support redirect to correct version URL

main.go

@@ -1,9 +1,9 @@
 package main
 
 import (
-	"bytes"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	"os"
 	"os/exec"
@@ -37,13 +37,18 @@ func main() {
 func mainHandler(inner http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if _, err := os.Stat(filepath.Join(cacheDir, r.URL.Path)); err != nil {
-			if strings.HasSuffix(r.URL.Path, ".info") {
+			var suffix string
+			if strings.HasSuffix(r.URL.Path, ".info") || strings.HasSuffix(r.URL.Path, ".mod") {
+				suffix = ".mod"
+				if strings.HasSuffix(r.URL.Path, ".info") {
+					suffix = ".info"
+				}
 				mod := strings.Split(r.URL.Path, "/@v/")
 				if len(mod) != 2 {
 					ReturnServerError(w, fmt.Errorf("bad module path:%s", r.URL.Path))
 					return
 				}
-				version := strings.TrimSuffix(mod[1], ".info")
+				version := strings.TrimSuffix(mod[1], suffix)
 				version, err = module.DecodeVersion(version)
 				if err != nil {
 					ReturnServerError(w, err)
@@ -55,9 +60,9 @@ func mainHandler(inner http.Handler) http.Handler {
 					ReturnServerError(w, err)
 					return
 				}
-				stdout, stderr, err := goGet(path + "@" + version)
+				err = goGet(path, version, suffix, w, r)
 				if err != nil {
-					ReturnServerError(w, fmt.Errorf("stdout: %s stderr: %s", stdout, stderr))
+					ReturnServerError(w, err)
 					return
 				}
 			}
@@ -71,13 +76,55 @@ func mainHandler(inner http.Handler) http.Handler {
 	})
 }
 
-func goGet(path string) (string, string, error) {
-	fmt.Fprintf(os.Stdout, "goproxy: download %s\n", path)
-	cmd := exec.Command("go", "get", "-d", path)
-	var stdout bytes.Buffer
-	var stderr bytes.Buffer
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
-	err := cmd.Run()
-	return string(stdout.Bytes()), string(stderr.Bytes()), err
+func goGet(path, version, suffix string, w http.ResponseWriter, r *http.Request) error {
+	cmd := exec.Command("go", "get", "-d", path+"@"+version)
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return err
+	}
+
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		return err
+	}
+
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+
+	bytesErr, err := ioutil.ReadAll(stderr)
+	if err != nil {
+		return err
+	}
+
+	bytesOut, err := ioutil.ReadAll(stdout)
+	if err != nil {
+		return err
+	}
+
+	if err := cmd.Wait(); err != nil {
+		return err
+	}
+	out := fmt.Sprintf("%s", bytesErr)
+
+	for _, line := range strings.Split(out, "\n") {
+		f := strings.Fields(line)
+		if len(f) != 4 {
+			continue
+		}
+		if f[1] == "downloading" && f[2] == path && f[3] != version {
+			h := r.Host
+			mod := strings.Split(r.URL.Path, "/@v/")
+			p := fmt.Sprintf("%s/@v/%s%s", mod[0], f[3], suffix)
+			scheme := "http:"
+			if r.TLS != nil {
+				scheme = "https:"
+			}
+			url := fmt.Sprintf("%s//%s/%s", scheme, h, p)
+			http.Redirect(w, r, url, 302)
+		}
+	}
+
+	fmt.Fprintf(os.Stdout, "goproxy: download %s stdout: %s stderr: %s\n", path, string(bytesOut), string(bytesErr))
+	return nil
 }