fix(goenv.GOMOD): never rely on the environment variable

The value of `go env GOMOD` changes depending on the working directory
where the command is executed. Instead of relying on the environment
variable, always spawn `go env GOMOD` in an explicitly selected
directory.

internal/ensure/requiredversion.go

@@ -14,6 +14,7 @@ import (
 	"runtime"
 	"syscall"
 
+	"github.com/DataDog/orchestrion/internal/goenv"
 	"github.com/DataDog/orchestrion/internal/log"
 	"github.com/DataDog/orchestrion/internal/version"
 	"golang.org/x/tools/go/packages"
@@ -128,10 +129,16 @@ func requiredVersion(
 // working directory is used. The version may be blank if a replace directive is in effect; in which
 // case the path value may indicate the location of the source code that is being used instead.
 func goModVersion(dir string) (moduleVersion string, moduleDir string, err error) {
+	gomod, err := goenv.GOMOD(dir)
+	if err != nil {
+		return "", "", err
+	}
+
 	cfg := &packages.Config{
-		Dir:  dir,
-		Mode: packages.NeedModule,
-		Logf: func(format string, args ...any) { log.Tracef(format+"\n", args...) },
+		BuildFlags: []string{"-modfile", gomod},
+		Dir:        dir,
+		Mode:       packages.NeedModule,
+		Logf:       func(format string, args ...any) { log.Tracef(format+"\n", args...) },
 	}
 	pkgs, err := packages.Load(cfg, orchestrionPkgPath)
 	if err != nil {

internal/ensure/requiredversion_test.go

@@ -87,9 +87,7 @@ func TestGoModVersion(t *testing.T) {
 	}
 
 	t.Run("no-go-mod", func(t *testing.T) {
-		tmp, err := os.MkdirTemp("", "ensure-*")
-		require.NoError(t, err, "failed to create temporary directory")
-		defer os.RemoveAll(tmp)
+		tmp := t.TempDir()
 
 		os.WriteFile(filepath.Join(tmp, "main.go"), []byte(`
 		package main
@@ -98,9 +96,9 @@ func TestGoModVersion(t *testing.T) {
 				`), 0o644)
 
 		require.NotPanics(t, func() {
-			_, _, err = goModVersion(tmp)
+			_, _, err := goModVersion(tmp)
+			require.ErrorIs(t, err, goenv.ErrNoGoMod)
 		})
-		require.ErrorContains(t, err, "go.mod file not found in current directory")
 	})
 }
 

internal/goenv/goenv.go

@@ -16,15 +16,13 @@ import (
 
 var (
 	// ErrNoGoMod is returned when no GOMOD value could be identified.
-	ErrNoGoMod = errors.New("`go mod GOMOD` returned a blank string")
+	ErrNoGoMod = errors.New("`go env GOMOD` returned a blank string")
 )
 
-// GOMOD returns the current GOMOD environment variable (possibly from running `go env GOMOD`).
-func GOMOD() (string, error) {
-	if goMod := os.Getenv("GOMOD"); goMod != "" {
-		return goMod, nil
-	}
+// GOMOD returns the current GOMOD environment variable (from running `go env GOMOD`).
+func GOMOD(dir string) (string, error) {
 	cmd := exec.Command("go", "env", "GOMOD")
+	cmd.Dir = dir
 	var stdout bytes.Buffer
 	cmd.Stdout = &stdout
 	if err := cmd.Run(); err != nil {

internal/goenv/goenv_test.go

@@ -6,39 +6,21 @@
 package goenv
 
 import (
-	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
 func TestGOMOD(t *testing.T) {
 	t.Run("without GOMOD environment variable", func(t *testing.T) {
-		t.Setenv("GOMOD", "")
-
-		gomod, err := GOMOD()
+		gomod, err := GOMOD("")
 		require.NoError(t, err)
 		require.NotEmpty(t, gomod)
 	})
 
 	t.Run("no GOMOD can be found", func(t *testing.T) {
-		t.Setenv("GOMOD", "")
-
-		wd, _ := os.Getwd()
-		defer os.Chdir(wd)
-		os.Chdir(os.TempDir())
-
-		val, err := GOMOD()
+		val, err := GOMOD(t.TempDir())
 		require.Empty(t, val)
 		require.ErrorIs(t, err, ErrNoGoMod)
 	})
-
-	t.Run("with GOMOD environment variable", func(t *testing.T) {
-		expected := "/fake/path/to/go.mod"
-		t.Setenv("GOMOD", expected)
-
-		gomod, err := GOMOD()
-		require.NoError(t, err)
-		require.EqualValues(t, expected, gomod)
-	})
 }

internal/pin/pin.go

@@ -91,7 +91,7 @@ func defaultOrchestrionToolGo() *dst.File {
 // PinOrchestrion applies or update the orchestrion pin file in the current
 // working directory, according to the supplied [Options].
 func PinOrchestrion(opts Options) error {
-	goMod, err := goenv.GOMOD()
+	goMod, err := goenv.GOMOD(".")
 	if err != nil {
 		return fmt.Errorf("getting GOMOD: %w", err)
 	}