Treat tags as an unordered set

Breaking change.  The `tags` attribute will change from string-type to
set-of-string type.  Users will need to make the following syntactic
change to their Terraform configuration files:

    // before:
    resource "pingdom_check" "example" {
      // ...
      tags = "foo,bar"
    }

    // after:
    resource "pingdom_check" "example" {
      // ...
      tags = ["foo", "bar"]  // order no longer relevant
    }

This commit includes a schema migration for existing user state.

---

The Pingdom API returns tag elements in an unpredictable order.
Ordering for this type is not documented in the Pingdom API spec.
Current plugin behaviour often leads to spurious diffs against state.

https://www.pingdom.com/api/2.1/#MethodGet+Check+List

Notionally, tags should be considered to be sets in the Pingdom data
model.  It does not make much practical sense for us to enforce strict
ordering on them.  Any implied ordering is likely an inadvertent
side-effect of JSON's lack of an unordered set type.

This change will hash tag names using the FNV-1a alternate algorithm.
(Tag names are the only labels that users see in the Pingdom UI and API.
Pingdom do not expose tag IDs in their API.)

http://www.isthe.com/chongo/tech/comp/fnv/index.html#FNV-1a

As far as I know, this algorithm is a good fit for set element hash
computation:  it is said to be fast, said to provide reasonably good
dispersion, and is a part of the Go standard library.

pingdom/resource_pingdom_check.go

@@ -2,6 +2,7 @@ package pingdom
 
 import (
 	"fmt"
+	"hash/fnv"
 	"log"
 	"strconv"
 	"strings"
@@ -146,10 +147,12 @@ func resourcePingdomCheck() *schema.Resource {
 				Optional: true,
 				ForceNew: false,
 			},
+
 			"tags": {
-				Type:     schema.TypeString,
+				Type:     schema.TypeSet,
 				Optional: true,
 				ForceNew: false,
+				Elem:     &schema.Schema{Type: schema.TypeString},
 			},
 
 			"probefilters": {
@@ -184,6 +187,15 @@ func resourcePingdomCheck() *schema.Resource {
 				ForceNew: false,
 			},
 		},
+
+		SchemaVersion: 1,
+		StateUpgraders: []schema.StateUpgrader{
+			{
+				Version: 0,
+				Type:    resourcePingdomCheckResourceV0().CoreConfigSchema().ImpliedType(),
+				Upgrade: resourcePingdomCheckStateUpgradeV0,
+			},
+		},
 	}
 }
 
@@ -315,7 +327,12 @@ func checkForResource(d *schema.ResourceData) (pingdom.Check, error) {
 		}
 	}
 	if v, ok := d.GetOk("tags"); ok {
-		checkParams.Tags = v.(string)
+		interfaceSlice := v.(*schema.Set).List()
+		tags := make([]string, len(interfaceSlice))
+		for i := range interfaceSlice {
+			tags[i] = interfaceSlice[i].(string)
+		}
+		checkParams.Tags = strings.Join(tags, ",")
 	}
 
 	if v, ok := d.GetOk("probefilters"); ok {
@@ -467,11 +484,18 @@ func resourcePingdomCheckRead(d *schema.ResourceData, meta interface{}) error {
 	d.Set("notifywhenbackup", ck.NotifyWhenBackup)
 	d.Set("publicreport", inPublicReport)
 
-	tags := []string{}
+	tags := schema.NewSet(
+		func(tagName interface{}) int {
+			h := fnv.New32a()
+			h.Write([]byte(tagName.(string)))
+			return int(h.Sum32())
+		},
+		[]interface{}{},
+	)
 	for _, tag := range ck.Tags {
-		tags = append(tags, tag.Name)
+		tags.Add(tag.Name)
 	}
-	d.Set("tags", strings.Join(tags, ","))
+	d.Set("tags", tags)
 
 	if ck.Status == "paused" {
 		d.Set("paused", true)
@@ -580,3 +604,187 @@ func resourcePingdomCheckDelete(d *schema.ResourceData, meta interface{}) error
 
 	return nil
 }
+
+func resourcePingdomCheckResourceV0() *schema.Resource {
+	return &schema.Resource{
+		Schema: map[string]*schema.Schema{
+			"name": {
+				Type:     schema.TypeString,
+				Required: true,
+				ForceNew: false,
+			},
+
+			"host": {
+				Type:     schema.TypeString,
+				Required: true,
+				ForceNew: false,
+			},
+
+			"type": {
+				Type:     schema.TypeString,
+				Required: true,
+				ForceNew: true,
+			},
+
+			"paused": {
+				Type:     schema.TypeBool,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"responsetime_threshold": {
+				Type:     schema.TypeInt,
+				Optional: true,
+				ForceNew: false,
+				Computed: true,
+			},
+
+			"publicreport": {
+				Type:     schema.TypeBool,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"resolution": {
+				Type:     schema.TypeInt,
+				Required: true,
+				ForceNew: false,
+			},
+
+			"sendnotificationwhendown": {
+				Type:     schema.TypeInt,
+				Optional: true,
+				ForceNew: false,
+				Computed: true,
+			},
+
+			"notifyagainevery": {
+				Type:     schema.TypeInt,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"notifywhenbackup": {
+				Type:     schema.TypeBool,
+				Optional: true,
+				ForceNew: false,
+				Computed: true,
+			},
+
+			"integrationids": {
+				Type:     schema.TypeSet,
+				Optional: true,
+				ForceNew: false,
+				Elem:     &schema.Schema{Type: schema.TypeInt},
+			},
+
+			"encryption": {
+				Type:     schema.TypeBool,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"url": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+				Default:  "/",
+			},
+
+			"port": {
+				Type:     schema.TypeInt,
+				Optional: true,
+				ForceNew: false,
+				Computed: true,
+			},
+
+			"username": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"password": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"shouldcontain": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"shouldnotcontain": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"postdata": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"requestheaders": {
+				Type:     schema.TypeMap,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"tags": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"probefilters": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"userids": {
+				Type:     schema.TypeSet,
+				Optional: true,
+				ForceNew: false,
+				Elem:     &schema.Schema{Type: schema.TypeInt},
+			},
+
+			"teamids": {
+				Type:     schema.TypeSet,
+				Optional: true,
+				ForceNew: false,
+				Elem:     &schema.Schema{Type: schema.TypeInt},
+			},
+
+			"stringtosend": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+
+			"stringtoexpect": {
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: false,
+			},
+		},
+	}
+}
+
+func resourcePingdomCheckStateUpgradeV0(rawState map[string]interface{}, meta interface{}) (map[string]interface{}, error) {
+	oldTags, ok := rawState["tags"]
+	if !ok {
+		return rawState, nil
+	}
+	newTags := []string{}
+	tags := strings.Split(oldTags.(string), ",")
+	for _, t := range tags {
+		newTags = append(newTags, t)
+	}
+	rawState["tags"] = newTags
+	return rawState, nil
+}