, void *rfds_in, void *wfds_in)
+channel_select_setup(
+	int maxfd_in,
+	void *rfds_in,
+	void *wfds_in,
+	struct timeval *tv,
+	struct timeval **tvp)
 {
     int		maxfd = maxfd_in;
     channel_T	*channel;
@@ -4057,9 +4082,25 @@ channel_select_setup(int maxfd_in, void *rfds_in, void *wfds_in)
 
 	    if (fd != INVALID_FD)
 	    {
-		FD_SET((int)fd, rfds);
+		if (channel->ch_keep_open)
-		if (maxfd < (int)fd)
+		{
-		    maxfd = (int)fd;
+		    /* For unknown reason select() returns immediately for a
+		     * keep-open channel.  Instead of adding it to the rfds add
+		     * a short timeout and check, like polling. */
+		    if (*tvp == NULL || tv->tv_sec > 0
+					|| tv->tv_usec > KEEP_OPEN_TIME * 1000)
+		    {
+			*tvp = tv;
+			tv->tv_sec = 0;
+			tv->tv_usec = KEEP_OPEN_TIME * 1000;
+		    }
+		}
+		else
+		{
+		    FD_SET((int)fd, rfds);
+		    if (maxfd < (int)fd)
+			maxfd = (int)fd;
+		}
 	    }
 	}
     }
@@ -4094,6 +4135,11 @@ channel_select_check(int ret_in, void *rfds_in, void *wfds_in)
 		FD_CLR(fd, rfds);
 		--ret;
 	    }
+	    else if (fd != INVALID_FD && channel->ch_keep_open)
+	    {
+		/* polling a keep-open channel */
+		channel_read(channel, part, "channel_select_check_keep_open");
+	    }
 	}
 
 	in_part = &channel->ch_part[PART_IN];

src/os_unix.c

@@ -5330,6 +5330,9 @@ mch_job_start(char **argv, job_T *job, jobopt_T *options)
 	    channel = add_channel();
 	if (channel == NULL)
 	    goto failed;
+	if (job->jv_tty_out != NULL)
+	    ch_log(channel, "using pty %s on fd %d",
+					       job->jv_tty_out, pty_master_fd);
     }
 
     BLOCK_SIGNALS(&curset);
@@ -5702,6 +5705,9 @@ mch_create_pty_channel(job_T *job, jobopt_T *options)
 	close(pty_master_fd);
 	return FAIL;
     }
+    if (job->jv_tty_out != NULL)
+	ch_log(channel, "using pty %s on fd %d",
+					       job->jv_tty_out, pty_master_fd);
     job->jv_channel = channel;  /* ch_refcount was set by add_channel() */
     channel->ch_keep_open = TRUE;
 
@@ -5969,7 +5975,7 @@ RealWaitForChar(int fd, long msec, int *check_for_gpm UNUSED, int *interrupted)
 	}
 # endif
 #ifdef FEAT_JOB_CHANNEL
-	nfd = channel_poll_setup(nfd, &fds);
+	nfd = channel_poll_setup(nfd, &fds, &towait);
 #endif
 	if (interrupted != NULL)
 	    *interrupted = FALSE;
@@ -6021,7 +6027,8 @@ RealWaitForChar(int fd, long msec, int *check_for_gpm UNUSED, int *interrupted)
 	}
 # endif
 #ifdef FEAT_JOB_CHANNEL
-	if (ret > 0)
+	/* also call when ret == 0, we may be polling a keep-open channel */
+	if (ret >= 0)
 	    ret = channel_poll_check(ret, &fds);
 #endif
 
@@ -6097,7 +6104,7 @@ select_eintr:
 	}
 # endif
 # ifdef FEAT_JOB_CHANNEL
-	maxfd = channel_select_setup(maxfd, &rfds, &wfds);
+	maxfd = channel_select_setup(maxfd, &rfds, &wfds, &tv, &tvp);
 # endif
 	if (interrupted != NULL)
 	    *interrupted = FALSE;
@@ -6183,7 +6190,8 @@ select_eintr:
 	}
 # endif
 #ifdef FEAT_JOB_CHANNEL
-	if (ret > 0)
+	/* also call when ret == 0, we may be polling a keep-open channel */
+	if (ret >= 0)
 	    ret = channel_select_check(ret, &rfds, &wfds);
 #endif
 

src/proto/channel.pro

@@ -40,9 +40,9 @@ void channel_set_nonblock(channel_T *channel, ch_part_T part);
 int channel_send(channel_T *channel, ch_part_T part, char_u *buf_arg, int len_arg, char *fun);
 void ch_expr_common(typval_T *argvars, typval_T *rettv, int eval);
 void ch_raw_common(typval_T *argvars, typval_T *rettv, int eval);
-int channel_poll_setup(int nfd_in, void *fds_in);
+int channel_poll_setup(int nfd_in, void *fds_in, int *towait);
 int channel_poll_check(int ret_in, void *fds_in);
-int channel_select_setup(int maxfd_in, void *rfds_in, void *wfds_in);
+int channel_select_setup(int maxfd_in, void *rfds_in, void *wfds_in, struct timeval *tv, struct timeval **tvp);
 int channel_select_check(int ret_in, void *rfds_in, void *wfds_in);
 int channel_parse_messages(void);
 int channel_any_readahead(void);

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1170,
 /**/
     1169,
 /**/