fix(fcm): encode durations without floating-point rounding errors

encode_ttl and encode_milliseconds computed the nanoseconds from the
float returned by timedelta.total_seconds(), so some durations were
sent with rounding errors: an AndroidConfig.ttl of 86400.9 seconds (or
timedelta(days=1, microseconds=900000)) was encoded as
"86400.899999999s", and 1.123457 as "1.123456999s". The Duration
string is now built from the exact days/seconds/microseconds of the
timedelta.

Author: Shubham-Padkonde

firebase_admin/_messaging_encoder.py

@@ -16,7 +16,6 @@
 
 import datetime
 import json
-import math
 import numbers
 import re
 import warnings
@@ -275,14 +274,9 @@ def encode_ttl(cls, ttl):
         if not isinstance(ttl, datetime.timedelta):
             raise ValueError('AndroidConfig.ttl must be a duration in seconds or an instance of '
                              'datetime.timedelta.')
-        total_seconds = ttl.total_seconds()
-        if total_seconds < 0:
+        if ttl < datetime.timedelta(0):
             raise ValueError('AndroidConfig.ttl must not be negative.')
-        seconds = int(math.floor(total_seconds))
-        nanos = int((total_seconds - seconds) * 1e9)
-        if nanos:
-            return f'{seconds}.{str(nanos).zfill(9)}s'
-        return f'{seconds}s'
+        return cls.encode_duration(ttl)
 
     @classmethod
     def encode_milliseconds(cls, label, msec):
@@ -294,11 +288,17 @@ def encode_milliseconds(cls, label, msec):
         if not isinstance(msec, datetime.timedelta):
             raise ValueError(
                 f'{label} must be a duration in milliseconds or an instance of datetime.timedelta.')
-        total_seconds = msec.total_seconds()
-        if total_seconds < 0:
+        if msec < datetime.timedelta(0):
             raise ValueError(f'{label} must not be negative.')
-        seconds = int(math.floor(total_seconds))
-        nanos = int((total_seconds - seconds) * 1e9)
+        return cls.encode_duration(msec)
+
+    @classmethod
+    def encode_duration(cls, duration):
+        """Encodes a non-negative ``datetime.timedelta`` into a protobuf Duration string."""
+        # Use the exact integer fields of the timedelta instead of total_seconds(), which
+        # is a float and can turn e.g. 86400.9 seconds into 86400.899999999s.
+        seconds = duration.days * 86400 + duration.seconds
+        nanos = duration.microseconds * 1000
         if nanos:
             return f'{seconds}.{str(nanos).zfill(9)}s'
         return f'{seconds}s'

tests/test_messaging.py

@@ -466,6 +466,9 @@ def test_android_config(self):
         (123, '123s'),
         (123.45, '123.450000000s'),
         (datetime.timedelta(days=1, seconds=100), '86500s'),
+        (1.123457, '1.123457000s'),
+        (86400.9, '86400.900000000s'),
+        (datetime.timedelta(days=1, microseconds=900000), '86400.900000000s'),
     ])
     def test_android_ttl(self, ttl):
         msg = messaging.Message(