cmd/go: group 'go get' update messages together near the end of output

In module mode, 'go get' prints a message for each version query it
resolves. This change groups those messages together near the end of
the output so they aren't mixed with other module "finding" and
"downloading" messages. They'll still be printed before build-related
messages.

Fixes #37982

Change-Id: I107a9f2b2f839e896399df906e20d6fc77f280c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/232578
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modget/get.go

@@ -6,8 +6,10 @@
 package modget
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"sort"
@@ -702,6 +704,12 @@ func runGet(cmd *base.Command, args []string) {
 	modload.AllowWriteGoMod()
 	modload.WriteGoMod()
 
+	// Print the changes we made.
+	// TODO(golang.org/issue/33284): include more information about changes to
+	// relevant module versions due to MVS upgrades and downgrades. For now,
+	// the log only contains messages for versions resolved with getQuery.
+	writeUpdateLog()
+
 	// If -d was specified, we're done after the module work.
 	// We've already downloaded modules by loading packages above.
 	// Otherwise, we need to build and install the packages matched by
@@ -1034,11 +1042,28 @@ func (r *lostUpgradeReqs) Required(mod module.Version) ([]module.Version, error)
 	return r.Reqs.Required(mod)
 }
 
-var loggedLines sync.Map
+var updateLog struct {
+	mu     sync.Mutex
+	buf    bytes.Buffer
+	logged map[string]bool
+}
 
 func logOncef(format string, args ...interface{}) {
 	msg := fmt.Sprintf(format, args...)
-	if _, dup := loggedLines.LoadOrStore(msg, true); !dup {
-		fmt.Fprintln(os.Stderr, msg)
+	updateLog.mu.Lock()
+	defer updateLog.mu.Unlock()
+	if updateLog.logged == nil {
+		updateLog.logged = make(map[string]bool)
+	}
+	if updateLog.logged[msg] {
+		return
 	}
+	updateLog.logged[msg] = true
+	fmt.Fprintln(&updateLog.buf, msg)
+}
+
+func writeUpdateLog() {
+	updateLog.mu.Lock()
+	defer updateLog.mu.Unlock()
+	io.Copy(os.Stderr, &updateLog.buf)
 }

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

@@ -0,0 +1,15 @@
+# Upgrades are reported.
+go get -d rsc.io/quote
+stderr '^go: rsc.io/quote upgrade => v1.5.2\n\z'
+
+# Downgrades are not reported.
+# TODO(golang.org/issue/33284): they should be.
+go get -d rsc.io/quote@v1.5.0
+stderr '^go: downloading.*\n\z'
+
+-- go.mod --
+module m
+
+go 1.15
+
+require rsc.io/quote v1.5.0