Connect-DbaInstance: Auto-retry with Initial Catalog=master for mirrored SQL Server instances (#10215)

Author: andreasjordan

public/Connect-DbaInstance.ps1

@@ -1130,6 +1130,37 @@ function Connect-DbaInstance {
                     }
                 }
 
+                # Check if the error is about Failover Partner requiring Initial Catalog.
+                # This happens when connecting to a SQL Server instance configured for database mirroring.
+                # The .NET SqlClient sends Failover Partner info from the server's TDS handshake to the
+                # connection pool, and the pool then requires Initial Catalog to be set in the connection string.
+                $isFailoverPartnerError = $errorMessage -match "Failover Partner" -and $errorMessage -match "Initial Catalog"
+                if ($isNewConnection -and $isFailoverPartnerError -and -not $connectionSucceeded -and $inputObjectType -eq 'String') {
+                    Write-Message -Level Verbose -Message "Connection failed because the server is configured for database mirroring (Failover Partner requires Initial Catalog). Retrying with Initial Catalog=master."
+                    Write-Message -Level Debug -Message "Original error: $errorMessage"
+
+                    try {
+                        # Add Initial Catalog=master to satisfy the Failover Partner connection string requirement
+                        if ($server.ConnectionContext.SqlConnectionObject.ConnectionString -notmatch "Initial Catalog" -and $server.ConnectionContext.SqlConnectionObject.ConnectionString -notmatch "Database=") {
+                            if ($server.ConnectionContext.SqlConnectionObject.ConnectionString -match ";$") {
+                                $server.ConnectionContext.SqlConnectionObject.ConnectionString += "Initial Catalog=master;"
+                            } else {
+                                $server.ConnectionContext.SqlConnectionObject.ConnectionString += ";Initial Catalog=master;"
+                            }
+                        }
+
+                        # Retry the connection
+                        Write-Message -Level Debug -Message "Retrying connection with Initial Catalog=master for server with database mirroring"
+                        $null = $server.ConnectionContext.ExecuteWithResults("SELECT 'dbatools is opening a new connection with Initial Catalog'")
+                        Write-Message -Level Verbose -Message "Connection succeeded with Initial Catalog=master for server with database mirroring"
+                        $connectionSucceeded = $true
+                    } catch {
+                        Write-Message -Level Debug -Message "Retry with Initial Catalog=master also failed: $($_.Exception.Message)"
+                        # Keep the original error for reporting
+                        $connectionError = $_
+                    }
+                }
+
                 if (-not $connectionSucceeded) {
                     Stop-Function -Target $instance -Message "Failure" -Category ConnectionError -ErrorRecord $connectionError -Continue
                 }