cmd/go: add go mod download

go mod download provides a way to force downloading
of a particular module version into the download cache
and also to locate its cached files.

Forcing downloads is useful for warming caches, such as
in base docker images.

Finding the cached files allows caching proxies to use
go mod download as the way to obtain module files
on cache miss.

Fixes #26577.
Fixes #26610.

Change-Id: Ib8065bcce07c9f5105868ec1d87887ef4871f07e
Reviewed-on: https://go-review.googlesource.com/128355
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

7 files changed, 291 insertions, 32 deletions

diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
new file mode 100644
index 0000000000..0a457a56f2
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -0,0 +1,123 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/module"
+	"cmd/go/internal/par"
+	"encoding/json"
+	"os"
+)
+
+var cmdDownload = &base.Command{
+	UsageLine: "go mod download [-dir] [-json] [modules]",
+	Short:     "download modules to local cache",
+	Long: `
+Download downloads the named modules, which can be module patterns selecting
+dependencies of the main module or module queries of the form path@version.
+With no arguments, download applies to all dependencies of the main module.
+
+The go command will automatically download modules as needed during ordinary
+execution. The "go mod download" command is useful mainly for pre-filling
+the local cache or to compute the answers for a Go module proxy.
+
+By default, download reports errors to standard error but is otherwise silent.
+The -json flag causes download to print a sequence of JSON objects
+to standard output, describing each downloaded module (or failure),
+corresponding to this Go struct:
+
+    type Module struct {
+        Path    string // module path
+        Version string // module version
+        Error   string // error loading module
+        Info    string // absolute path to cached .info file
+        GoMod   string // absolute path to cached .mod file
+        Zip     string // absolute path to cached .zip file
+        Dir     string // absolute path to cached source root directory
+    }
+
+See 'go help module' for more about module queries.
+	`,
+}
+
+var downloadJSON = cmdDownload.Flag.Bool("json", false, "")
+
+func init() {
+	cmdDownload.Run = runDownload // break init cycle
+}
+
+type moduleJSON struct {
+	Path    string `json:",omitempty"`
+	Version string `json:",omitempty"`
+	Error   string `json:",omitempty"`
+	Info    string `json:",omitempty"`
+	GoMod   string `json:",omitempty"`
+	Zip     string `json:",omitempty"`
+	Dir     string `json:",omitempty"`
+}
+
+func runDownload(cmd *base.Command, args []string) {
+	if len(args) == 0 {
+		args = []string{"all"}
+	}
+
+	var mods []*moduleJSON
+	var work par.Work
+	listU := false
+	listVersions := false
+	for _, info := range modload.ListModules(args, listU, listVersions) {
+		if info.Replace != nil {
+			info = info.Replace
+		}
+		if info.Version == "" {
+			continue
+		}
+		m := &moduleJSON{
+			Path:    info.Path,
+			Version: info.Version,
+		}
+		mods = append(mods, m)
+		work.Add(m)
+	}
+
+	work.Do(10, func(item interface{}) {
+		m := item.(*moduleJSON)
+		var err error
+		m.Info, err = modfetch.InfoFile(m.Path, m.Version)
+		if err != nil {
+			m.Error = err.Error()
+			return
+		}
+		m.GoMod, err = modfetch.GoModFile(m.Path, m.Version)
+		if err != nil {
+			m.Error = err.Error()
+			return
+		}
+		mod := module.Version{Path: m.Path, Version: m.Version}
+		m.Zip, err = modfetch.DownloadZip(mod)
+		if err != nil {
+			m.Error = err.Error()
+			return
+		}
+		m.Dir, err = modfetch.Download(mod)
+		if err != nil {
+			m.Error = err.Error()
+			return
+		}
+	})
+
+	if *downloadJSON {
+		for _, m := range mods {
+			b, err := json.MarshalIndent(m, "", "\t")
+			if err != nil {
+				base.Fatalf("%v", err)
+			}
+			os.Stdout.Write(append(b, '\n'))
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modcmd/mod.go b/src/cmd/go/internal/modcmd/mod.go
index c1a0ddc7e1..0f78cc3b41 100644
--- a/src/cmd/go/internal/modcmd/mod.go
+++ b/src/cmd/go/internal/modcmd/mod.go
@@ -19,6 +19,7 @@ See 'go help modules' for an overview of module functionality.
 	`,
 
 	Commands: []*base.Command{
+		cmdDownload,
 		cmdEdit,
 		cmdFix,
 		cmdGraph,
diff --git a/src/cmd/go/internal/modfetch/cache.go b/src/cmd/go/internal/modfetch/cache.go
index b801f6485c..efcd4854e8 100644
--- a/src/cmd/go/internal/modfetch/cache.go
+++ b/src/cmd/go/internal/modfetch/cache.go
@@ -232,6 +232,23 @@ func Stat(path, rev string) (*RevInfo, error) {
 	return repo.Stat(rev)
 }
 
