Background

During the review of PR #1899, it was identified that MongoException constructors behave inconsistently with respect to
copying error labels from a cause exception:

• MongoException(final int code, final String msg, final Throwable t) — copies error labels from the cause.
• MongoException(@Nullable final String msg, @Nullable final Throwable t) — does not copy error labels from the cause.

Problem

This inconsistency affects the withTransaction timeout error behavior introduced in DRIVERS-3391. When withTransaction creates a timeout error wrapping the last retried error, error labels (such as
TransientTransactionError and UnknownTransactionCommitResult) from the wrapped exception may not be propagated to the timeout error, depending on which constructor is used.

Since these error labels are publicly documented, they must be reliably accessible on the top-level error
returned to callers.

Action Items

1. Investigate whether the omission of label copying in MongoException(@Nullable final String msg, @Nullable final Throwable t) was intentional or a bug.
2. If it was intentional, document the rationale and determine if withTransaction needs a different approach to propagate labels.
3. If it was a bug, fix the constructor to copy labels from the cause, consistent with the other constructor.

References

• PR #1899 discussion
• Transactions spec — TransientTransactionError
• DRIVERS-3391