fix json stream decode performance issue

string slicing is expensive as python copies the (immutable) string...

pqm

@@ -184,6 +184,21 @@ class QueueName(enum.Enum):
 ALL_QUEUE_NAMES: set[QueueName] = set(QueueName)
 
 
+def json_decode_stream(data: str):
+    decoder = json.JSONDecoder()
+    data_len = len(data)
+    data_pos = 0
+    while data_len > data_pos and data[data_len-1].isspace():
+        data_len -= 1
+    while True:
+        while data_pos < data_len and data[data_pos].isspace():
+            data_pos += 1
+        if data_pos >= data_len:
+            return
+        obj, data_pos = decoder.raw_decode(data, data_pos)
+        yield obj
+
+
 @dataclasses.dataclass
 class Mail:
     """Metadata for mail in postfix queue"""
@@ -209,11 +224,7 @@ class Mail:
     @staticmethod
     def read_postqueue_json(data: str, id_prefix: str = '') -> list[Mail]:
         queue = []
-        decoder = json.JSONDecoder()
-        data = data.strip()
-        while data:
-            obj, end = decoder.raw_decode(data, 0)
-            data = data[end:].lstrip()
+        for obj in json_decode_stream(data):
             mail = Mail.from_json(obj)
             mail.queue_id = id_prefix + mail.queue_id
             queue.append(mail)