worker: use openapi spec and generated code

Write an openapi spec for the worker API and use `deepmap/oapi-codegen`
to generate scaffolding for the server-side using the `labstack/echo`
server.

Incidentally, echo by default returns the errors in the same format that
worker API always has:

    { "message": "..." }

The API itself is unchanged to make this change easier to understand. It
will be changed to better suit our needs in future commits.

vendor/github.com/deepmap/oapi-codegen/pkg/runtime/bindstring.go

@@ -0,0 +1,116 @@
+// Copyright 2019 DeepMap, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package runtime
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"strconv"
+	"time"
+
+	"github.com/deepmap/oapi-codegen/pkg/types"
+)
+
+// This function takes a string, and attempts to assign it to the destination
+// interface via whatever type conversion is necessary. We have to do this
+// via reflection instead of a much simpler type switch so that we can handle
+// type aliases. This function was the easy way out, the better way, since we
+// know the destination type each place that we use this, is to generate code
+// to read each specific type.
+func BindStringToObject(src string, dst interface{}) error {
+	var err error
+
+	v := reflect.ValueOf(dst)
+	t := reflect.TypeOf(dst)
+
+	// We need to dereference pointers
+	if t.Kind() == reflect.Ptr {
+		v = reflect.Indirect(v)
+		t = v.Type()
+	}
+
+	// The resulting type must be settable. reflect will catch issues like
+	// passing the destination by value.
+	if !v.CanSet() {
+		return errors.New("destination is not settable")
+	}
+
+	switch t.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		var val int64
+		val, err = strconv.ParseInt(src, 10, 64)
+		if err == nil {
+			v.SetInt(val)
+		}
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		var val uint64
+		val, err = strconv.ParseUint(src, 10, 64)
+		if err == nil {
+			v.SetUint(val)
+		}
+	case reflect.String:
+		v.SetString(src)
+		err = nil
+	case reflect.Float64, reflect.Float32:
+		var val float64
+		val, err = strconv.ParseFloat(src, 64)
+		if err == nil {
+			v.SetFloat(val)
+		}
+	case reflect.Bool:
+		var val bool
+		val, err = strconv.ParseBool(src)
+		if err == nil {
+			v.SetBool(val)
+		}
+	case reflect.Struct:
+		switch dstType := dst.(type) {
+		case *time.Time:
+			// Don't fail on empty string.
+			if src == "" {
+				return nil
+			}
+			// Time is a special case of a struct that we handle
+			parsedTime, err := time.Parse(time.RFC3339Nano, src)
+			if err != nil {
+				parsedTime, err = time.Parse(types.DateFormat, src)
+				if err != nil {
+					return fmt.Errorf("error parsing '%s' as RFC3339 or 2006-01-02 time: %s", src, err)
+				}
+			}
+			*dstType = parsedTime
+			return nil
+		case *types.Date:
+			// Don't fail on empty string.
+			if src == "" {
+				return nil
+			}
+			parsedTime, err := time.Parse(types.DateFormat, src)
+			if err != nil {
+				return fmt.Errorf("error parsing '%s' as date: %s", src, err)
+			}
+			dstType.Time = parsedTime
+			return nil
+		}
+		fallthrough
+	default:
+		// We've got a bunch of types unimplemented, don't fail silently.
+		err = fmt.Errorf("can not bind to destination of type: %s", t.Kind())
+	}
+	if err != nil {
+		return fmt.Errorf("error binding string parameter: %s", err)
+	}
+	return nil
+}