microsoft/FASTER · error · FasterException

Can spin-wait for commit (checkpoint completion) only if…

Error message

Can spin-wait for commit (checkpoint completion) only if wait is true

What it means

CompletePending(spinWaitForCommit: true) blocks by spinning until a checkpoint/commit finishes, which is only meaningful when the call itself blocks (wait: true). Passing spinWaitForCommit with wait=false would spin on a non-blocking call, so FASTER rejects the combination immediately.

Solutions

Pass wait: true when using spinWaitForCommit: true.
If you do not need to block, drop spinWaitForCommit and call CompletePending(wait: false).
For async scenarios use CompletePendingAsync(waitForCommit: true) instead of spinning.

Example fix

// before
session.CompletePending(wait: false, spinWaitForCommit: true); // throws

// after
session.CompletePending(wait: true, spinWaitForCommit: true);

Defensive patterns

Strategy: validation

Validate before calling

bool wait = true, spinWaitForCommit = true;
if (spinWaitForCommit && !wait)
    throw new ArgumentException("spinWaitForCommit requires wait: true");
session.CompletePending(wait, spinWaitForCommit);

Try / catch

try
{
    session.CompletePending(wait, spinWaitForCommit);
}
catch (FasterException ex) when (ex.Message.Contains("spin-wait"))
{
    session.CompletePending(wait: true, spinWaitForCommit: true);
}

Prevention

Avoid positional booleans; use named arguments for CompletePending.
Wrap session completion in a helper with a valid default combination.
Review call sites that toggle 'wait' to false for leftover spinWaitForCommit flags.

When it happens

Trigger: Calling session.CompletePending(wait: false, spinWaitForCommit: true) (or the CompletePendingWithoutCommit spinWait variant) on a ClientSession.

Common situations: Developers copy a CompletePending(wait: true, spinWaitForCommit: true) call and change wait to false to avoid blocking, forgetting the spinWaitForCommit flag still set; boolean flags passed positionally in the wrong order.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/b1853cb2f2117a2c. Report an issue: GitHub.

Appendix: source

Thrown at cs/src/core/ClientSession/ClientSession.cs:856

            try
            {
                return UnsafeCompletePending(FasterSession, getOutputs, wait, spinWaitForCommit);
            }
            finally
            {
                UnsafeSuspendThread();
            }
        }

        internal bool UnsafeCompletePending<FasterSession>(FasterSession fasterSession, bool getOutputs, bool wait, bool spinWaitForCommit)
            where FasterSession : IFasterSession<Key, Value, Input, Output, Context>
        {
            var requestedOutputs = getOutputs ? this.completedOutputs : default;
            var result = fht.InternalCompletePending(fasterSession, wait, requestedOutputs);
            if (spinWaitForCommit)
            {
                if (!wait)
                    throw new FasterException("Can spin-wait for commit (checkpoint completion) only if wait is true");
                do
                {
                    fht.InternalCompletePending(fasterSession, wait, requestedOutputs);
                    if (fht.InRestPhase())
                    {
                        fht.InternalCompletePending(fasterSession, wait, requestedOutputs);
                        return true;
                    }
                } while (wait);
            }
            return result;
        }

        /// <inheritdoc/>
        public ValueTask CompletePendingAsync(bool waitForCommit = false, CancellationToken token = default)
            => CompletePendingAsync(false, waitForCommit, token);

        /// <inheritdoc/>

View on GitHub (pinned to 321d872eab)