From 5c7413f5f174ff124ea5e8b3b9a4717ce054820d Mon Sep 17 00:00:00 2001
From: Maurice Makaay <maurice@h2b.nl>
Date: Fri, 22 Aug 2025 12:55:49 +0200
Subject: [PATCH] Fix generating monotonic ULIDs in the same ms

Due to a race condition, the intended behaviour to provide monotonic
incrementing ULID values, even when they are generated during the same
timestamp in milliseconds, is not reliable.

This patch fixes the issue.
For more information, see the issue report at:

https://github.com/mdomke/python-ulid/issues/45
---
 tests/test_ulid.py |  8 ++++++++
 ulid/__init__.py   | 12 ++++++------
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/tests/test_ulid.py b/tests/test_ulid.py
index 11ae933..f9e0e8e 100644
--- a/tests/test_ulid.py
+++ b/tests/test_ulid.py
@@ -74,6 +74,14 @@ def test_same_millisecond_overflow() -> None:
         ULID()
 
 
+def test_same_millisecond_monotonic_sorting_without_frozen_time() -> None:
+    previous_ulid = "" 
+    for i in range(10000):
+        new_ulid = ULID()
+        assert new_ulid > previous_ulid
+        previous_ulid = new_ulid
+
+
 def assert_sorted(seq: list) -> None:
     last = seq[0]
     for item in seq[1:]:
diff --git a/ulid/__init__.py b/ulid/__init__.py
index d327385..c22b342 100644
--- a/ulid/__init__.py
+++ b/ulid/__init__.py
@@ -69,10 +69,9 @@ def timestamp(self, value: float | None = None) -> int:
             raise ValueError("Value exceeds maximum possible timestamp")
         return value
 
-    def randomness(self) -> bytes:
+    def randomness(self, timestamp: int) -> bytes:
         with self.lock:
-            current_timestamp = self.timestamp()
-            if current_timestamp == self.prev_timestamp:
+            if timestamp == self.prev_timestamp:
                 if self.prev_randomness == constants.MAX_RANDOMNESS:
                     raise ValueError("Randomness within same millisecond exhausted")
                 randomness = self.increment_bytes(self.prev_randomness)
@@ -80,7 +79,7 @@ def randomness(self) -> bytes:
                 randomness = os.urandom(constants.RANDOMNESS_LEN)
 
             self.prev_randomness = randomness
-            self.prev_timestamp = current_timestamp
+            self.prev_timestamp = timestamp
         return randomness
 
     def increment_bytes(self, value: bytes) -> bytes:
@@ -148,8 +147,9 @@ def from_timestamp(cls, value: float) -> Self:
             >>> ULID.from_timestamp(time.time())
             ULID(01E75QWN5HKQ0JAVX9FG1K4YP4)
         """
-        timestamp = int.to_bytes(cls.provider.timestamp(value), constants.TIMESTAMP_LEN, "big")
-        randomness = cls.provider.randomness()
+        timestamp_value = cls.provider.timestamp(value)
+        timestamp = int.to_bytes(timestamp_value, constants.TIMESTAMP_LEN, "big")
+        randomness = cls.provider.randomness(timestamp_value)
         return cls.from_bytes(timestamp + randomness)
 
     @classmethod