SqlConnection.OpenAsync() may block on network i/o during connection creation

please see the stack trace below.

SqlConnection.OpenAsync() has a code path that does synchronous blocking network i/o to the sql server.

my use case is a sql server reboot during a client request. the stack below was called from a u/i thread which subsequently hung the application due to OpenAsync() doing synchronous network i/o rather than awaiting.

(version 2.1.0 netcore)

```
[Managed to Native Transition]
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIOpenSyncEx(Microsoft.Data.SqlClient.SNINativeMethodWrapper.ConsumerInfo consumerInfo, string constring, ref System.IntPtr pConn, byte[] spnBuffer, byte[] instanceName, bool fOverrideCache, bool fSync, int timeout, bool fParallel, Microsoft.Data.SqlClient.SQLDNSInfo cachedDNSInfo) Line 405 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SNIHandle.SNIHandle(Microsoft.Data.SqlClient.SNINativeMethodWrapper.ConsumerInfo myInfo, string serverName, byte[] spnBuffer, bool ignoreSniOpenTimeout, int timeout, out byte[] instanceName, bool flushCache, bool fSync, bool fParallel, Microsoft.Data.SqlClient.SQLDNSInfo cachedDNSInfo) Line 161 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.TdsParserStateObjectNative.CreatePhysicalSNIHandle(string serverName, bool ignoreSniOpenTimeout, long timerExpire, out byte[] instanceName, ref byte[] spnBuffer, bool flushCache, bool async, bool fParallel, string cachedFQDN, ref Microsoft.Data.SqlClient.SQLDNSInfo pendingDNSInfo, bool isIntegratedSecurity) Line 175 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.TdsParser.Connect(Microsoft.Data.SqlClient.ServerInfo serverInfo, Microsoft.Data.SqlClient.SqlInternalConnectionTds connHandler, bool ignoreSniOpenTimeout, long timerExpire, bool encrypt, bool trustServerCert, bool integratedSecurity, bool withFailover, Microsoft.Data.SqlClient.SqlAuthenticationMethod authType) Line 445 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(Microsoft.Data.SqlClient.ServerInfo serverInfo, string newPassword, System.Security.SecureString newSecurePassword, bool ignoreSniOpenTimeout, Microsoft.Data.ProviderBase.TimeoutTimer timeout, bool withFailover) Line 1880 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(Microsoft.Data.SqlClient.ServerInfo serverInfo, string newPassword, System.Security.SecureString newSecurePassword, bool redirectedUserInstance, Microsoft.Data.SqlClient.SqlConnectionString connectionOptions, Microsoft.Data.SqlClient.SqlCredential credential, Microsoft.Data.ProviderBase.TimeoutTimer timeout) Line 1511 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(Microsoft.Data.ProviderBase.TimeoutTimer timeout, Microsoft.Data.SqlClient.SqlConnectionString connectionOptions, Microsoft.Data.SqlClient.SqlCredential credential, string newPassword, System.Security.SecureString newSecurePassword, bool redirectedUserInstance) Line 1414 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlInternalConnectionTds.SqlInternalConnectionTds(Microsoft.Data.ProviderBase.DbConnectionPoolIdentity identity, Microsoft.Data.SqlClient.SqlConnectionString connectionOptions, Microsoft.Data.SqlClient.SqlCredential credential, object providerInfo, string newPassword, System.Security.SecureString newSecurePassword, bool redirectedUserInstance, Microsoft.Data.SqlClient.SqlConnectionString userConnectionOptions, Microsoft.Data.SqlClient.SessionData reconnectSessionData, bool applyTransientFaultHandling, string accessToken, Microsoft.Data.ProviderBase.DbConnectionPool pool) Line 507 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlConnectionFactory.CreateConnection(Microsoft.Data.Common.DbConnectionOptions options, Microsoft.Data.Common.DbConnectionPoolKey poolKey, object poolGroupProviderInfo, Microsoft.Data.ProviderBase.DbConnectionPool pool, System.Data.Common.DbConnection owningConnection, Microsoft.Data.Common.DbConnectionOptions userOptions) Line 133 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(Microsoft.Data.ProviderBase.DbConnectionPool pool, System.Data.Common.DbConnection owningObject, Microsoft.Data.Common.DbConnectionOptions options, Microsoft.Data.Common.DbConnectionPoolKey poolKey, Microsoft.Data.Common.DbConnectionOptions userOptions) Line 138 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionPool.CreateObject(System.Data.Common.DbConnection owningObject, Microsoft.Data.Common.DbConnectionOptions userOptions, Microsoft.Data.ProviderBase.DbConnectionInternal oldConnection) Line 726 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionPool.UserCreateRequest(System.Data.Common.DbConnection owningObject, Microsoft.Data.Common.DbConnectionOptions userOptions, Microsoft.Data.ProviderBase.DbConnectionInternal oldConnection) Line 1737 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionPool.TryGetConnection(System.Data.Common.DbConnection owningObject, uint waitForMultipleObjectsTimeout, bool allowCreate, bool onlyOneCheckConnection, Microsoft.Data.Common.DbConnectionOptions userOptions, out Microsoft.Data.ProviderBase.DbConnectionInternal connection) Line 1256 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionPool.TryGetConnection(System.Data.Common.DbConnection owningObject, System.Threading.Tasks.TaskCompletionSource retry, Microsoft.Data.Common.DbConnectionOptions userOptions, out Microsoft.Data.ProviderBase.DbConnectionInternal connection) Line 1114 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionFactory.TryGetConnection(System.Data.Common.DbConnection owningConnection, System.Threading.Tasks.TaskCompletionSource retry, Microsoft.Data.Common.DbConnectionOptions userOptions, Microsoft.Data.ProviderBase.DbConnectionInternal oldConnection, out Microsoft.Data.ProviderBase.DbConnectionInternal connection) Line 121 C#
> Microsoft.Data.SqlClient.dll!Microsoft.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(System.Data.Common.DbConnection outerConnection, Microsoft.Data.ProviderBase.DbConnectionFactory connectionFactory, System.Threading.Tasks.TaskCompletionSource retry, Microsoft.Data.Common.DbConnectionOptions userOptions) Line 338 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlConnection.TryOpen(System.Threading.Tasks.TaskCompletionSource retry, Microsoft.Data.SqlClient.SqlConnectionOverrides overrides) Line 1721 C#
Microsoft.Data.SqlClient.dll!Microsoft.Data.SqlClient.SqlConnection.OpenAsync(System.Threading.CancellationToken cancellationToken) Line 1474 C#
... my code
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