Add regex fetcher

Signed-off-by: Knut Ahlers <knut@ahlers.me>

internal/fetcher/regex.go

@@ -0,0 +1,89 @@
+package fetcher
+
+import (
+	"context"
+	"io/ioutil"
+	"net/http"
+	"regexp"
+	"time"
+
+	"github.com/pkg/errors"
+
+	"github.com/Luzifer/go-latestver/internal/database"
+	"github.com/Luzifer/go_helpers/v2/fieldcollection"
+)
+
+const (
+	httpStatus3xx               = 300
+	regexpFetcherExpectedLength = 2
+)
+
+type (
+	RegexFetcher struct{}
+)
+
+func init() { registerFetcher("regex", func() Fetcher { return &RegexFetcher{} }) }
+
+func (h RegexFetcher) FetchVersion(ctx context.Context, attrs *fieldcollection.FieldCollection) (string, time.Time, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, attrs.MustString("url", nil), nil)
+	if err != nil {
+		return "", time.Time{}, errors.Wrap(err, "creating request")
+	}
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", time.Time{}, errors.Wrap(err, "executing request")
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= httpStatus3xx {
+		return "", time.Time{}, errors.Errorf("HTTP status %d", resp.StatusCode)
+	}
+
+	data, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return "", time.Time{}, errors.Wrap(err, "reading response body")
+	}
+
+	matches := regexp.MustCompile(attrs.MustString("regex", nil)).FindStringSubmatch(string(data))
+	if matches == nil {
+		return "", time.Time{}, errors.New("regex did not match the response")
+	}
+
+	if l := len(matches); l != regexpFetcherExpectedLength {
+		return "", time.Time{}, errors.Errorf("unpexected number of matches: %d != 2", l)
+	}
+
+	return matches[1], time.Now(), nil
+}
+
+func (h RegexFetcher) Links(attrs *fieldcollection.FieldCollection) []database.CatalogLink {
+	return []database.CatalogLink{
+		{
+			IconClass: "fas fa-globe",
+			Name:      "Website",
+			URL:       attrs.MustString("url", nil),
+		},
+	}
+}
+
+func (h RegexFetcher) Validate(attrs *fieldcollection.FieldCollection) error {
+	if v, err := attrs.String("url"); err != nil || v == "" {
+		return errors.New("url is expected to be non-empty string")
+	}
+
+	if v, err := attrs.String("regex"); err != nil || v == "" {
+		return errors.New("regex is expected to be non-empty string")
+	}
+
+	r, err := regexp.Compile(attrs.MustString("regex", nil))
+	if err != nil {
+		return errors.Wrap(err, "compiling regex expression")
+	}
+
+	if n := r.NumSubexp(); n != 1 {
+		return errors.Errorf("regex must have 1 submatch, has %d", n)
+	}
+
+	return nil
+}