cmd/go: add a Latest field to the output of 'go mod download -json'

Fixes #32239

Change-Id: I5723abaa9b6bed7e8fb2d95f749a4e03ecc8741b
Reviewed-on: https://go-review.googlesource.com/c/go/+/183841
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modcmd/download.go

@@ -43,6 +43,7 @@ corresponding to this Go struct:
         Dir      string // absolute path to cached source root directory
         Sum      string // checksum for path, version (as in go.sum)
         GoModSum string // checksum for go.mod (as in go.sum)
+        Latest   bool   // would @latest resolve to this version?
     }
 
 See 'go help modules' for more about module queries.
@@ -65,6 +66,7 @@ type moduleJSON struct {
 	Dir      string `json:",omitempty"`
 	Sum      string `json:",omitempty"`
 	GoModSum string `json:",omitempty"`
+	Latest   bool   `json:",omitempty"`
 }
 
 func runDownload(cmd *base.Command, args []string) {
@@ -98,6 +100,26 @@ func runDownload(cmd *base.Command, args []string) {
 		work.Add(m)
 	}
 
+	latest := map[string]string{} // path → version
+	if *downloadJSON {
+		// We need to populate the Latest field, but if the main module depends on a
+		// version newer than latest — or if the version requested on the command
+		// line is itself newer than latest — that's not trivial to determine from
+		// the info returned by ListModules. Instead, we issue a separate
+		// ListModules request for "latest", which should be inexpensive relative to
+		// downloading the modules.
+		var latestArgs []string
+		for _, m := range mods {
+			latestArgs = append(latestArgs, m.Path+"@latest")
+		}
+
+		for _, info := range modload.ListModules(latestArgs, listU, listVersions) {
+			if info.Version != "" {
+				latest[info.Path] = info.Version
+			}
+		}
+	}
+
 	work.Do(10, func(item interface{}) {
 		m := item.(*moduleJSON)
 		var err error
@@ -128,6 +150,9 @@ func runDownload(cmd *base.Command, args []string) {
 			m.Error = err.Error()
 			return
 		}
+		if latest[m.Path] == m.Version {
+			m.Latest = true
+		}
 	})
 
 	if *downloadJSON {

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

@@ -17,6 +17,7 @@ stderr 'this.domain.is.invalid'
 stdout '"Error": ".*this.domain.is.invalid.*"'
 
 # download -json with version should print JSON
+# and download the .info file for the 'latest' version.
 go mod download -json 'rsc.io/quote@<=v1.5.0'
 stdout '^\t"Path": "rsc.io/quote"'
 stdout '^\t"Version": "v1.5.0"'
@@ -27,13 +28,14 @@ stdout '^\t"Sum": "h1:6fJa6E\+wGadANKkUMlZ0DhXFpoKlslOQDCo259XtdIE="'  # hash of
 stdout '^\t"GoModSum": "h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe\+TKr0="'
 ! stdout '"Error"'
 
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+
 # 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
 

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

@@ -0,0 +1,20 @@
+env GO111MODULE=on
+
+# If the module is the latest version of itself,
+# the Latest field should be set.
+go mod download -json rsc.io/quote@v1.5.2
+stdout '"Latest":\s*true'
+
+# If the module is older than latest, the field should be unset.
+go mod download -json rsc.io/quote@v1.5.1
+! stdout '"Latest":'
+
+# If the module is newer than "latest", the field should be unset...
+go mod download -json rsc.io/quote@v1.5.3-pre1
+! stdout '"Latest":'
+
+# ...even if that version is also what is required by the main module.
+go mod init example.com
+go mod edit -require rsc.io/quote@v1.5.3-pre1
+go mod download -json rsc.io/quote@v1.5.3-pre1
+! stdout '"Latest":'

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

@@ -1,8 +1,28 @@
 env GO111MODULE=on
 
+# If the current version is not latest, 'go list -u' should include its upgrade.
 go list -m -u all
 stdout 'rsc.io/quote v1.2.0 \[v1\.5\.2\]'
 
+# If the current version is latest, 'go list -u' should omit the upgrade.
+go get -d rsc.io/quote@v1.5.2
+go list -m -u all
+stdout 'rsc.io/quote v1.5.2$'
+
+# If the current version is newer than latest, 'go list -u' should
+# omit the upgrade.
+go get -d rsc.io/quote@v1.5.3-pre1
+go list -m -u all
+stdout 'rsc.io/quote v1.5.3-pre1$'
+
+# If the current build list has a higher version and the user asks about
+# a lower one, -u should report the upgrade for the lower one
+# but leave the build list unchanged.
+go list -m -u rsc.io/quote@v1.5.1
+stdout 'rsc.io/quote v1.5.1 \[v1.5.2\]$'
+go list -m -u rsc.io/quote
+stdout 'rsc.io/quote v1.5.3-pre1$'
+
 -- go.mod --
 module x
 require rsc.io/quote v1.2.0