Fix SanitizeLabelName for certain invalid labels

SanitizeLabelName was not correctly sanitizing label names that:

1. Started with a digit (0-9)
2. Were empty

This commit changes the santization code to catch both of these edge
cases and adds new tests to validate it works correctly in them both. In
the first case, a leading digit will be replaced with an underscore, and
in the latter, the function will return a single underscore.

Signed-off-by: Nick Moore <nicholas.moore@grafana.com>

util/strutil/strconv.go

@@ -16,12 +16,9 @@ package strutil
 import (
 	"fmt"
 	"net/url"
-
-	"github.com/grafana/regexp"
+	"strings"
 )
 
-var invalidLabelCharRE = regexp.MustCompile(`[^a-zA-Z0-9_]`)
-
 // TableLinkForExpression creates an escaped relative link to the table view of
 // the provided expression.
 func TableLinkForExpression(expr string) string {
@@ -36,8 +33,19 @@ func GraphLinkForExpression(expr string) string {
 	return fmt.Sprintf("/graph?g0.expr=%s&g0.tab=0", escapedExpression)
 }
 
-// SanitizeLabelName replaces anything that doesn't match
-// client_label.LabelNameRE with an underscore.
+// SanitizeLabelName replaces any invalid character with an underscore, and if
+// given an empty string, returns a string containing a single underscore.
 func SanitizeLabelName(name string) string {
-	return invalidLabelCharRE.ReplaceAllString(name, "_")
+	if len(name) == 0 {
+		return "_"
+	}
+	var validSb strings.Builder
+	for i, b := range name {
+		if !((b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z') || b == '_' || (b >= '0' && b <= '9' && i > 0)) {
+			validSb.WriteRune('_')
+		} else {
+			validSb.WriteRune(b)
+		}
+	}
+	return validSb.String()
 }

util/strutil/strconv_test.go

@@ -58,4 +58,12 @@ func TestSanitizeLabelName(t *testing.T) {
 	actual = SanitizeLabelName("barClient.LABEL$$##")
 	expected = "barClient_LABEL____"
 	require.Equal(t, expected, actual, "SanitizeLabelName failed for label (%s)", expected)
+
+	actual = SanitizeLabelName("0zerothClient1LABEL")
+	expected = "_zerothClient1LABEL"
+	require.Equal(t, expected, actual, "SanitizeLabelName failed for label (%s)", expected)
+
+	actual = SanitizeLabelName("")
+	expected = "_"
+	require.Equal(t, expected, actual, "SanitizeLabelName failed for the empty label")
 }