Improve error output for master discovery (#2720)

Make MasterNotFoundError exception more precise in the case of
ConnectionError and TimeoutError to help the user to identify
configuration errors

Co-authored-by: Marc Schöchlin <marc.schoechlin@flipapp.de>

CHANGES

@@ -42,6 +42,7 @@
     * Fix Sentinel.execute_command doesn't execute across the entire sentinel cluster bug (#2458)
     * Added a replacement for the default cluster node in the event of failure (#2463)
     * Fix for Unhandled exception related to self.host with unix socket (#2496)
+    * Improve error output for master discovery 
 
 * 4.1.3 (Feb 8, 2022)
     * Fix flushdb and flushall (#1926)

redis/asyncio/sentinel.py

@@ -254,10 +254,12 @@ async def discover_master(self, service_name: str):
         Returns a pair (address, port) or raises MasterNotFoundError if no
         master is found.
         """
+        collected_errors = list()
         for sentinel_no, sentinel in enumerate(self.sentinels):
             try:
                 masters = await sentinel.sentinel_masters()
-            except (ConnectionError, TimeoutError):
+            except (ConnectionError, TimeoutError) as e:
+                collected_errors.append(f"{sentinel} - {e!r}")
                 continue
             state = masters.get(service_name)
             if state and self.check_master_state(state, service_name):
@@ -267,7 +269,11 @@ async def discover_master(self, service_name: str):
                     self.sentinels[0],
                 )
                 return state["ip"], state["port"]
-        raise MasterNotFoundError(f"No master found for {service_name!r}")
+
+        error_info = ""
+        if len(collected_errors) > 0:
+            error_info = f" : {', '.join(collected_errors)}"
+        raise MasterNotFoundError(f"No master found for {service_name!r}{error_info}")
 
     def filter_slaves(
         self, slaves: Iterable[Mapping]

redis/sentinel.py

@@ -230,10 +230,12 @@ def discover_master(self, service_name):
         Returns a pair (address, port) or raises MasterNotFoundError if no
         master is found.
         """
+        collected_errors = list()
         for sentinel_no, sentinel in enumerate(self.sentinels):
             try:
                 masters = sentinel.sentinel_masters()
-            except (ConnectionError, TimeoutError):
+            except (ConnectionError, TimeoutError) as e:
+                collected_errors.append(f"{sentinel} - {e!r}")
                 continue
             state = masters.get(service_name)
             if state and self.check_master_state(state, service_name):
@@ -243,7 +245,11 @@ def discover_master(self, service_name):
                     self.sentinels[0],
                 )
                 return state["ip"], state["port"]
-        raise MasterNotFoundError(f"No master found for {service_name!r}")
+
+        error_info = ""
+        if len(collected_errors) > 0:
+            error_info = f" : {', '.join(collected_errors)}"
+        raise MasterNotFoundError(f"No master found for {service_name!r}{error_info}")
 
     def filter_slaves(self, slaves):
         "Remove slaves that are in an ODOWN or SDOWN state"