cmd/go/internal/modfetch: cache latest revinfo in Versions func

The responses have been cached by the web2 package before removed
it in CL 170879. This change add latest revinfo cache in Versions
func.

Fixes #51391

Change-Id: I73597e0a6b4938238e69d85e1cbbaa9007776db3
Reviewed-on: https://go-review.googlesource.com/c/go/+/403335
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Lee Baokun <bk@golangcn.org>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 26 insertions, 8 deletions

diff --git a/src/cmd/go/internal/modfetch/proxy.go b/src/cmd/go/internal/modfetch/proxy.go
index 4d63dd5648..2491b7d185 100644
--- a/src/cmd/go/internal/modfetch/proxy.go
+++ b/src/cmd/go/internal/modfetch/proxy.go
@@ -187,6 +187,10 @@ type proxyRepo struct {
 	url         *url.URL
 	path        string
 	redactedURL string
+
+	listLatestOnce sync.Once
+	listLatest     *RevInfo
+	listLatestErr  error
 }
 
 func newProxyRepo(baseURL, path string) (Repo, error) {
@@ -214,7 +218,7 @@ func newProxyRepo(baseURL, path string) (Repo, error) {
 	redactedURL := base.Redacted()
 	base.Path = strings.TrimSuffix(base.Path, "/") + "/" + enc
 	base.RawPath = strings.TrimSuffix(base.RawPath, "/") + "/" + pathEscape(enc)
-	return &proxyRepo{base, path, redactedURL}, nil
+	return &proxyRepo{base, path, redactedURL, sync.Once{}, nil, nil}, nil
 }
 
 func (p *proxyRepo) ModulePath() string {
@@ -278,32 +282,46 @@ func (p *proxyRepo) getBody(path string) (r io.ReadCloser, err error) {
 func (p *proxyRepo) Versions(prefix string) ([]string, error) {
 	data, err := p.getBytes("@v/list")
 	if err != nil {
+		p.listLatestOnce.Do(func() {
+			p.listLatest, p.listLatestErr = nil, p.versionError("", err)
+		})
 		return nil, p.versionError("", err)
 	}
 	var list []string
-	for _, line := range strings.Split(string(data), "\n") {
+	allLine := strings.Split(string(data), "\n")
+	for _, line := range allLine {
 		f := strings.Fields(line)
 		if len(f) >= 1 && semver.IsValid(f[0]) && strings.HasPrefix(f[0], prefix) && !module.IsPseudoVersion(f[0]) {
 			list = append(list, f[0])
 		}
 	}
+	p.listLatestOnce.Do(func() {
+		p.listLatest, p.listLatestErr = p.latestFromList(allLine)
+	})
 	semver.Sort(list)
 	return list, nil
 }
 
 func (p *proxyRepo) latest() (*RevInfo, error) {
-	data, err := p.getBytes("@v/list")
-	if err != nil {
-		return nil, p.versionError("", err)
-	}
+	p.listLatestOnce.Do(func() {
+		data, err := p.getBytes("@v/list")
+		if err != nil {
+			p.listLatestErr = p.versionError("", err)
+			return
+		}
+		list := strings.Split(string(data), "\n")
+		p.listLatest, p.listLatestErr = p.latestFromList(list)
+	})
+	return p.listLatest, p.listLatestErr
+}
 
+func (p *proxyRepo) latestFromList(allLine []string) (*RevInfo, error) {
 	var (
 		bestTime             time.Time
 		bestTimeIsFromPseudo bool
 		bestVersion          string
 	)
-
-	for _, line := range strings.Split(string(data), "\n") {
+	for _, line := range allLine {
 		f := strings.Fields(line)
 		if len(f) >= 1 && semver.IsValid(f[0]) {
 			// If the proxy includes timestamps, prefer the timestamp it reports.