+// InfoFile is like Stat but returns the name of the file containing
+// the cached information.
+func InfoFile(path, version string) (string, error) {
+	if !semver.IsValid(version) {
+		return "", fmt.Errorf("invalid version %q", version)
+	}
+	if _, err := Stat(path, version); err != nil {
+		return "", err
+	}
+	// Stat should have populated the disk cache for us.
+	file, _, err := readDiskStat(path, version)
+	if err != nil {
+		return "", err
+	}
+	return file, nil
+}
+
 // GoMod is like Lookup(path).GoMod(rev) but avoids the
 // repository path resolution in Lookup if the result is
 // already cached on local disk.
@@ -256,6 +273,23 @@ func GoMod(path, rev string) ([]byte, error) {
 	return repo.GoMod(rev)
 }
 
+// GoModFile is like GoMod but returns the name of the file containing
+// the cached information.
+func GoModFile(path, version string) (string, error) {
+	if !semver.IsValid(version) {
+		return "", fmt.Errorf("invalid version %q", version)
+	}
+	if _, err := GoMod(path, version); err != nil {
+		return "", err
+	}
+	// GoMod should have populated the disk cache for us.
+	file, _, err := readDiskGoMod(path, version)
+	if err != nil {
+		return "", err
+	}
+	return file, nil
+}
+
 var errNotCached = fmt.Errorf("not in cache")
 
 // readDiskStat reads a cached stat result from disk,
@@ -274,6 +308,13 @@ func readDiskStat(path, rev string) (file string, info *RevInfo, err error) {
 	if err := json.Unmarshal(data, info); err != nil {
 		return file, nil, errNotCached
 	}
+	// The disk might have stale .info files that have Name and Short fields set.
+	// We want to canonicalize to .info files with those fields omitted.
+	// Remarshal and update the cache file if needed.
+	data2, err := json.Marshal(info)
+	if err == nil && !bytes.Equal(data2, data) {
+		writeDiskCache(file, data)
+	}
 	return file, info, nil
 }
 
diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
index b4944af8c2..480579156f 100644
--- a/src/cmd/go/internal/modfetch/fetch.go
+++ b/src/cmd/go/internal/modfetch/fetch.go
@@ -17,6 +17,7 @@ import (
 	"sync"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/dirhash"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
@@ -46,24 +47,10 @@ func Download(mod module.Version) (dir string, err error) {
 			return cached{"", err}
 		}
 		if files, _ := ioutil.ReadDir(dir); len(files) == 0 {
-			zipfile, err := CachePath(mod, "zip")
+			zipfile, err := DownloadZip(mod)
 			if err != nil {
 				return cached{"", err}
 			}
-			if _, err := os.Stat(zipfile); err == nil {
-				// Use it.
-				// This should only happen if the mod/cache directory is preinitialized
-				// or if pkg/mod/path was removed but not pkg/mod/cache/download.
-				fmt.Fprintf(os.Stderr, "go: extracting %s %s\n", mod.Path, mod.Version)
-			} else {
-				if err := os.MkdirAll(filepath.Dir(zipfile), 0777); err != nil {
-					return cached{"", err}
-				}
-				fmt.Fprintf(os.Stderr, "go: downloading %s %s\n", mod.Path, mod.Version)
-				if err := downloadZip(mod, zipfile); err != nil {
-					return cached{"", err}
-				}
-			}
 			modpath := mod.Path + "@" + mod.Version
 			if err := Unzip(dir, zipfile, modpath, 0); err != nil {
 				fmt.Fprintf(os.Stderr, "-> %s\n", err)
@@ -76,6 +63,46 @@ func Download(mod module.Version) (dir string, err error) {
 	return c.dir, c.err
 }
 
