virCommand: Properly handle POLLHUP

It is a good practise to set revents to zero before doing any poll().
Moreover, we should check if event we waited for really occurred or
if any of fds we were polling on didn't encountered hangup.

src/util/command.c

@@ -1620,16 +1620,19 @@ virCommandProcessIO(virCommandPtr cmd)
         if (infd != -1) {
             fds[nfds].fd = infd;
             fds[nfds].events = POLLOUT;
+            fds[nfds].revents = 0;
             nfds++;
         }
         if (outfd != -1) {
             fds[nfds].fd = outfd;
             fds[nfds].events = POLLIN;
+            fds[nfds].revents = 0;
             nfds++;
         }
         if (errfd != -1) {
             fds[nfds].fd = errfd;
             fds[nfds].events = POLLIN;
+            fds[nfds].revents = 0;
             nfds++;
         }
 
@@ -1645,8 +1648,8 @@ virCommandProcessIO(virCommandPtr cmd)
         }
 
         for (i = 0; i < nfds ; i++) {
-            if (fds[i].fd == errfd ||
-                fds[i].fd == outfd) {
+            if (fds[i].revents & POLLIN &&
+                (fds[i].fd == errfd || fds[i].fd == outfd)) {
                 char data[1024];
                 char **buf;
                 size_t *len;
@@ -1684,7 +1687,10 @@ virCommandProcessIO(virCommandPtr cmd)
                     memcpy(*buf + *len, data, done);
                     *len += done;
                 }
-            } else {
+            }
+
+            if (fds[i].revents & POLLOUT &&
+                fds[i].fd == infd) {
                 int done;
 
                 /* Coverity 5.3.0 can't see that we only get here if
@@ -1710,6 +1716,18 @@ virCommandProcessIO(virCommandPtr cmd)
                 }
             }
 
+            if (fds[i].revents & (POLLHUP | POLLERR)) {
+                if (fds[i].fd == errfd) {
+                    VIR_DEBUG("hangup on stderr");
+                    errfd = -1;
+                } else if (fds[i].fd == outfd) {
+                    VIR_DEBUG("hangup on stdout");
+                    outfd = -1;
+                } else {
+                    VIR_DEBUG("hangup on stdin");
+                    infd = -1;
+                }
+            }
         }
     }