Do a large chunk of the move to go

This commit moves a huge chunk of the code to go. The TODO.md document
indicates what is left to be done.

internal/config/validators.go

@@ -0,0 +1,105 @@
+package config
+
+import (
+	"net/url"
+	"regexp"
+	"strconv"
+	"strings"
+
+	"github.com/FAU-CDI/wisski-distillery/internal/fsx"
+	"github.com/pkg/errors"
+)
+
+// Validator reads from the configuration file
+type Validator func(s string) (interface{}, error)
+
+var knownValidators map[string]Validator = map[string]Validator{
+	"is_valid_abspath":   IsValidAbspath,
+	"is_valid_domain":    IsValidDomain,
+	"is_valid_domains":   IsValidDomains,
+	"is_valid_number":    IsValidNumber,
+	"is_valid_https_url": IsValidHttpsURL,
+	"is_valid_slug":      IsValidSlug,
+	"is_valid_file":      IsValidFile,
+	"is_valid_email":     IsValidEmail,
+	"is_nonempty":        IsNonEmpty,
+}
+
+func IsValidAbspath(s string) (interface{}, error) {
+	if !fsx.IsDirectory(s) {
+		return nil, errors.Errorf("%q does not exist or is not a directory", s)
+	}
+	return s, nil
+}
+
+func IsValidFile(s string) (interface{}, error) {
+	if !fsx.IsFile(s) {
+		return nil, errors.Errorf("%q does not exist or is not a regular file", s)
+	}
+	return s, nil
+}
+
+func IsNonEmpty(s string) (interface{}, error) {
+	if s == "" {
+		return nil, errors.New("value is empty")
+	}
+	return s, nil
+}
+
+var regexpDomain = regexp.MustCompile(`^([a-zA-Z0-9][-a-zA-Z0-9]*\.)*[a-zA-Z0-9][-a-zA-Z0-9]*$`) // TODO: Make this regexp nicer!
+
+func IsValidDomain(s string) (interface{}, error) {
+	if !regexpDomain.MatchString(s) {
+		return nil, errors.Errorf("%q is not a valid domain", s)
+	}
+	return s, nil
+}
+func IsValidDomains(s string) (interface{}, error) {
+	if len(s) == 0 {
+		return []string{}, nil
+	}
+	domains := strings.Split(s, ",")
+	for _, d := range domains {
+		if !regexpDomain.MatchString(d) {
+			return nil, errors.Errorf("%q is not a valid domain", d)
+		}
+	}
+	return domains, nil
+}
+
+func IsValidNumber(s string) (interface{}, error) {
+	value, err := strconv.ParseInt(s, 10, 64)
+	return int(value), err
+}
+
+func IsValidHttpsURL(s string) (interface{}, error) {
+	url, err := url.Parse(s)
+	if err != nil {
+		return nil, errors.Wrapf(err, "%q is not a valid URL", s)
+	}
+	if url.Scheme != "https" {
+		return nil, errors.Errorf("%q is not a valid https URL (%q)", s, url.Scheme)
+	}
+	return url, nil
+}
+
+var regexpEmail = regexp.MustCompile(`^([-a-zA-Z0-9]+)\@([a-zA-Z0-9][-a-zA-Z0-9]*\.)*[a-zA-Z0-9][-a-zA-Z0-9]*$`) // TODO: Make this regexp nicer!
+
+func IsValidEmail(s string) (interface{}, error) {
+	if s == "" { // no email provided
+		return "", nil
+	}
+	if !regexpEmail.MatchString(s) {
+		return nil, errors.Errorf("%q is not a valid email", s)
+	}
+	return s, nil
+}
+
+var regexpSlug = regexp.MustCompile(`^[a-zA-Z0-9][-a-zA-Z0-9]*$`) // TODO: Make this regexp nicer!
+
+func IsValidSlug(s string) (interface{}, error) {
+	if !regexpSlug.MatchString(s) {
+		return nil, errors.Errorf("%q is not a valid slug", s)
+	}
+	return s, nil
+}