infinite loop in xenstat_qmp.c

["Reiser, Hans" <hr@xxxxxxxxxxxxxxxxx>]

Hi,

I have seen several occasions with "dead" xentop processes consuming 100% CPU 
time, and tracked this down
to the following problem:

When the QEMU process the qmp_read function is communicating with terminates, 
qmp_read may enter an
infinite loop:  poll signals EOF (POLLIN and POLLHUP set), the subsequent 
read() call returns 0, and then the
function calls poll again, which still sees the EOF condition and will return 
again immediately with POLLIN and
POLLHUP set, repeating ad infinitum.

A simple fix is to terminate the loop when read returns 0 (under "normal" 
instances, poll will return with POLLIN
set only if there is data to read, so read will always read >0 bytes, except if 
the socket has been closed).

Cheers, Hans

diff --git a/tools/xenstat/libxenstat/src/xenstat_qmp.c 
b/tools/xenstat/libxenstat/src/xenstat_qmp.c
index 19b236e7b6..0c5748ba68 100644
--- a/tools/xenstat/libxenstat/src/xenstat_qmp.c
+++ b/tools/xenstat/libxenstat/src/xenstat_qmp.c
@@ -298,7 +298,7 @@ static int qmp_read(int qfd, unsigned char **qstats)
        pfd[0].events = POLLIN;
        while ((n = poll(pfd, 1, 10)) > 0) {
                if (pfd[0].revents & POLLIN) {
-                       if ((n = read(qfd, buf, sizeof(buf))) < 0) {
+                       if ((n = read(qfd, buf, sizeof(buf))) <= 0) {
                                free(*qstats);
                                return 0;
                        }