conf: check for buffer errors before virBufferUse

After an OOM error, virBuffer* APIs set buf->use to zero.
Adding a buffer to the parent buffer only if use is non-zero
would quietly drop data on error.

Check the error beforehand to make sure buf->use is zero
because we have not attempted to add anything to it.

src/conf/capabilities.c

@@ -931,6 +931,9 @@ virCapabilitiesFormatCaches(virBufferPtr buf,
                               bank->controls[j]->max_allocation);
         }
 
+        if (virBufferCheckError(&controlBuf) < 0)
+            return -1;
+
         if (virBufferUse(&controlBuf)) {
             virBufferAddLit(buf, ">\n");
             virBufferAddBuffer(buf, &controlBuf);

src/conf/cpu_conf.c

@@ -646,6 +646,10 @@ virCPUDefFormatBufFull(virBufferPtr buf,
     if (virDomainNumaDefCPUFormat(&childrenBuf, numa) < 0)
         goto cleanup;
 
+    if (virBufferCheckError(&attributeBuf) < 0 ||
+        virBufferCheckError(&childrenBuf) < 0)
+        goto cleanup;
+
     /* Put it all together */
     if (virBufferUse(&attributeBuf) || virBufferUse(&childrenBuf)) {
         virBufferAddLit(buf, "<cpu");

src/conf/domain_conf.c

@@ -21573,6 +21573,9 @@ virDomainDiskDefFormat(virBufferPtr buf,
 
     virDomainVirtioOptionsFormat(&driverBuf, def->virtio);
 
+    if (virBufferCheckError(&driverBuf) < 0)
+        return -1;
+
     if (virBufferUse(&driverBuf)) {
         virBufferAddLit(buf, "<driver");
         virBufferAddBuffer(buf, &driverBuf);
@@ -21757,7 +21760,7 @@ virDomainControllerDriverFormat(virBufferPtr buf,
 
     virDomainVirtioOptionsFormat(&driverBuf, def->virtio);
 
-    if (virBufferUse(&driverBuf)) {
+    if (virBufferError(&driverBuf) != 0 || virBufferUse(&driverBuf)) {
         virBufferAddLit(buf, "<driver");
         virBufferAddBuffer(buf, &driverBuf);
         virBufferAddLit(buf, "/>\n");
@@ -21904,6 +21907,9 @@ virDomainControllerDefFormat(virBufferPtr buf,
                           "pcihole64>\n", def->opts.pciopts.pcihole64size);
     }
 
+    if (virBufferCheckError(&childBuf) < 0)
+        return -1;
+
     if (virBufferUse(&childBuf)) {
         virBufferAddLit(buf, ">\n");
         virBufferAddBuffer(buf, &childBuf);
@@ -21975,6 +21981,9 @@ virDomainFSDefFormat(virBufferPtr buf,
 
     virDomainVirtioOptionsFormat(&driverBuf, def->virtio);
 
+    if (virBufferCheckError(&driverBuf) < 0)
+        return -1;
+
     if (virBufferUse(&driverBuf)) {
         virBufferAddLit(buf, "<driver");
         virBufferAddBuffer(buf, &driverBuf);
@@ -23239,6 +23248,9 @@ virDomainMemballoonDefFormat(virBufferPtr buf,
         }
     }
 
+    if (virBufferCheckError(&childrenBuf) < 0)
+        return -1;
+
     if (!virBufferUse(&childrenBuf)) {
         virBufferAddLit(buf, "/>\n");
     } else {
@@ -23325,6 +23337,10 @@ static int virDomainPanicDefFormat(virBufferPtr buf,
     virBufferAdjustIndent(&childrenBuf, indent + 2);
     if (virDomainDeviceInfoFormat(&childrenBuf, &def->info, 0) < 0)
         return -1;
+
+    if (virBufferCheckError(&childrenBuf) < 0)
+        return -1;
+
     if (virBufferUse(&childrenBuf)) {
         virBufferAddLit(buf, ">\n");
         virBufferAddBuffer(buf, &childrenBuf);
@@ -23671,6 +23687,9 @@ virDomainInputDefFormat(virBufferPtr buf,
     if (virDomainDeviceInfoFormat(&childbuf, &def->info, flags) < 0)
         return -1;
 
+    if (virBufferCheckError(&childbuf) < 0)
+        return -1;
+
     if (!virBufferUse(&childbuf)) {
         virBufferAddLit(buf, "/>\n");
     } else {
@@ -24612,6 +24631,9 @@ virDomainCputuneDefFormat(virBufferPtr buf,
                                  def->iothreadids[i]->iothread_id);
     }
 
+    if (virBufferCheckError(&childrenBuf) < 0)
+        return -1;
+
     if (virBufferUse(&childrenBuf)) {
         virBufferAddLit(buf, "<cputune>\n");
         virBufferAddBuffer(buf, &childrenBuf);
@@ -24725,7 +24747,8 @@ virDomainIOMMUDefFormat(virBufferPtr buf,
 
     virBufferAsprintf(buf, "<iommu model='%s'",
                       virDomainIOMMUModelTypeToString(iommu->model));
-    if (virBufferUse(&childBuf)) {
+
+    if (virBufferError(&childBuf) != 0 || virBufferUse(&childBuf)) {
         virBufferAddLit(buf, ">\n");
         virBufferAddBuffer(buf, &childBuf);
         virBufferAddLit(buf, "</iommu>\n");
@@ -24863,6 +24886,10 @@ virDomainDefFormatInternal(virDomainDefPtr def,
         virBufferAdjustIndent(&childrenBuf, -2);
         virBufferAddLit(&childrenBuf, "</device>\n");
     }
+
+    if (virBufferCheckError(&childrenBuf) < 0)
+        goto error;
+
     if (virBufferUse(&childrenBuf)) {
         virBufferAddLit(buf, "<blkiotune>\n");
         virBufferAddBuffer(buf, &childrenBuf);