DRIVERS-2782 Re-work phrasing and tests concerning variable snapshotTime access (#1873)

Author: prestonvasquez

source/sessions/snapshot-sessions.md

@@ -137,8 +137,8 @@ class ClientSession {
 }
 ```
 
-The `snapshotTime` field MUST be read-only; in APIs that expose `snapshotTime` with a getter, attempting to read it on a
-non-snapshot session MUST raise an error.
+On a non-snapshot session, the value of `snapshotTime` is undefined. Drivers MUST either raise an error when the
+property is accessed or return an unset value.
 
 Transactions are not allowed with snapshot sessions. Calling `session.startTransaction(options)` on a snapshot session
 MUST raise an error.
@@ -257,6 +257,12 @@ don't have to be changed. This goal is met by defining a `SessionOptions` field
 it aligns better with the existing API, and requires minimal API changes. Future extensibility for snapshot reads would
 be best served by a session-based approach, as no API changes will be required.
 
+### `snapshotTimeout` must be read-only
+
+Allowing callers to mutate `snapshotTime` enables misuse patterns where applications overwrite `snapshotTime` on an
+existing session and then use that session in transactions, combining an inconsistent timestamp with an existing server
+session and triggering hard-to-diagnose write conflicts.
+
 ## Backwards Compatibility
 
 The API changes to support snapshot reads extend the existing API but do not introduce any backward breaking changes.
@@ -271,7 +277,7 @@ C# driver will provide the reference implementation. The corresponding ticket is
 
 ## Changelog
 
-- 2025-12-17: Clarify snapshotTime semantics: the field is either read-only or validated.
+- 2025-12-18: Make snapshot getter optional.
 - 2025-09-23: Exposed snapshotTime to applications.
 - 2024-05-08: Migrated from reStructuredText to Markdown.
 - 2021-06-15: Initial version.

source/sessions/tests/README.md

@@ -270,22 +270,27 @@ Snapshot sessions tests require server of version 5.0 or higher and replica set
 
 Snapshot sessions tests require server of version 5.0 or higher and replica set or a sharded cluster deployment.
 
-- Start a session by calling `startSession` with `snapshot = false`.
-
-Drivers SHOULD implement one of the following approaches:
-
-**Approach 1: Validation (preferred if idiomatic)**
-
+- Start a session by calling `startSession` on with `snapshot = false`.
 - Try to access the session's snapshot time.
 - Assert that a client side error was raised.
+- Assert that the session's `snapshotTime` is unset.
+
+Drivers MAY skip this test if they choose not to implement a getter for `snapshotTime`.
 
-**Approach 2: Ensure Read-Only (if error-returning getters are not idiomatic)**
+### 23. Ensure `snapshotTime` is Read-Only
 
+Snapshot sessions tests require server of version 5.0 or higher and replica set or a sharded cluster deployment.
+
+- Start a session by calling `startSession` on with `snapshot = false`.
+- Assert that the session's `snapshotTime` is unset.
 - Attempt to mutate the session's `snapshotTime` field through any publicly accessible API.
 - Assert that the original session's `snapshotTime` remains unchanged.
 
+Drivers MAY skip this test if they choose to expose a read-only `snapshotTime` property using an accessor method only.
+
 ## Changelog
 
+- 2025-12-18: Make snapshot getter prose test optional.
 - 2025-09-25: Added tests for snapshotTime.
 - 2025-02-24: Test drivers do not gossip $clusterTime on SDAM.
 - 2024-05-08: Migrated from reStructuredText to Markdown.