Instance name on process log lines and every metric (closes #91)

Process log lines carry instance, as request lines do; the instance name
is read before the other settings, so the line saying a setting is
invalid carries it too. Every metric, Go's and the process's included,
carries the label instance, set once on the registry. README.md says so,
and that Prometheus keeps it as exported_instance unless the scrape sets
honor_labels. An instance name that is not valid UTF-8 stops the start,
as the metrics library panics on such a label.

Tests that read metrics expect the label; one helper replaces the alert
tests' loops that wait for them.

Judgement call: the label is named instance, as in the log lines and
alerts, although Prometheus gives each target a label of that name.

Model: opus-5-5

README.md

@@ -213,7 +213,8 @@ effective settings are logged at start.
   the name every log line and alert gives as `instance`, and every metric
   carries as its label `instance` (see "Metrics" below). Set it, for example to
   `fsn1app1/gitea`, for a name that stays the same when a deploy replaces the
-  container, and that tells instances apart when several log to one place.
+  container, and that tells instances apart when several log to one place. A
+  name that is not valid UTF-8, such as one saved in Latin-1, stops the start.
 - `SWWAF_MODE` (default `enforce`): `enforce`, or `observe` to pass on the
   requests `smallwebwaf` would refuse and log what it would have done (see "What
   it does so far" above).

internal/config/config.go

@@ -268,6 +268,7 @@ var (
 		"is not ban, permanent_ban, waf_block, anomaly, reputation_hit, " +
 			"source_failure or file_error")
 	errNotNumberOrOff = errors.New("is not a whole number above zero, such as 60, or off")
+	errNotUTF8        = errors.New("is not valid UTF-8")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -670,11 +671,17 @@ func (e *environment) facility(name, defaultValue string) int {
 	return number
 }
 
-// instanceName reads SWWAF_INSTANCE_NAME, by default the host's name.
+// instanceName reads SWWAF_INSTANCE_NAME, by default the host's name. It
+// must be valid UTF-8: the metrics library panics on a label that is not.
 func (e *environment) instanceName() string {
 	hostname, _ := os.Hostname() // "" when the host has no name to give
 
-	return e.value("SWWAF_INSTANCE_NAME", hostname)
+	value := e.value("SWWAF_INSTANCE_NAME", hostname)
+	if !utf8.ValidString(value) {
+		e.check("SWWAF_INSTANCE_NAME", fmt.Errorf("%q %w", value, errNotUTF8))
+	}
+
+	return value
 }
 
 // appName reads the setting that is the APP-NAME of the records the log

internal/config/config_test.go

@@ -737,6 +737,25 @@ func TestInstanceNameWithAControlCharacterStopsTheStartOnlyWithNtfySet(t *testin
 	}
 }
 
+func TestInstanceNameNotUTF8StopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	// café saved in Latin-1.
+	const latin1 = "caf\xe9"
+
+	for name, env := range map[string]environment{
+		"set":       {instanceName: latin1},
+		"in a file": {instanceName + "_FILE": writeFile(t, latin1+"\n")},
+	} {
+		_, err := config.FromEnvironment(env.lookupEnv)
+
+		want := instanceName + `: "caf\xe9" is not valid UTF-8`
+		if err == nil || err.Error() != want {
+			t.Errorf("%s: error %v, want %s", name, err, want)
+		}
+	}
+}
+
 func TestCodeOnBothCountryListsStopsTheStart(t *testing.T) {
 	t.Parallel()
 

internal/smallwebwaf/smallwebwaf_test.go

@@ -597,6 +597,13 @@ func TestStalledRemoteLogEndpointHoldsUpNoRequest(t *testing.T) {
 	})
 
 	out.line(t, "type", "request")
+
+	// While the lines are sent, the process's lines give the instance name
+	// too.
+	line := out.line(t, "msg", "starting")
+	if line["instance"] != instance {
+		t.Errorf("start logged with instance %v, want %s", line["instance"], instance)
+	}
 }
 
 func TestBanIsAlertedAndAnAlertNotSentIsKeptAcrossARestart(t *testing.T) {