add a test for num_threads using a c++ program which runs 42 threads

metric/system/process/process_linux_test.go

@@ -20,10 +20,15 @@
 package process
 
 import (
+	"bytes"
 	"os"
+	"os/exec"
 	"os/user"
+	"path"
 	"strconv"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -133,3 +138,53 @@ func TestParseProcStat(t *testing.T) {
 
 	assert.Equal(t, want, got, "")
 }
+
+func runThreads(t *testing.T) *exec.Cmd {
+	t.Helper()
+
+	out, err := exec.Command("go", "env", "CXX").CombinedOutput()
+	if err != nil {
+		t.Fatalf("go env CXX failed: %v", err)
+	}
+	cxx := strings.TrimSpace(string(out))
+	if len(strings.Fields(cxx)) != 1 {
+		t.Fatalf("CXX=%q, does not look like a c++ compiler, "+
+			"it's got more than 1 string", cxx)
+	}
+	_, err = exec.LookPath(cxx)
+	if err != nil {
+		t.Fatalf("could not find %q: %x\v", cxx, err)
+	}
+
+	threads := path.Join(t.TempDir(), "threads")
+	wd, err := os.Getwd()
+	require.NoErrorf(t, err, "could not get current working directory")
+
+	theadscpp := path.Join(wd, "testdata", "threads.cpp")
+
+	out, err = exec.Command(cxx, "-o", threads, theadscpp).CombinedOutput()
+	if err != nil {
+		t.Fatalf("could not compile %q : %v. Output: %s",
+			threads, err, string(out))
+	}
+
+	var b bytes.Buffer
+	cmd := exec.Command(threads)
+	cmd.Stdout = &b
+	cmd.Stderr = &b
+
+	err = cmd.Start()
+	require.NoErrorf(t, err, "failed to start %q", threads)
+
+	var log string
+	require.Eventually(t,
+		func() bool {
+			log += b.String()
+			return strings.Contains(log, "running")
+		},
+		time.Second, 50*time.Millisecond,
+		"could not determine if %q is running. Output: %q",
+		threads, log)
+
+	return cmd
+}

metric/system/process/process_test.go

@@ -242,6 +242,27 @@ func TestGetProcess(t *testing.T) {
 	}
 }
 
+func TestGetInfoForPid_numThreads(t *testing.T) {
+	want := 42
+	cmd := runThreads(t)
+
+	got, err := GetInfoForPid(
+		resolve.NewTestResolver("/"), cmd.Process.Pid)
+	require.NoError(t, err, "failed to GetInfoForPid")
+
+	if !got.NumThreads.Exists() {
+		bs, err := json.Marshal(got)
+		if err != nil {
+			t.Logf("could not marshal ProcState: %v", err)
+		}
+		t.Fatalf("num_thread was not collected. Collected info: %s", bs)
+	}
+
+	numThreads := got.NumThreads.ValueOr(-1)
+	assert.Equalf(t, want, numThreads,
+		"want %d threads, got %d", want, numThreads)
+}
+
 func TestMatchProcs(t *testing.T) {
 	var procStats = Stats{}
 

metric/system/process/testdata/threads.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+#include <thread>
+#include <vector>
+
+void importantStuff() {
+    std::this_thread::sleep_for(std::chrono::minutes(42));
+}
+
+int main(int argc, char* argv[]) {
+    std::vector<std::thread> threads;
+    for (int i = 0; i < 41; ++i) {
+        threads.emplace_back(importantStuff);
+    }
+
+    std::cout << "running...\n" << std::flush;
+    importantStuff();
+
+    return 0;
+}