all: add logging to TestGovulncheck

Fixes golang/go#64254

Change-Id: If3b5280fbb239981dec38035c5e2f0ad5f5c6fd7
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/545335
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: zpavlinovic

all_test.go

@@ -9,6 +9,7 @@ package main
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"io/fs"
 	"os"
@@ -78,21 +79,28 @@ func TestGovulncheck(t *testing.T) {
 	skipIfShort(t)
 	testenv.NeedsGoBuild(t)
 
+	var o string
+	out := bytes.NewBufferString(o)
 	ctx := context.Background()
 
 	cmd := scan.Command(ctx, "./...")
+	cmd.Stdout = out
+	cmd.Stderr = out
 	err := cmd.Start()
 	if err == nil {
 		err = cmd.Wait()
 	}
+
+	t.Logf("govulncheck finished with std out/err:\n%s", out.String())
 	switch err := err.(type) {
 	case nil:
+		t.Log("govulncheck: no vulnerabilities detected")
 	case interface{ ExitCode() int }:
 		if err.ExitCode() != 0 {
-			t.Error("govulncheck found problems")
+			t.Errorf("govulncheck ./... failed with exit code: %d", err.ExitCode())
 		}
 	default:
-		t.Error(err)
+		t.Errorf("unrecognized govulncheck command error: %v", err)
 	}
 }
 

internal/vulncheck/packages.go

@@ -7,6 +7,7 @@ package vulncheck
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os/exec"
 	"strings"
@@ -41,15 +42,20 @@ func (g *PackageGraph) LoadModules(cfg *packages.Config) (mods []*packages.Modul
 	cmd.Dir = cfg.Dir
 	out, err := cmd.Output()
 	if err != nil {
-		return nil, err
+		if ee := (*exec.ExitError)(nil); errors.As(err, &ee) && len(ee.Stderr) > 0 {
+			return nil, fmt.Errorf("%v: %v\n%s", cmd, err, ee.Stderr)
+		}
+		return nil, fmt.Errorf("%v: %v", cmd, err)
 	}
 
 	dec := json.NewDecoder(bytes.NewReader(out))
 	for dec.More() {
 		var m *packages.Module
 		err = dec.Decode(&m)
 		if err != nil {
-			return nil, err
+			if err != nil {
+				return nil, fmt.Errorf("decoding output of %v: %v", cmd, err)
+			}
 		}
 		mods = append(mods, m)
 	}