do not use go list to find go.mod file (#56)

The go list command fails if the go.mod file isn't well formed (like for
example one of the replace directives refers to a non-existent target).
This can be a problem, so find the go.mod file by looking directly for
it instead.

gomodcmd.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"io"
 	"io/ioutil"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -56,29 +57,35 @@ func listModules(modules ...string) (mods map[string]*listModule, err error) {
 	return mods, nil
 }
 
-func goModInfo() (*modfile.File, error) {
-	// Get info on the current module so that we can find the
-	// path to the current go.mod file.
-	mods, err := listModules()
+// goModInfo returns the main module's root directory
+// and the parsed contents of the go.mod file.
+func goModInfo() (string, *modfile.File, error) {
+	goModPath, err := findGoMod(cwd)
 	if err != nil {
-		return nil, errors.Wrap(err)
-	}
-	if len(mods) != 1 {
-		return nil, errors.Newf("unexpected module list count (want 1 got %d)", len(mods))
-	}
-	var goModPath string
-	for _, m := range mods {
-		goModPath = m.GoMod
+		return "", nil, errors.Notef(err, nil, "cannot find main module")
 	}
+	rootDir := filepath.Dir(goModPath)
 	data, err := ioutil.ReadFile(goModPath)
 	if err != nil {
-		return nil, errors.Notef(err, nil, "cannot read current go.mod file")
+		return "", nil, errors.Notef(err, nil, "cannot read main go.mod file")
 	}
 	modf, err := modfile.Parse(goModPath, data, nil)
 	if err != nil {
-		return nil, errors.Wrap(err)
+		return "", nil, errors.Wrap(err)
+	}
+	return rootDir, modf, nil
+}
+
+func findGoMod(dir string) (string, error) {
+	out, err := runCmd(dir, "go", "env", "GOMOD")
+	if err != nil {
+		return "", err
+	}
+	out = strings.TrimSpace(out)
+	if out == "" {
+		return "", errors.New("no go.mod file found in any parent directory")
 	}
-	return modf, nil
+	return strings.TrimSpace(out), nil
 }
 
 func writeModFile(modf *modfile.File) error {

main.go

@@ -104,7 +104,7 @@ func main1() int {
 		return 2
 	}
 
-	if mf, err := goModInfo(); err == nil {
+	if _, mf, err := goModInfo(); err == nil {
 		mainModFile = mf
 	} else {
 		return errorf("cannot determine main module: %v", err)

script_test.go

@@ -46,7 +46,10 @@ func TestScripts(t *testing.T) {
 	p := testscript.Params{
 		Dir: "testdata",
 		Setup: func(e *testscript.Env) error {
-			e.Vars = append(e.Vars, "GOPROXY="+proxyURL)
+			e.Vars = append(e.Vars,
+				"GOPROXY="+proxyURL,
+				"GONOSUMDB=*",
+			)
 			return nil
 		},
 	}

testdata/get-no-main-mod.txt

@@ -0,0 +1,2 @@
+! gohack get nothing
+stderr 'cannot find main module: no go.mod file found in any parent directory'

testdata/undo-not-existent.txt

@@ -0,0 +1,29 @@
+cd repo
+go get rsc.io/[email protected]
+env GOHACK=$WORK/gohack
+gohack get rsc.io/quote
+stdout '^rsc.io/quote => .*/gohack/rsc.io/quote$'
+! stderr .+
+
+exists $GOHACK/rsc.io/quote
+rm $GOHACK/rsc.io/quote
+
+gohack undo
+stdout '^dropped rsc\.io/quote$'
+! stderr .+
+
+-- repo/main.go --
+package main
+import (
+	"fmt"
+	"rsc.io/quote"
+	"rsc.io/sampler"
+)
+
+func main() {
+	sampler.DefaultUserPrefs()
+	fmt.Println(quote.Glass())
+}
+
+-- repo/go.mod --
+module example.com/repo