util: error: Add helpers for saving and restoring of last error

Some cleanup paths overwrite a usefull error message with a less useful
one and we then try to preserve the original message. The handlers added
in this patch will simplify the operations since they are designed right
for the purpose.

src/libvirt_private.syms

@@ -1605,6 +1605,8 @@ ebtablesRemoveForwardAllowIn;
 virDispatchError;
 virErrorCopyNew;
 virErrorInitialize;
+virErrorPreserveLast;
+virErrorRestore;
 virErrorSetErrnoFromLastError;
 virLastErrorIsSystemErrno;
 virRaiseErrorFull;

src/util/virerror.c

@@ -371,6 +371,51 @@ virSaveLastError(void)
     return to;
 }
 
+
+/**
+ * virErrorPreserveLast:
+ * @saveerr: pointer to virErrorPtr for storing last error object
+ *
+ * Preserves the currently set last error (for the thread) into @saveerr so that
+ * it can be restored via virErrorRestore(). @saveerr must be passed to
+ * virErrorRestore()
+ */
+void
+virErrorPreserveLast(virErrorPtr *saveerr)
+{
+    int saved_errno = errno;
+    virErrorPtr lasterr = virGetLastError();
+
+    *saveerr = NULL;
+
+    if (lasterr)
+        *saveerr = virErrorCopyNew(lasterr);
+
+    errno = saved_errno;
+}
+
+
+/**
+ * virErrorRestore:
+ * @savederr: error object holding saved error
+ *
+ * Restores the error passed via @savederr and clears associated memory.
+ */
+void
+virErrorRestore(virErrorPtr *savederr)
+{
+    int saved_errno = errno;
+
+    if (!*savederr)
+        return;
+
+    virSetError(*savederr);
+    virFreeError(*savederr);
+    *savederr = NULL;
+    errno = saved_errno;
+}
+
+
 /**
  * virResetError:
  * @err: pointer to the virError to clean up

src/util/virerror.h

@@ -196,4 +196,7 @@ void virErrorSetErrnoFromLastError(void);
 
 bool virLastErrorIsSystemErrno(int errnum);
 
+void virErrorPreserveLast(virErrorPtr *saveerr);
+void virErrorRestore(virErrorPtr *savederr);
+
 #endif