libxl: fix libvirtd segfault

Commit d72ef888 introduced a bug in the libxl driver that will
segfault libvirtd if libxl reports an error message, e.g. when
attempting to initialize the driver on a non-Xen system.  I
assumed it was valid to pass a NULL logger to libxl_ctx_alloc(),
but that is not the case since any errors associated with the ctx
that are emitted by libxl will dereference the logger and crash
libvirtd.

Errors associated with the libxl driver-wide ctx could be useful
for debugging anyway, so create a 'libxl-driver.log' to capture
these errors.

src/libxl/libxl_conf.h

@@ -57,6 +57,9 @@ struct _libxlDriverPrivate {
     virDomainXMLOptionPtr xmlopt;
     unsigned int version;
 
+    /* log stream for driver-wide libxl ctx */
+    FILE *logger_file;
+    xentoollog_logger *logger;
     /* libxl ctx for driver wide ops; getVersion, getNodeInfo, ... */
     libxl_ctx *ctx;
 

src/libxl/libxl_driver.c

@@ -1180,6 +1180,9 @@ libxlStateCleanup(void)
     virObjectUnref(libxl_driver->xmlopt);
     virObjectUnref(libxl_driver->domains);
     libxl_ctx_free(libxl_driver->ctx);
+    xtl_logger_destroy(libxl_driver->logger);
+    if (libxl_driver->logger_file)
+        VIR_FORCE_FCLOSE(libxl_driver->logger_file);
 
     virObjectUnref(libxl_driver->reservedVNCPorts);
 
@@ -1229,6 +1232,7 @@ libxlStateInitialize(bool privileged,
                      void *opaque ATTRIBUTE_UNUSED)
 {
     const libxl_version_info *ver_info;
+    char *log_file = NULL;
     virCommandPtr cmd;
     int status, ret = 0;
     unsigned int free_mem;
@@ -1308,6 +1312,17 @@ libxlStateInitialize(bool privileged,
         goto error;
     }
 
+    if (virAsprintf(&log_file, "%s/libxl-driver.log", libxl_driver->logDir) < 0)
+        goto error;
+
+    if ((libxl_driver->logger_file = fopen(log_file, "a")) == NULL)  {
+        virReportSystemError(errno,
+                             _("failed to create logfile %s"),
+                             log_file);
+        goto error;
+    }
+    VIR_FREE(log_file);
+
     /* read the host sysinfo */
     if (privileged)
         libxl_driver->hostsysinfo = virSysinfoRead();
@@ -1316,8 +1331,18 @@ libxlStateInitialize(bool privileged,
     if (!libxl_driver->domainEventState)
         goto error;
 
-    if (libxl_ctx_alloc(&libxl_driver->ctx, LIBXL_VERSION, 0, NULL)) {
-        VIR_INFO("cannot initialize libxenlight context, probably not running in a Xen Dom0, disabling driver");
+    libxl_driver->logger =
+            (xentoollog_logger *)xtl_createlogger_stdiostream(libxl_driver->logger_file, XTL_DEBUG, 0);
+    if (!libxl_driver->logger) {
+        VIR_INFO("cannot create logger for libxenlight, disabling driver");
+        goto fail;
+    }
+
+    if (libxl_ctx_alloc(&libxl_driver->ctx,
+                       LIBXL_VERSION, 0,
+                       libxl_driver->logger)) {
+        VIR_INFO("cannot initialize libxenlight context, probably not running "
+                 "in a Xen Dom0, disabling driver");
         goto fail;
     }
 
@@ -1383,6 +1408,7 @@ libxlStateInitialize(bool privileged,
 error:
     ret = -1;
 fail:
+    VIR_FREE(log_file);
     if (libxl_driver)
         libxlDriverUnlock(libxl_driver);
     libxlStateCleanup();