NEW PROVIDER: HETZNER DNS Console (#904)

* HETZNER: implement the provider for Hetzner DNS Console

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: apply review feedback

- add domain into error messages
- insert sub-strings using `%q`
- insert sub-errors using `%w`
- change api.getZone() signature to return a (potentially `nil`) Zone
   pointer instead of a (potentially empty) Zone value
- sort imports and confirm with `$ goimports -w providers/hetzner/`
- use exact 'api_key' term in error message of settings validation
- add blank line for logic separation
- drop internal record id from correction messages

Co-Authored-By: Tom Limoncelli <tlimoncelli@stackoverflow.com>
Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: add request rate-limiting handling

There are a limited number of data-points on how their rate-limiting
 works at this time. I deduce from my account to others and use a fixed/
 constant backoff of 1s as the initial delay. Thereafter exponential
 increase with factor 2 (not needed at this time).
Hetzner has not made any official statements on rate-limiting, so this
 is guesswork only.

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: address golint complaints

- baseUrl -> baseURL
- mark Record as private -> record
- mark Zone as private -> zone
- mark RequestRateLimiter as private -> requestRateLimiter
- capitalize Id fields as ID
- keep delay logic on same level, move return out of branch

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: rate_limited: init the response timestamp on requestRateLimiter

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: requestRateLimiter: align local variable with struct name

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

Co-authored-by: Tom Limoncelli <tlimoncelli@stackoverflow.com>

providers/hetzner/hetznerProvider.go

@@ -0,0 +1,173 @@
+package hetzner
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/StackExchange/dnscontrol/v3/models"
+	"github.com/StackExchange/dnscontrol/v3/pkg/diff"
+	"github.com/StackExchange/dnscontrol/v3/providers"
+)
+
+var features = providers.DocumentationNotes{
+	providers.DocCreateDomains:       providers.Can(),
+	providers.DocDualHost:            providers.Can(),
+	providers.DocOfficiallySupported: providers.Cannot(),
+	providers.CanGetZones:            providers.Can(),
+	providers.CanUseAlias:            providers.Cannot(),
+	providers.CanUseCAA:              providers.Can(),
+	providers.CanUseDS:               providers.Cannot(),
+	providers.CanUsePTR:              providers.Cannot(),
+	providers.CanUseSRV:              providers.Can(),
+	providers.CanUseSSHFP:            providers.Cannot(),
+	providers.CanUseTLSA:             providers.Cannot(),
+	providers.CanUseTXTMulti:         providers.Cannot(),
+}
+
+func init() {
+	providers.RegisterDomainServiceProviderType("HETZNER", New, features)
+}
+
+// New creates a new API handle.
+func New(settings map[string]string, _ json.RawMessage) (providers.DNSServiceProvider, error) {
+	if settings["api_key"] == "" {
+		return nil, fmt.Errorf("missing HETZNER api_key")
+	}
+
+	api := &api{}
+
+	api.apiKey = settings["api_key"]
+
+	if settings["rate_limited"] == "true" {
+		api.startRateLimited()
+	}
+
+	return api, nil
+}
+
+// EnsureDomainExists creates the domain if it does not exist.
+func (api *api) EnsureDomainExists(domain string) error {
+	domains, err := api.ListZones()
+	if err != nil {
+		return err
+	}
+
+	for _, d := range domains {
+		if d == domain {
+			return nil
+		}
+	}
+
+	return api.createZone(domain)
+}
+
+// GetDomainCorrections returns the corrections for a domain.
+func (api *api) GetDomainCorrections(dc *models.DomainConfig) ([]*models.Correction, error) {
+	dc, err := dc.Copy()
+	if err != nil {
+		return nil, err
+	}
+
+	err = dc.Punycode()
+	if err != nil {
+		return nil, err
+	}
+	domain := dc.Name
+
+	// Get existing records
+	existingRecords, err := api.GetZoneRecords(domain)
+	if err != nil {
+		return nil, err
+	}
+
+	// Normalize
+	models.PostProcessRecords(existingRecords)
+
+	differ := diff.New(dc)
+	_, create, del, modify, err := differ.IncrementalDiff(existingRecords)
+	if err != nil {
+		return nil, err
+	}
+
+	var corrections []*models.Correction
+
+	zone, err := api.getZone(domain)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, m := range del {
+		record := m.Existing.Original.(*record)
+		corr := &models.Correction{
+			Msg: m.String(),
+			F: func() error {
+				return api.deleteRecord(*record)
+			},
+		}
+		corrections = append(corrections, corr)
+	}
+
+	for _, m := range create {
+		record := fromRecordConfig(m.Desired, zone)
+		corr := &models.Correction{
+			Msg: m.String(),
+			F: func() error {
+				return api.createRecord(*record)
+			},
+		}
+		corrections = append(corrections, corr)
+	}
+
+	for _, m := range modify {
+		id := m.Existing.Original.(*record).ID
+		record := fromRecordConfig(m.Desired, zone)
+		record.ID = id
+		corr := &models.Correction{
+			Msg: m.String(),
+			F: func() error {
+				return api.updateRecord(*record)
+			},
+		}
+		corrections = append(corrections, corr)
+	}
+
+	return corrections, nil
+}
+
+// GetNameservers returns the nameservers for a domain.
+func (api *api) GetNameservers(domain string) ([]*models.Nameserver, error) {
+	zone, err := api.getZone(domain)
+	if err != nil {
+		return nil, err
+	}
+	nameserver := make([]*models.Nameserver, len(zone.NameServers))
+	for i := range zone.NameServers {
+		nameserver[i] = &models.Nameserver{Name: zone.NameServers[i]}
+	}
+	return nameserver, nil
+}
+
+// GetZoneRecords gets the records of a zone and returns them in RecordConfig format.
+func (api *api) GetZoneRecords(domain string) (models.Records, error) {
+	records, err := api.getAllRecords(domain)
+	if err != nil {
+		return nil, err
+	}
+	existingRecords := make([]*models.RecordConfig, len(records))
+	for i := range records {
+		existingRecords[i] = toRecordConfig(domain, &records[i])
+	}
+	return existingRecords, nil
+}
+
+// ListZones lists the zones on this account.
+func (api *api) ListZones() ([]string, error) {
+	if err := api.getAllZones(); err != nil {
+		return nil, err
+	}
+	var zones []string
+	for i := range api.zones {
+		zones = append(zones, i)
+	}
+	return zones, nil
+}