main: support installing github.com/myitcv/gobin (#17)

This is initial support that is unlikely to work on Windows, hence the
TODO in the code.

Unfortunately, I've had to mark the test as skip for now because we
can't install "self" from the module under test. I.e. it's not possible
to run go install for a package in the module under test, i.e.
github.com/myitcv/gobin.

main.go

@@ -280,6 +280,10 @@ func mainerr() error {
 			gobin := filepath.Join(gobinCache, mp.Dir)
 			target := filepath.Join(gobin, path.Base(mp.ImportPath))
 
+			// optimistically remove our target in case we are installing over self
+			// TODO work out what to do for Windows
+			_ = os.Remove(target)
+
 			proxy := "file://" + modDlCache
 
 			var stdout, stderr bytes.Buffer
@@ -320,7 +324,12 @@ func mainerr() error {
 				}
 				defer src.Close()
 				bin := filepath.Join(installBin, path.Base(mp.ImportPath))
-				dest, err := os.Create(bin)
+
+				// optimistically remove our target in case we are installing over self
+				// TODO work out what to do for Windows
+				_ = os.Remove(bin)
+
+				dest, err := os.OpenFile(bin, os.O_CREATE|os.O_WRONLY|os.O_EXCL, 0755)
 				if err != nil {
 					return fmt.Errorf("failed to open %v for writing: %v", bin, err)
 				}

script_test.go

@@ -3,14 +3,17 @@ package main
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"testing"
 
 	"github.com/rogpeppe/go-internal/goproxytest"
 	"github.com/rogpeppe/go-internal/gotooltest"
 	"github.com/rogpeppe/go-internal/testscript"
 )
 
-var proxyURL string
+var (
+	proxyURL string
+)
 
 func TestMain(m *testing.M) {
 	os.Exit(testscript.RunMain(gobinMain{m}, map[string]func() int{
@@ -23,14 +26,6 @@ type gobinMain struct {
 }
 
 func (m gobinMain) Run() int {
-	if os.Getenv("GO_GCFLAGS") != "" {
-		fmt.Fprintf(os.Stderr, "testing: warning: no tests to run\n") // magic string for cmd/go
-		fmt.Printf("cmd/go test is not compatible with $GO_GCFLAGS being set\n")
-		fmt.Printf("SKIP\n")
-		return 0
-	}
-	os.Unsetenv("GOROOT_FINAL")
-
 	// Start the Go proxy server running for all tests.
 	srv, err := goproxytest.NewServer("testdata/mod", "")
 	if err != nil {
@@ -43,10 +38,38 @@ func (m gobinMain) Run() int {
 }
 
 func TestScripts(t *testing.T) {
+	var (
+		pathToMod     string // local path to this module
+		modTestGOPATH string // GOPATH set when running tests in this module
+	)
+
+	// set pathToMod
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("failed to get working directory for test: %v", err)
+	}
+	pathToMod = wd
+
+	// set modTestGOPATH
+	cmd := exec.Command("go", "env", "GOPATH")
+	out, err := cmd.Output()
+	if err != nil {
+		var stderr []byte
+		if err, ok := err.(*exec.ExitError); ok {
+			stderr = err.Stderr
+		}
+		t.Fatalf("failed to get GOPATH for test: %v\n%s", err, stderr)
+	}
+	modTestGOPATH = string(out)
+
 	p := testscript.Params{
 		Dir: "testdata",
 		Setup: func(e *testscript.Env) error {
-			e.Vars = append(e.Vars, "GOPROXY="+proxyURL)
+			e.Vars = append(e.Vars,
+				"TESTGOPATH="+modTestGOPATH,
+				"GOBINMODPATH="+pathToMod,
+				"GOPROXY="+proxyURL,
+			)
 			return nil
 		},
 	}

testdata/install-self.txt

@@ -0,0 +1,21 @@
+skip 'Skipping because of https://github.com/golang/go/issues/26241'
+
+env GOPROXY=file://$TESTGOPATH/pkg/mod/cache/download
+env GOBIN=$WORK/install
+env PATH=$GOBIN:$PATH
+
+env GOPATH=$TESTGOPATH
+cd $GOBINMODPATH
+go install
+exists $GOBIN/gobin
+exec which gobin
+stdout ^$GOBIN/gobin'$'
+
+cd $WORK/repo
+exec go mod edit -replace=myitcv.io=$GOBINMODPATH
+exec gobin -m github.com/myitcv/gobin
+
+-- repo/go.mod --
+module mod
+
+require github.com/myitcv/gobin v0.0.0