qemu: Report cmdline output if VM dies early

qemuReadLogOutput early VM death detection is racy and won't always work.
Startup then errors when connecting to the VM monitor. This won't report
the emulator cmdline output which is typically the most useful diagnostic.

Check if the VM has died at the very end of the monitor connection step,
and if so, report the cmdline output.

See also: https://bugzilla.redhat.com/show_bug.cgi?id=581381

src/qemu/qemu_driver.c

@@ -2034,39 +2034,47 @@ static void qemudFreePtyPath(void *payload, const char *name ATTRIBUTE_UNUSED)
     VIR_FREE(payload);
 }
 
+static void
+qemuReadLogFD(int logfd, char *buf, int maxlen, int off)
+{
+    int ret;
+    char *tmpbuf = buf + off;
+
+    ret = saferead(logfd, tmpbuf, maxlen - off - 1);
+    if (ret < 0) {
+        ret = 0;
+    }
+
+    tmpbuf[ret] = '\0';
+}
+
 static int
 qemudWaitForMonitor(struct qemud_driver* driver,
                     virDomainObjPtr vm, off_t pos)
 {
-    char buf[4096]; /* Plenty of space to get startup greeting */
+    char buf[4096] = ""; /* Plenty of space to get startup greeting */
     int logfd;
     int ret = -1;
+    virHashTablePtr paths = NULL;
 
-    if ((logfd = qemudLogReadFD(driver->logDir, vm->def->name, pos))
-        < 0)
+    if ((logfd = qemudLogReadFD(driver->logDir, vm->def->name, pos)) < 0)
         return -1;
 
-    ret = qemudReadLogOutput(vm, logfd, buf, sizeof(buf),
-                             qemudFindCharDevicePTYs,
-                             "console", 30);
-    if (close(logfd) < 0) {
-        char ebuf[4096];
-        VIR_WARN(_("Unable to close logfile: %s"),
-                 virStrerror(errno, ebuf, sizeof ebuf));
-    }
-
-    if (ret < 0)
-        return -1;
+    if (qemudReadLogOutput(vm, logfd, buf, sizeof(buf),
+                           qemudFindCharDevicePTYs,
+                           "console", 30) < 0)
+        goto closelog;
 
     VIR_DEBUG("Connect monitor to %p '%s'", vm, vm->def->name);
-    if (qemuConnectMonitor(driver, vm) < 0)
-        return -1;
+    if (qemuConnectMonitor(driver, vm) < 0) {
+        goto cleanup;
+    }
 
     /* Try to get the pty path mappings again via the monitor. This is much more
      * reliable if it's available.
      * Note that the monitor itself can be on a pty, so we still need to try the
      * log output method. */
-    virHashTablePtr paths = virHashCreate(0);
+    paths = virHashCreate(0);
     if (paths == NULL) {
         virReportOOMError();
         goto cleanup;
@@ -2087,6 +2095,23 @@ cleanup:
         virHashFree(paths, qemudFreePtyPath);
     }
 
+    if (kill(vm->pid, 0) == -1 && errno == ESRCH) {
+        /* VM is dead, any other error raised in the interim is probably
+         * not as important as the qemu cmdline output */
+        qemuReadLogFD(logfd, buf, sizeof(buf), strlen(buf));
+        qemuReportError(VIR_ERR_INTERNAL_ERROR,
+                        _("process exited while connecting to monitor: %s"),
+                        buf);
+        ret = -1;
+    }
+
+closelog:
+    if (close(logfd) < 0) {
+        char ebuf[4096];
+        VIR_WARN(_("Unable to close logfile: %s"),
+                 virStrerror(errno, ebuf, sizeof ebuf));
+    }
+
     return ret;
 }