procstats: fix tests on Macs

Previously they would fail with an anticipated error; instead, skip
the failing test on Darwin.

procstats/proc.go

@@ -187,11 +187,19 @@ type ProcInfo struct {
 	Threads ThreadInfo
 }
 
-// CollectProcInfo return a ProcInfo and error (if any) for a given PID.
+// CollectProcInfo returns a ProcInfo and error (if any) for a given PID.
 func CollectProcInfo(pid int) (ProcInfo, error) {
 	return collectProcInfo(pid)
 }
 
+type OSUnsupportedError struct {
+	Msg string
+}
+
+func (o *OSUnsupportedError) Error() string {
+	return o.Msg
+}
+
 // CPUInfo holds statistics and configuration details for a process.
 type CPUInfo struct {
 	User time.Duration // user cpu time used by the process

procstats/proc_darwin.go

@@ -23,7 +23,6 @@ static mach_port_t mach_task_self() { return mach_task_self_; }
 import "C"
 
 import (
-	"errors"
 	"fmt"
 	"os"
 	"syscall"
@@ -35,7 +34,7 @@ func collectProcInfo(pid int) (info ProcInfo, err error) {
 	defer func() { err = convertPanicToError(recover()) }()
 
 	if pid != os.Getpid() {
-		panic(errors.New("on darwin systems only metrics of the current process can be collected"))
+		panic(&OSUnsupportedError{Msg: "on darwin systems only metrics of the current process can be collected"})
 	}
 
 	self := C.mach_port_name_t(C.mach_task_self())

procstats/proc_test.go

@@ -1,9 +1,11 @@
 package procstats
 
 import (
-	"io/ioutil"
+	"errors"
+	"io"
 	"os"
 	"os/exec"
+	"runtime"
 	"testing"
 	"time"
 
@@ -18,23 +20,36 @@ func TestProcMetrics(t *testing.T) {
 	t.Run("child", func(t *testing.T) {
 		cmd := exec.Command("yes")
 		cmd.Stdin = os.Stdin
-		cmd.Stdout = ioutil.Discard
-		cmd.Stderr = ioutil.Discard
+		cmd.Stdout = io.Discard
+		cmd.Stderr = io.Discard
 
-		cmd.Start()
+		if err := cmd.Start(); err != nil {
+			t.Fatal(err)
+		}
 		time.Sleep(200 * time.Millisecond)
 		testProcMetrics(t, cmd.Process.Pid)
 		cmd.Process.Signal(os.Interrupt)
-		cmd.Wait()
+		if err := cmd.Wait(); err != nil {
+			t.Fatal(err)
+		}
 	})
 }
 
 func testProcMetrics(t *testing.T, pid int) {
+	t.Helper()
 	h := &statstest.Handler{}
 	e := stats.NewEngine("", h)
 
 	proc := NewProcMetricsWith(e, pid)
 
+	// for darwin - catch the "can't collect child metrics" error before
+	// starting the test
+	_, err := CollectProcInfo(proc.pid)
+	var o *OSUnsupportedError
+	if errors.As(err, &o) {
+		t.Skipf("can't run test because current OS is unsupported: %v", runtime.GOOS)
+	}
+
 	for i := 0; i != 10; i++ {
 		t.Logf("collect number %d", i)
 		proc.Collect()