public static async ValueTask <bool> ParseExitCodeAsync(int exitCode, TimeoutValue timeout, CancellationToken cancellationToken)
{
// sp_getapplock exit codes documented at
// https://msdn.microsoft.com/en-us/library/ms189823.aspx
switch (exitCode)
{
case 0:
case 1:
return(true);
case TimeoutExitCode:
return(false);
case -2: // canceled
throw new OperationCanceledException(GetErrorMessage(exitCode, "canceled"));
case -3: // deadlock
throw new DeadlockException(GetErrorMessage(exitCode, "deadlock"));
case -999: // parameter / unknown
throw new ArgumentException(GetErrorMessage(exitCode, "parameter validation or other error"));
case InvalidUpgradeExitCode:
// should never happen unless something goes wrong (e. g. user manually releases the lock on an externally-owned connection)
throw new InvalidOperationException("Cannot upgrade to an exclusive lock because the update lock is not held");
case AlreadyHeldExitCode:
return(timeout.IsZero ? false
: timeout.IsInfinite ? throw new DeadlockException("Attempted to acquire a lock that is already held on the same connection")
: await WaitThenReturnFalseAsync().ConfigureAwait(false));
default:
if (exitCode <= 0)
{
throw new InvalidOperationException(GetErrorMessage(exitCode, "unknown"));
}
return(true); // unknown success code
}
async ValueTask <bool> WaitThenReturnFalseAsync()
{
await SyncViaAsync.Delay(timeout, cancellationToken).ConfigureAwait(false);
return(false);
}
}
public async ValueTask <object?> TryAcquireAsync(DatabaseConnection connection, string resourceName, TimeoutValue timeout, CancellationToken cancellationToken)
{
const string SavePointName = "medallion_threading_postgres_advisory_lock_acquire";
var key = new PostgresAdvisoryLockKey(resourceName);
var hasTransaction = await HasTransactionAsync(connection).ConfigureAwait(false);
if (hasTransaction)
{
// Our acquire command will use SET LOCAL to set up statement timeouts. This lasts until the end
// of the current transaction instead of just the current batch if we're in a transaction. To make sure
// we don't leak those settings, in the case of a transaction we first set up a save point which we can
// later roll back (taking the settings changes with it but NOT the lock). Because we can't confidently
// roll back a save point without knowing that it has been set up, we start the save point in its own
// query before we try-catch
using var setSavePointCommand = connection.CreateCommand();
setSavePointCommand.SetCommandText("SAVEPOINT " + SavePointName);
await setSavePointCommand.ExecuteNonQueryAsync(cancellationToken).ConfigureAwait(false);
}
using var acquireCommand = this.CreateAcquireCommand(connection, key, timeout);
int acquireCommandResult;
try
{
acquireCommandResult = (int)await acquireCommand.ExecuteScalarAsync(cancellationToken).ConfigureAwait(false);
}
catch (Exception ex)
{
await RollBackTransactionTimeoutVariablesIfNeededAsync().ConfigureAwait(false);
if (ex is PostgresException postgresException)
{
switch (postgresException.SqlState)
{
// lock_timeout error code from https://www.postgresql.org/docs/10/errcodes-appendix.html
case "55P03":
return(null);
// deadlock_detected error code from https://www.postgresql.org/docs/10/errcodes-appendix.html
case "40P01":
throw new DeadlockException($"The request for the distributed lock failed with exit code '{postgresException.SqlState}' (deadlock_detected)", ex);
}
}
if (ex is OperationCanceledException && cancellationToken.IsCancellationRequested)
{
// if we bailed in the middle of an acquire, make sure we didn't leave a lock behind
await this.ReleaseAsync(connection, key, isTry : true).ConfigureAwait(false);
}
throw;
}
await RollBackTransactionTimeoutVariablesIfNeededAsync().ConfigureAwait(false);
switch (acquireCommandResult)
{
case 0: return(null);
case 1: return(Cookie);
case AlreadyHeldReturnCode:
if (timeout.IsZero)
{
return(null);
}
if (timeout.IsInfinite)
{
throw new DeadlockException("Attempted to acquire a lock that is already held on the same connection");
}
await SyncViaAsync.Delay(timeout, cancellationToken).ConfigureAwait(false);
return(null);
default:
throw new InvalidOperationException($"Unexpected return code {acquireCommandResult}");
}
async ValueTask RollBackTransactionTimeoutVariablesIfNeededAsync()
{
if (hasTransaction)
{
// attempt to clear the timeout variables we set
using var rollBackSavePointCommand = connection.CreateCommand();
rollBackSavePointCommand.SetCommandText("ROLLBACK TO SAVEPOINT " + SavePointName);
await rollBackSavePointCommand.ExecuteNonQueryAsync(CancellationToken.None).ConfigureAwait(false);
}
}
}
private static ValueTask <bool> ProcessAcquireResultAsync(
IDataParameterCollection parameters,
TimeoutValue timeout,
CancellationToken cancellationToken,
out string?markerTableName,
out string?ticketLockName)
{
var resultCode = (int)((IDbDataParameter)parameters[ResultCodeParameter]).Value;
switch (resultCode)
{
case SuccessCode:
ticketLockName = (string)((IDbDataParameter)parameters[TicketLockNameParameter]).Value;
markerTableName = (string)((IDbDataParameter)parameters[MarkerTableNameParameter]).Value;
return(true.AsValueTask());
case FinishedPreambleWithoutAcquiringCode:
ticketLockName = null;
markerTableName = (string)((IDbDataParameter)parameters[MarkerTableNameParameter]).Value;
return(false.AsValueTask());
case FailedToAcquireWithSpaceRemainingCode:
throw new InvalidOperationException($"An internal semaphore algorithm error ({resultCode}) occurred: failed to acquire a ticket despite indication that tickets are available");
case BusyWaitTimeoutCode:
ticketLockName = markerTableName = null;
return(false.AsValueTask());
case AllTicketsHeldByCurrentSessionCode:
// whenever we hit this case, it's a deadlock. If the user asked us to wait forever, we just throw. However,
// if the user asked us to wait a specified amount of time we will wait in C#. There are other justifiable policies
// but this one seems relatively safe and likely to do what you want. It seems reasonable that no one intends to hang
// forever but also reasonable that someone should be able to test for lock acquisition without getting a throw
if (timeout.IsInfinite)
{
throw new DeadlockException("Deadlock detected: attempt to acquire the semaphore cannot succeed because all tickets are held by the current connection");
}
ticketLockName = markerTableName = null;
async ValueTask <bool> DelayFalseAsync()
{
await SyncViaAsync.Delay(timeout, cancellationToken).ConfigureAwait(false);
return(false);
}
return(DelayFalseAsync());
case SqlApplicationLock.TimeoutExitCode:
ticketLockName = markerTableName = null;
return(false.AsValueTask());
default:
ticketLockName = markerTableName = null;
return(FailAsync());
async ValueTask <bool> FailAsync()
{
if (resultCode < 0)
{
await SqlApplicationLock.ParseExitCodeAsync(resultCode, timeout, cancellationToken).ConfigureAwait(false);
}
throw new InvalidOperationException($"Unexpected semaphore algorithm result code {resultCode}");
}
}
}