Alerting: Sanitize invalid label/annotation names for external alertmanagers (#54537)

* Alerting: Sanitize invalid label/annotation names for external alertmanagers

Grafana's built-in Alertmanager supports both Unicode label keys and values; however, if using an external
Prometheus Alertmanager label keys must be compatible with their data model.
This means label keys must only contain ASCII letters, numbers, as well as underscores and match the regex
`[a-zA-Z_][a-zA-Z0-9_]*`.

Any invalid characters will now be removed or replaced by the Grafana alerting engine before being sent to
the external Alertmanager according to the following rules:

- `Whitespace` will be removed.
- `ASCII characters` will be replaced with `_`.
- `All other characters` will be replaced with their lower-case hex representation.

* Prefix hex replacements with `0x`

* Refactor for clarity

* Apply suggestions from code review

Co-authored-by: brendamuir <100768211+brendamuir@users.noreply.github.com>

Co-authored-by: brendamuir <100768211+brendamuir@users.noreply.github.com>

pkg/services/ngalert/sender/sender_test.go

@@ -0,0 +1,103 @@
+package sender
+
+import (
+	"testing"
+
+	"github.com/prometheus/alertmanager/api/v2/models"
+	"github.com/prometheus/prometheus/pkg/labels"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSanitizeLabelName(t *testing.T) {
+	cases := []struct {
+		desc           string
+		labelName      string
+		expectedResult string
+		expectedErr    string
+	}{
+		{
+			desc:           "Remove whitespace",
+			labelName:      "   a\tb\nc\vd\re\ff   ",
+			expectedResult: "abcdef",
+		},
+		{
+			desc:           "Replace ASCII with underscore",
+			labelName:      " !\"#$%&\\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~",
+			expectedResult: "________________0123456789_______ABCDEFGHIJKLMNOPQRSTUVWXYZ______abcdefghijklmnopqrstuvwxyz____",
+		},
+		{
+			desc:           "Replace non-ASCII unicode with hex",
+			labelName:      "_€_ƒ_„_†_‡_œ_Ÿ_®_º_¼_×_ð_þ_¿_±_四_十_二_🔥",
+			expectedResult: "_0x20ac_0x192_0x201e_0x2020_0x2021_0x153_0x178_0xae_0xba_0xbc_0xd7_0xf0_0xfe_0xbf_0xb1_0x56db_0x5341_0x4e8c_0x1f525",
+		},
+		{ // labels starting with a number are invalid, so we have to make sure we don't sanitize to another invalid label.
+			desc:           "If first character is replaced with hex, prefix with underscore",
+			labelName:      "😍😍😍",
+			expectedResult: "_0x1f60d0x1f60d0x1f60d",
+		},
+		{
+			desc:        "Empty string should error",
+			labelName:   "",
+			expectedErr: "label name cannot be empty",
+		},
+		{
+			desc:        "Only whitespace should error",
+			labelName:   "   \t\n\v\n\f   ",
+			expectedErr: "label name is empty after removing invalids chars",
+		},
+	}
+
+	for _, tc := range cases {
+		am, _ := NewExternalAlertmanagerSender()
+		t.Run(tc.desc, func(t *testing.T) {
+			res, err := am.sanitizeLabelName(tc.labelName)
+
+			if tc.expectedErr != "" {
+				require.EqualError(t, err, tc.expectedErr)
+			}
+
+			require.Equal(t, tc.expectedResult, res)
+		})
+	}
+}
+
+func TestSanitizeLabelSet(t *testing.T) {
+	cases := []struct {
+		desc           string
+		labelset       models.LabelSet
+		expectedResult labels.Labels
+	}{
+		{
+			desc: "Duplicate labels after sanitizations, append short has as suffix to duplicates",
+			labelset: models.LabelSet{
+				"test-alert": "42",
+				"test_alert": "43",
+				"test+alert": "44",
+			},
+			expectedResult: labels.Labels{
+				labels.Label{Name: "test_alert", Value: "44"},
+				labels.Label{Name: "test_alert_ed6237", Value: "42"},
+				labels.Label{Name: "test_alert_a67b5e", Value: "43"},
+			},
+		},
+		{
+			desc: "If sanitize fails for a label, skip it",
+			labelset: models.LabelSet{
+				"test-alert":       "42",
+				"   \t\n\v\n\f   ": "43",
+				"test+alert":       "44",
+			},
+			expectedResult: labels.Labels{
+				labels.Label{Name: "test_alert", Value: "44"},
+				labels.Label{Name: "test_alert_ed6237", Value: "42"},
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		am, _ := NewExternalAlertmanagerSender()
+		t.Run(tc.desc, func(t *testing.T) {
+			require.Equal(t, tc.expectedResult, am.sanitizeLabelSet(tc.labelset))
+		})
+	}
+}