workload validation before registering elektron.

Added a utility to help with validating structs. This utility accepts
validators and runs them. If any of the validators fail, then the
error is wrapped with a given base message and returned.

Added validators for checking different attributes of a task
definition.

Added test code to test task validators.

Retrofitted scheduler.go to just log the task validation error and
terminate. If task validation does not report any error, then the
tasks are provided to the scheduler and elektron registers itself
with Mesos.

def/taskValidators.go

@@ -0,0 +1,91 @@
+// Copyright (C) 2018 spdfg
+//
+// This file is part of Elektron.
+//
+// Elektron is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Elektron is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Elektron.  If not, see <http://www.gnu.org/licenses/>.
+//
+
+package def
+
+import (
+	"github.com/pkg/errors"
+	"github.com/spdfg/elektron/utilities/validation"
+	"regexp"
+)
+
+// taskValidator is a validator that validates one or more attributes of a task.
+type taskValidator func(Task) error
+
+// ValidatorForTask returns a validator that runs all provided taskValidators and
+// returns an error corresponding to the first taskValidator that failed.
+func ValidatorForTask(t Task, taskValidators ...taskValidator) validation.Validator {
+	return func() error {
+		for _, tv := range taskValidators {
+			if err := tv(t); err != nil {
+				return err
+			}
+		}
+
+		return nil
+	}
+}
+
+// withNameValidator returns a taskValidator that checks whether the task name is valid.
+func withNameValidator() taskValidator {
+	return func(t Task) error {
+		// Task name cannot be empty string.
+		if t.Name == "" {
+			return errors.New("task name cannot be empty string")
+		}
+
+		// Task name cannot contain tabs or spaces.
+		matched, _ := regexp.MatchString("\\t+|\\s+", t.Name)
+		if matched {
+			return errors.New("task name cannot contain tabs or spaces")
+		}
+
+		return nil
+	}
+}
+
+// withResourceValidator returns a taskValidator that checks whether the resource requirements are valid.
+// Currently, only requirements for traditional resources (cpu and memory) are validated.
+func withResourceValidator() taskValidator {
+	return func(t Task) error {
+		// CPU value cannot be 0.
+		if t.CPU == 0.0 {
+			return errors.New("cpu resource for task cannot be 0")
+		}
+
+		// RAM value cannot be 0.
+		if t.RAM == 0.0 {
+			return errors.New("memory resource for task cannot be 0")
+		}
+
+		return nil
+	}
+}
+
+// withImageValidator returns a taskValidator that checks whether a valid image has been
+// provided for the task.
+func withImageValidator() taskValidator {
+	return func(t Task) error {
+		// Image cannot be empty.
+		if t.Image == "" {
+			return errors.New("valid image needs to be provided for task")
+		}
+
+		return nil
+	}
+}