fix: running in Go workspace (#63)

testdata/src/go.work

@@ -0,0 +1,7 @@
+go 1.18
+
+use (
+	./builtins
+	./custom
+	./tests
+)

utils.go

@@ -2,29 +2,56 @@ package musttag
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io"
+	"os"
 	"os/exec"
 	"strings"
 )
 
+var (
+	getwd = os.Getwd
+
+	commandOutput = func(name string, args ...string) (string, error) {
+		output, err := exec.Command(name, args...).Output()
+		return string(output), err
+	}
+)
+
 func getMainModule() (string, error) {
 	args := []string{"go", "list", "-m", "-json"}
 
-	data, err := exec.Command(args[0], args[1:]...).Output()
+	output, err := commandOutput(args[0], args[1:]...)
 	if err != nil {
 		return "", fmt.Errorf("running `%s`: %w", strings.Join(args, " "), err)
 	}
 
-	var module struct {
-		Path      string `json:"Path"`
-		Main      bool   `json:"Main"`
-		Dir       string `json:"Dir"`
-		GoMod     string `json:"GoMod"`
-		GoVersion string `json:"GoVersion"`
-	}
-	if err := json.Unmarshal(data, &module); err != nil {
-		return "", fmt.Errorf("decoding json: %w: %s", err, string(data))
+	cwd, err := getwd()
+	if err != nil {
+		return "", fmt.Errorf("getting wd: %w", err)
 	}
 
-	return module.Path, nil
+	decoder := json.NewDecoder(strings.NewReader(output))
+
+	for {
+		// multiple JSON objects will be returned when using Go workspaces; see #63 for details.
+		var module struct {
+			Path      string `json:"Path"`
+			Main      bool   `json:"Main"`
+			Dir       string `json:"Dir"`
+			GoMod     string `json:"GoMod"`
+			GoVersion string `json:"GoVersion"`
+		}
+		if err := decoder.Decode(&module); err != nil {
+			if errors.Is(err, io.EOF) {
+				return "", fmt.Errorf("main module not found\n%s", output)
+			}
+			return "", fmt.Errorf("decoding json: %w\n%s", err, output)
+		}
+
+		if module.Main && strings.HasPrefix(cwd, module.Dir) {
+			return module.Path, nil
+		}
+	}
 }

utils_test.go

@@ -0,0 +1,54 @@
+package musttag
+
+import (
+	"testing"
+
+	"go-simpler.org/assert"
+	. "go-simpler.org/assert/dotimport"
+)
+
+func Test_getMainModule(t *testing.T) {
+	test := func(name, want, output string) {
+		t.Helper()
+		t.Run(name, func(t *testing.T) {
+			t.Helper()
+
+			gwd := getwd
+			co := commandOutput
+			defer func() {
+				getwd = gwd
+				commandOutput = co
+			}()
+
+			getwd = func() (string, error) {
+				return "/path/to/module1/pkg", nil
+			}
+			commandOutput = func(name string, args ...string) (string, error) {
+				return output, nil
+			}
+
+			got, err := getMainModule()
+			assert.NoErr[F](t, err)
+			assert.Equal[E](t, got, want)
+		})
+	}
+
+	test("single module", "module1", `
+{
+	"Path": "module1",
+	"Main": true,
+	"Dir": "/path/to/module1"
+}`)
+
+	test("multiple modules", "module1", `
+{
+	"Path": "module1",
+	"Main": true,
+	"Dir": "/path/to/module1"
+}
+{
+	"Path": "module2",
+	"Main": true,
+	"Dir": "/path/to/module2"
+}`)
+}