UUID: add nil value for UUIDs

Add `BLI_uuid_nil()` that returns the nil UUID (used to indicate "not
set") and `BLI_uuid_is_nil(uuid)` to do an equality test with the nil
value.

source/blender/blenlib/BLI_uuid.h

@@ -37,6 +37,14 @@ extern "C" {
  * This function is not thread-safe. */
 UUID BLI_uuid_generate_random(void);
 
+/**
+ * Return the UUID nil value, consisting of all-zero fields.
+ */
+UUID BLI_uuid_nil(void);
+
+/** Return true iff this is the nil UUID. */
+bool BLI_uuid_is_nil(UUID uuid);
+
 /** Compare two UUIDs, return true if they are equal. */
 bool BLI_uuid_equal(UUID uuid1, UUID uuid2);
 

source/blender/blenlib/intern/uuid.cc

@@ -78,6 +78,17 @@ UUID BLI_uuid_generate_random()
   return uuid;
 }
 
+UUID BLI_uuid_nil(void)
+{
+  const UUID nil = {0, 0, 0, 0, 0, 0};
+  return nil;
+}
+
+bool BLI_uuid_is_nil(UUID uuid)
+{
+  return BLI_uuid_equal(BLI_uuid_nil(), uuid);
+}
+
 bool BLI_uuid_equal(const UUID uuid1, const UUID uuid2)
 {
   return std::memcmp(&uuid1, &uuid2, sizeof(uuid1)) == 0;

source/blender/blenlib/tests/BLI_uuid_test.cc

@@ -48,6 +48,19 @@ TEST(BLI_uuid, generate_many_random)
   }
 }
 
+TEST(BLI_uuid, nil_value)
+{
+  const UUID nil_uuid = BLI_uuid_nil();
+  const UUID zeroes_uuid = {0, 0, 0, 0, 0, 0};
+
+  EXPECT_TRUE(BLI_uuid_equal(nil_uuid, zeroes_uuid));
+  EXPECT_TRUE(BLI_uuid_is_nil(nil_uuid));
+
+  std::string buffer(36, '\0');
+  BLI_uuid_format(buffer.data(), nil_uuid);
+  EXPECT_EQ("00000000-0000-0000-0000-000000000000", buffer);
+}
+
 TEST(BLI_uuid, equality)
 {
   const UUID uuid1 = BLI_uuid_generate_random();