Add initial test for net_linux.go

More tests needed based on iwlist output, but the side-effect-free
part of GetSupportedFrequencies() has been been broken out into a function
that can now be tested without calling core.Exec().

network/net_linux.go

@@ -59,13 +59,12 @@ func SetInterfaceChannel(iface string, channel int) error {
 	return nil
 }
 
-func GetSupportedFrequencies(iface string) ([]int, error) {
+func processSupportedFrequencies(output string, err error) ([]int, error) {
 	freqs := make([]int, 0)
-	out, err := core.Exec("iwlist", []string{iface, "freq"})
 	if err != nil {
 		return freqs, err
-	} else if out != "" {
-		scanner := bufio.NewScanner(strings.NewReader(out))
+	} else if output != "" {
+		scanner := bufio.NewScanner(strings.NewReader(output))
 		for scanner.Scan() {
 			line := scanner.Text()
 			matches := WiFiFreqParser.FindStringSubmatch(line)
@@ -78,3 +77,8 @@ func GetSupportedFrequencies(iface string) ([]int, error) {
 	}
 	return freqs, nil
 }
+
+func GetSupportedFrequencies(iface string) ([]int, error) {
+	out, err := core.Exec("iwlist", []string{iface, "freq"})
+	return processSupportedFrequencies(out, err)
+}

network/net_linux_test.go

@@ -0,0 +1,40 @@
+package network
+
+import (
+	"errors"
+	"reflect"
+	"testing"
+)
+
+func TestProcessSupportedFrequencies(t *testing.T) {
+	// Actually test processSupportedFrequencies; IO is lifted out.
+	cases := []struct {
+		Name          string
+		InputString   string
+		InputError    error
+		ExpectedFreqs []int
+		ExpectedError bool
+	}{
+		{
+			"Returns empty with an error",
+			"Shouldn't matter",
+			errors.New("iwlist must have failed"),
+			[]int{},
+			true,
+		},
+	}
+	for _, test := range cases {
+		t.Run(test.Name, func(t *testing.T) {
+			freqs, err := processSupportedFrequencies(test.InputString, test.InputError)
+			if err != nil && !test.ExpectedError {
+				t.Errorf("unexpected error: %s", err)
+			}
+			if err == nil && test.ExpectedError {
+				t.Error("expected error, but got none")
+			}
+			if !test.ExpectedError && !reflect.DeepEqual(freqs, test.ExpectedFreqs) {
+				t.Errorf("got %v, want %v", freqs, test.ExpectedFreqs)
+			}
+		})
+	}
+}