+var downloadZipCache par.Cache
+
+// DownloadZip downloads the specific module version to the
+// local zip cache and returns the name of the zip file.
+func DownloadZip(mod module.Version) (zipfile string, err error) {
+	// The par.Cache here avoids duplicate work but also
+	// avoids conflicts from simultaneous calls by multiple goroutines
+	// for the same version.
+	type cached struct {
+		zipfile string
+		err     error
+	}
+	c := downloadZipCache.Do(mod, func() interface{} {
+		zipfile, err := CachePath(mod, "zip")
+		if err != nil {
+			return cached{"", err}
+		}
+		if _, err := os.Stat(zipfile); err == nil {
+			// Use it.
+			// This should only happen if the mod/cache directory is preinitialized
+			// or if pkg/mod/path was removed but not pkg/mod/cache/download.
+			if cfg.CmdName != "mod download" {
+				fmt.Fprintf(os.Stderr, "go: extracting %s %s\n", mod.Path, mod.Version)
+			}
+		} else {
+			if err := os.MkdirAll(filepath.Dir(zipfile), 0777); err != nil {
+				return cached{"", err}
+			}
+			if cfg.CmdName != "mod download" {
+				fmt.Fprintf(os.Stderr, "go: downloading %s %s\n", mod.Path, mod.Version)
+			}
+			if err := downloadZip(mod, zipfile); err != nil {
+				return cached{"", err}
+			}
+		}
+		return cached{zipfile, nil}
+	}).(cached)
+	return c.zipfile, c.err
+}
+
 func downloadZip(mod module.Version, target string) error {
 	repo, err := Lookup(mod.Path)
 	if err != nil {
diff --git a/src/cmd/go/internal/modfetch/repo.go b/src/cmd/go/internal/modfetch/repo.go
index f6f47bb998..003479461c 100644
--- a/src/cmd/go/internal/modfetch/repo.go
+++ b/src/cmd/go/internal/modfetch/repo.go
@@ -55,9 +55,12 @@ type Repo interface {
 // A Rev describes a single revision in a module repository.
 type RevInfo struct {
 	Version string    // version string
-	Name    string    // complete ID in underlying repository
-	Short   string    // shortened ID, for use in pseudo-version
 	Time    time.Time // commit time
+
+	// These fields are used for Stat of arbitrary rev,
+	// but they are not recorded when talking about module versions.
+	Name  string `json:"-"` // complete ID in underlying repository
+	Short string `json:"-"` // shortened ID, for use in pseudo-version
 }
 
 // Re: module paths, import paths, repository roots, and lookups
diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
index b989af28da..5893db14aa 100644
--- a/src/cmd/go/internal/modload/build.go
+++ b/src/cmd/go/internal/modload/build.go
@@ -144,23 +144,25 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 
 	complete(info)
 
-	if r := Replacement(m); r.Path != "" {
-		info.Replace = &modinfo.ModulePublic{
-			Path:      r.Path,
-			Version:   r.Version,
-			GoVersion: info.GoVersion,
-		}
-		if r.Version == "" {
-			if filepath.IsAbs(r.Path) {
-				info.Replace.Dir = r.Path
-			} else {
-				info.Replace.Dir = filepath.Join(ModRoot, r.Path)
+	if fromBuildList {
+		if r := Replacement(m); r.Path != "" {
+			info.Replace = &modinfo.ModulePublic{
+				Path:      r.Path,
+				Version:   r.Version,
+				GoVersion: info.GoVersion,
+			}
+			if r.Version == "" {
+				if filepath.IsAbs(r.Path) {
+					info.Replace.Dir = r.Path
+				} else {
+					info.Replace.Dir = filepath.Join(ModRoot, r.Path)
+				}
 			}
+			complete(info.Replace)
+			info.Dir = info.Replace.Dir
+			info.GoMod = filepath.Join(info.Dir, "go.mod")
+			info.Error = nil // ignore error loading original module version (it has been replaced)
 		}
-		complete(info.Replace)
-		info.Dir = info.Replace.Dir
-		info.GoMod = filepath.Join(info.Dir, "go.mod")
-		info.Error = nil // ignore error loading original module version (it has been replaced)
 	}
 
 	return info
diff --git a/src/cmd/go/testdata/script/mod_download.txt b/src/cmd/go/testdata/script/mod_download.txt
new file mode 100644
index 0000000000..ef931cfd30
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download.txt
@@ -0,0 +1,62 @@
+env GO111MODULE=on
+
+# download with version should print nothing
+go mod download rsc.io/quote@v1.5.0
+! stdout .
+
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+# download -json with version should print JSON
+go mod download -json 'rsc.io/quote@<=v1.5.0'
+stdout '^\t"Path": "rsc.io/quote"'
+stdout '^\t"Version": "v1.5.0"'
+stdout '^\t"Info": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.0.info"'
+stdout '^\t"GoMod": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.0.mod"'
+stdout '^\t"Zip": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.0.zip"'
+! stdout '"Error"'
+
+# download queries above should not have added to go.mod.
+go list -m all
+! stdout rsc.io
+
+# add to go.mod so we can test non-query downloads
+go mod edit -require rsc.io/quote@v1.5.2
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+# module loading will page in the info and mod files
+go list -m all
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+# download will fetch and unpack the zip file
+go mod download
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+
+go mod download -json
+stdout '^\t"Path": "rsc.io/quote"'
+stdout '^\t"Version": "v1.5.2"'
+stdout '^\t"Info": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.2.info"'
+stdout '^\t"GoMod": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.2.mod"'
+stdout '^\t"Zip": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.2.zip"'
+stdout '^\t"Dir": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)rsc.io(\\\\|/)quote@v1.5.2"'
+
+# download will follow replacements
+go mod edit -require rsc.io/quote@v1.5.1 -replace rsc.io/quote@v1.5.1=rsc.io/quote@v1.5.3-pre1
+go mod download
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.zip
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
+
+# download will not follow replacements for explicit module queries
+go mod download -json rsc.io/quote@v1.5.1
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.zip
+
+-- go.mod --
+module m