Catch exception in download (PaddlePaddle#28090)

* cat exeception in request download; test=develop

* add re-try in download; test=develop

python/paddle/dataset/common.py

@@ -88,27 +88,31 @@ def download(url, module_name, md5sum, save_name=None):
         sys.stderr.write("Cache file %s not found, downloading %s \n" %
                          (filename, url))
         sys.stderr.write("Begin to download\n")
-        r = requests.get(url, stream=True)
-        total_length = r.headers.get('content-length')
-
-        if total_length is None:
-            with open(filename, 'wb') as f:
-                shutil.copyfileobj(r.raw, f)
-        else:
-            with open(filename, 'wb') as f:
-                chunk_size = 4096
-                total_length = int(total_length)
-                total_iter = total_length / chunk_size + 1
-                log_interval = total_iter / 20 if total_iter > 20 else 1
-                log_index = 0
-                for data in r.iter_content(chunk_size=chunk_size):
-                    if six.PY2:
-                        data = six.b(data)
-                    f.write(data)
-                    log_index += 1
-                    if log_index % log_interval == 0:
-                        sys.stderr.write(".")
-                    sys.stdout.flush()
+        try:
+            r = requests.get(url, stream=True)
+            total_length = r.headers.get('content-length')
+
+            if total_length is None:
+                with open(filename, 'wb') as f:
+                    shutil.copyfileobj(r.raw, f)
+            else:
+                with open(filename, 'wb') as f:
+                    chunk_size = 4096
+                    total_length = int(total_length)
+                    total_iter = total_length / chunk_size + 1
+                    log_interval = total_iter / 20 if total_iter > 20 else 1
+                    log_index = 0
+                    for data in r.iter_content(chunk_size=chunk_size):
+                        if six.PY2:
+                            data = six.b(data)
+                        f.write(data)
+                        log_index += 1
+                        if log_index % log_interval == 0:
+                            sys.stderr.write(".")
+                        sys.stdout.flush()
+        except Exception as e:
+            # re-try
+            continue
     sys.stderr.write("\nDownload finished\n")
     sys.stdout.flush()
     return filename