54783: improve checkptycmd

With the previous (and now fallback) version of checkptycmd(), doing
this:
zpty foo 'while read; do echo hi; done'
zpty (or zpty -t foo)

would hang forever, which seems counterintuitive to the purpose of
listing processes (or testing if a process is running).

3 files changed, 78 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index e220499de..49dab9d42 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2026-07-04  Mikael Magnusson  <mikachu@gmail.com>
+
+	* 54783: Src/Modules/zpty.c, Test/V08zpty.ztst: improve
+	checkptycmd
+
 2026-07-03  Oliver Kiddle  <opk@zsh.org>
 
 	* unposted: Completion/X/Command/_mplayer, Completion/X/Command/_xv:
diff --git a/Src/Modules/zpty.c b/Src/Modules/zpty.c
index 6e201e76b..2f9b80f40 100644
--- a/Src/Modules/zpty.c
+++ b/Src/Modules/zpty.c
@@ -37,6 +37,13 @@
 #endif
 #endif
 
+#ifdef HAVE_POLL_H
+# include <poll.h>
+#endif
+#if defined(HAVE_POLL) && !defined(POLLIN)
+# undef HAVE_POLL
+#endif
+
 /* The number of bytes we normally read when given no pattern and the
  * upper bound on the number of bytes we read (even if we are give a
  * pattern). */
@@ -529,6 +536,59 @@ deleteallptycmds(void)
 
 /**** a better process handling would be nice */
 
+#ifdef HAVE_POLL
+static void
+checkptycmd(Ptycmd cmd)
+{
+    struct pollfd pfd;
+
+    if (cmd->fin)
+        return;
+    pfd.fd = cmd->fd;
+    pfd.events = POLLIN;
+    pfd.revents = 0;
+    if (poll(&pfd, 1, 0) <= 0)
+        return;  /* no events: slave still open, not finished */
+    if (pfd.revents & POLLIN)
+        return;  /* data available, not finished */
+    /* POLLHUP without POLLIN: slave closed, no data */
+    cmd->fin = 1;
+    zclose(cmd->fd);
+}
+#elif defined(FIONREAD)
+static void
+checkptycmd(Ptycmd cmd)
+{
+    int val = 0, select_ret = 0;
+
+    if (cmd->fin)
+        return;
+#ifdef HAVE_SELECT
+    {
+	fd_set fds;
+	struct timeval tv;
+	int ret;
+
+	FD_ZERO(&fds);
+	FD_SET(cmd->fd, &fds);
+	tv.tv_sec = 0;
+	tv.tv_usec = 0;
+	ret = select(cmd->fd + 1, (SELECT_ARG_2_T) &fds, NULL, NULL, &tv);
+	if (ret > 0 && FD_ISSET(cmd->fd, &fds)) {
+	    /* either there are bytes, or the process exited */
+	    select_ret = 1;
+	}
+    }
+#endif
+    if (ioctl(cmd->fd, FIONREAD, (char *) &val) == 0 && val > 0)
+        return;  /* data available, not finished */
+    /* No data (or ioctl failed): check if process is dead */
+    if (select_ret || kill(cmd->pid, 0) < 0) {
+        cmd->fin = 1;
+        zclose(cmd->fd);
+    }
+}
+#else
 static void
 checkptycmd(Ptycmd cmd)
 {
@@ -545,6 +605,7 @@ checkptycmd(Ptycmd cmd)
     }
     cmd->read = (int) c;
 }
+#endif
 
 static int
 ptyread(char *nam, Ptycmd cmd, char **args, int noblock, int mustmatch)
diff --git a/Test/V08zpty.ztst b/Test/V08zpty.ztst
index 057db2e18..cad6bb9bd 100644
--- a/Test/V08zpty.ztst
+++ b/Test/V08zpty.ztst
@@ -25,3 +25,15 @@
   zpty -d cat
 0:zpty with a process that does not set up the terminal: write via stdin
 >a line of text
+
+  zpty loop 'while read; do echo hello; done'
+  zpty -w loop hi
+  zpty -rt loop | tr -d $'\r'
+  zpty -w loop hi
+  zpty -t loop
+  zpty -rt loop | tr -d $'\r'
+  zpty -rt loop
+  zpty -d loop
+0:zpty doesn't duplicate data
+>hello
+>hello