v35-0002-Mix-in-2-bits-of-entropy-into-timestampt-of-UUID.patch

From 328032b5ec850e7eacc135e00e8d5f63c23d8058 Mon Sep 17 00:00:00 2001
From: Andrey Borodin <amborodin@acm.org>
Date: Tue, 19 Nov 2024 22:41:32 +0500
Subject: [PATCH v35 2/2] Mix in 2 bits of entropy into timestampt of UUID on
 MacOS

---
 src/backend/utils/adt/uuid.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/backend/utils/adt/uuid.c b/src/backend/utils/adt/uuid.c
index f8b8b59021..25cbe62803 100644
--- a/src/backend/utils/adt/uuid.c
+++ b/src/backend/utils/adt/uuid.c
@@ -455,7 +455,12 @@ static inline uint64 get_real_time_ns_ascending()
 	ns = tmp.tv_sec * 1000000000L + tmp.tv_nsec;
 
 	/* minimum amount of ns that guarantees step of UUID increased clock precision */
-#define SUB_MILLISECOND_STEP ((NS_PER_MS / (1 << 12)) + 1)
+#if defined(__darwin__) || _MSC_VER
+#define SUB_MILLISECOND_BITS 10
+#else
+#define SUB_MILLISECOND_BITS 12
+#endif
+#define SUB_MILLISECOND_STEP ((NS_PER_MS / (1 << SUB_MILLISECOND_BITS)) + 1)
 	if (previous_ns + SUB_MILLISECOND_STEP >= ns)
 		ns = previous_ns + SUB_MILLISECOND_STEP;
 	previous_ns = ns;
@@ -503,6 +508,17 @@ generate_uuidv7(uint64 ns)
 				(errcode(ERRCODE_INTERNAL_ERROR),
 				 errmsg("could not generate random values")));
 
+#if defined(__darwin__) || _MSC_VER
+	/*
+	 * On MacOS real time is truncted to microseconds. Thus, 2 least
+	 * significant bits of increased_clock_precision are neither random
+	 * (CSPRNG), nor time-dependent (in a sense - truly random). These 2 bits
+	 * are dependent on other time-specific bits, thus they do not contribute
+	 * to uniqueness. To make these bit random we mix in two bits from CSPRNG.
+	 */
+	uuid->data[7] = uuid->data[7] ^ (uuid->data[8] >> 6);
+#endif
+
 	/*
 	 * Set magic numbers for a "version 7" (pseudorandom) UUID, see
 	 * https://www.rfc-editor.org/rfc/rfc9562#name-version-field
-- 
2.39.5 (Apple Git-154)