[libvirt] [PATCHv2 2/4] qemu: monitor: Add infrastructure to access VM logs for better err msgs

[Peter Krempa]

Early VM startup errors usually produce a better error message in the
machine log file. Currently we were accessing it only when the process
exited during certain phases of startup. This will help adding a more
comprehensive error extraction for early qemu startup phases.

This patch adds infrastructure to keep a file descriptor for the machine
log file that will be used in case an error happens.
---

Notes:
    Version 2:
    - no changes, just explained motivation in reply to previous version

 src/qemu/qemu_monitor.c | 27 +++++++++++++++++++++++++++
 src/qemu/qemu_monitor.h |  2 ++
 2 files changed, 29 insertions(+)

diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
index ecec8e2..c3701fe 100644
--- a/src/qemu/qemu_monitor.c
+++ b/src/qemu/qemu_monitor.c
@@ -88,6 +88,9 @@ struct _qemuMonitor {
     /* If found, path to the virtio memballoon driver */
     char *balloonpath;
     bool ballooninit;
+
+    /* Log file fd of the qemu process to dig for usable info */
+    int logfd;
 };

 static virClassPtr qemuMonitorClass;
@@ -254,6 +257,7 @@ static void qemuMonitorDispose(void *obj)
     VIR_FREE(mon->buffer);
     virJSONValueFree(mon->options);
     VIR_FREE(mon->balloonpath);
+    VIR_FORCE_CLOSE(mon->logfd);
 }


@@ -715,6 +719,7 @@ qemuMonitorOpenInternal(virDomainObjPtr vm,
         return NULL;

     mon->fd = -1;
+    mon->logfd = -1;
     if (virCondInit(&mon->notify) < 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("cannot initialize monitor condition"));
@@ -3843,3 +3848,25 @@ qemuMonitorGetDeviceAliases(qemuMonitorPtr mon,

     return qemuMonitorJSONGetDeviceAliases(mon, aliases);
 }
+
+
+/**
+ * qemuMonitorSetDomainLog:
+ * Set the file descriptor of the open VM log file to report potential
+ * early startup errors of qemu.
+ *
+ * @mon: Monitor object to set the log file reading on
+ * @logfd: File descriptor of the already open log file
+ */
+int
+qemuMonitorSetDomainLog(qemuMonitorPtr mon, int logfd)
+{
+    VIR_FORCE_CLOSE(mon->logfd);
+    if (logfd >= 0 &&
+        (mon->logfd = dup(logfd)) < 0) {
+        virReportSystemError(errno, "%s", _("failed to duplicate log fd"));
+        return -1;
+    }
+
+    return 0;
+}
diff --git a/src/qemu/qemu_monitor.h b/src/qemu/qemu_monitor.h
index 5fe33db..06ba7e8 100644
--- a/src/qemu/qemu_monitor.h
+++ b/src/qemu/qemu_monitor.h
@@ -761,6 +761,8 @@ int qemuMonitorDetachCharDev(qemuMonitorPtr mon,
 int qemuMonitorGetDeviceAliases(qemuMonitorPtr mon,
                                 char ***aliases);

+int qemuMonitorSetDomainLog(qemuMonitorPtr mon, int logfd);
+
 /**
  * When running two dd process and using <> redirection, we need a
  * shell that will not truncate files.  These two strings serve that
-- 
1.8.3.2