src/cmd/go/testdata/script/mod_download_concurrent_read.txt

# This test simulates a process watching for changes and reading files in
# module cache as a module is extracted.
#
# By default, we unzip a downloaded module into a temporary directory with a
# random name, then rename the directory into place. On Windows, this fails
# with ERROR_ACCESS_DENIED if another process (e.g., antivirus) opens files
# in the directory.
#
# Setting GODEBUG=modcacheunzipinplace=1 opts into new behavior: a downloaded
# module is unzipped in place. A .partial file is created elsewhere to indicate
# that the extraction is incomplete.
#
# Verifies golang.org/issue/36568.

[!windows] skip
[short] skip

# Control case: check that the default behavior fails.
# This is commented out to avoid flakiness. We can't reproduce the failure
# 100% of the time.
# ! go run downloader.go

# Experiment: check that the new behavior does not fail.
env GODEBUG=modcacheunzipinplace=1
go run downloader.go

-- go.mod --
module example.com/m

go 1.14

-- downloader.go --
package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"path/filepath"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

// run repeatedly downloads a module while opening files in the module cache
// in a background goroutine.
//
// run uses a different temporary module cache in each iteration so that we
// don't need to clean the cache or synchronize closing files after each
// iteration.
func run() (err error) {
	tmpDir, err := ioutil.TempDir("", "")
	if err != nil {
		return err
	}
	defer func() {
		if rmErr := os.RemoveAll(tmpDir); err == nil && rmErr != nil {
			err = rmErr
		}
	}()
	for i := 0; i < 10; i++ {
    gopath := filepath.Join(tmpDir, fmt.Sprintf("gopath%d", i))
		var err error
		done := make(chan struct{})
		go func() {
			err = download(gopath)
			close(done)
		}()
		readCache(gopath, done)
		if err != nil {
			return err
		}
	}
	return nil
}

// download downloads a module into the given cache using 'go mod download'.
func download(gopath string) error {
	cmd := exec.Command("go", "mod", "download", "-modcacherw", "rsc.io/quote@v1.5.2")
	cmd.Stderr = os.Stderr
	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
	return cmd.Run()
}

// readCache repeatedly globs for go.mod files in the given cache, then opens
// those files for reading. When the done chan is closed, readCache closes
// files and returns.
func readCache(gopath string, done <-chan struct{}) {
	files := make(map[string]*os.File)
	defer func() {
		for _, f := range files {
			f.Close()
		}
	}()

	pattern := filepath.Join(gopath, "pkg/mod/rsc.io/quote@v1.5.2*/go.mod")
	for {
		select {
		case <-done:
			return
		default:
		}

		names, _ := filepath.Glob(pattern)
		for _, name := range names {
			if files[name] != nil {
				continue
			}
			f, _ := os.Open(name)
			if f != nil {
				files[name] = f
			}
		}
	}
}