Connect backend refactor (#801)

* Move connect backend code to its own package
* Move errors into the connect package
* Add NewBackend func

pkg/connect/backend.go

@@ -0,0 +1,92 @@
+package connect
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/sirupsen/logrus"
+)
+
+type Backend struct {
+	Endpoint    string
+	Token       string
+	PassHeaders []string
+
+	logger *logrus.Logger
+}
+
+func NewBackend(endpoint string, token string) Backend {
+	return Backend{
+		Endpoint: endpoint,
+		Token:    token,
+		logger:   logrus.StandardLogger(),
+	}
+}
+
+func (be *Backend) SetLogger(logger *logrus.Logger) {
+	be.logger = logger
+}
+
+func (be *Backend) SetPassHeaders(headers []string) {
+	be.PassHeaders = headers
+}
+
+func (be *Backend) FetchCredential(ctx context.Context, resource string, headers http.Header) (*Credential, error) {
+	be.logger.WithField("resource", resource).Debug("fetching database credential")
+
+	request := Request{
+		Resource: resource,
+		Token:    be.Token,
+		Headers:  map[string]string{},
+	}
+
+	// Pass allow-listed client headers to the backend request
+	for _, name := range be.PassHeaders {
+		request.Headers[strings.ToLower(name)] = headers.Get(name)
+	}
+
+	body, err := json.Marshal(request)
+	if err != nil {
+		be.logger.WithField("resource", resource).Error("backend request serialization error:", err)
+		return nil, err
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, be.Endpoint, bytes.NewReader(body))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("content-type", "application/json")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		be.logger.WithField("resource", resource).Error("backend credential fetch failed:", err)
+		return nil, errBackendConnectError
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		err = fmt.Errorf("backend credential fetch received HTTP status code %v", resp.StatusCode)
+
+		be.logger.
+			WithField("resource", request.Resource).
+			WithField("status", resp.StatusCode).
+			Error(err)
+
+		return nil, err
+	}
+
+	cred := &Credential{}
+	if err := json.NewDecoder(resp.Body).Decode(cred); err != nil {
+		return nil, err
+	}
+
+	if cred.DatabaseURL == "" {
+		return nil, errConnStringRequired
+	}
+
+	return cred, nil
+}