[PATCH v2 6/7] qemufile: Always return a verbose error

[Peter Xu <peterx@redhat.com>]

There're a lot of cases where we only have an errno set in last_error but
without a detailed error description.  When this happens, try to generate
an error contains the errno as a descriptive error.

This will be helpful in cases where one relies on the Error*.  E.g.,
migration state only caches Error* in MigrationState.error.  With this,
we'll display correct error messages in e.g. query-migrate when the error
was only set by qemu_file_set_error().

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/qemu-file.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index acc282654a..419b4092e7 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -156,15 +156,24 @@ void qemu_file_set_hooks(QEMUFile *f, const QEMUFileHooks *hooks)
  *
  * Return negative error value if there has been an error on previous
  * operations, return 0 if no error happened.
- * Optional, it returns Error* in errp, but it may be NULL even if return value
- * is not 0.
  *
+ * If errp is specified, a verbose error message will be copied over.
  */
 int qemu_file_get_error_obj(QEMUFile *f, Error **errp)
 {
+    if (!f->last_error) {
+        return 0;
+    }
+
+    /* There is an error */
     if (errp) {
-        *errp = f->last_error_obj ? error_copy(f->last_error_obj) : NULL;
+        if (f->last_error_obj) {
+            *errp = error_copy(f->last_error_obj);
+        } else {
+            error_setg_errno(errp, -f->last_error, "Channel error");
+        }
     }
+
     return f->last_error;
 }
 
-- 
2.41.0