1 files changed, 121 insertions, 0 deletions
diff --git a/src/cmd/go/internal/vcweb/hg.go b/src/cmd/go/internal/vcweb/hg.go
new file mode 100644
index 0000000..e78f850
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/hg.go
@@ -0,0 +1,121 @@
+// Copyright 2022 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 vcweb
+
+import (
+	"bufio"
+	"errors"
+	"io"
+	"log"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+)
+
+type hgHandler struct {
+	once      sync.Once
+	hgPath    string
+	hgPathErr error
+}
+
+func (h *hgHandler) Available() bool {
+	h.once.Do(func() {
+		h.hgPath, h.hgPathErr = exec.LookPath("hg")
+	})
+	return h.hgPathErr == nil
+}
+
+func (h *hgHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	if !h.Available() {
+		return nil, ServerNotInstalledError{name: "hg"}
+	}
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		// Mercurial has a CGI server implementation (called hgweb). In theory we
+		// could use that — however, assuming that hgweb is even installed, the
+		// configuration for hgweb varies by Python version (2 vs 3), and we would
+		// rather not go rooting around trying to find the right Python version to
+		// run.
+		//
+		// Instead, we'll take a somewhat more roundabout approach: we assume that
+		// if "hg" works at all then "hg serve" works too, and we'll execute that as
+		// a subprocess, using a reverse proxy to forward the request and response.
+
+		cmd := exec.Command(h.hgPath, "serve", "--port", "0", "--address", "localhost", "--accesslog", os.DevNull, "--name", "vcweb", "--print-url")
+		cmd.Dir = dir
+		cmd.Env = append(env[:len(env):len(env)], "PWD="+dir)
+
+		stderr := new(strings.Builder)
+		cmd.Stderr = stderr
+
+		stdout, err := cmd.StdoutPipe()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		readDone := make(chan struct{})
+		defer func() {
+			stdout.Close()
+			<-readDone
+		}()
+
+		hgURL := make(chan *url.URL, 1)
+		hgURLError := make(chan error, 1)
+		go func() {
+			defer close(readDone)
+			r := bufio.NewReader(stdout)
+			for {
+				line, err := r.ReadString('\n')
+				if err != nil {
+					return
+				}
+				u, err := url.Parse(strings.TrimSpace(line))
+				if err == nil {
+					hgURL <- u
+				} else {
+					hgURLError <- err
+				}
+				break
+			}
+			io.Copy(io.Discard, r)
+		}()
+
+		if err := cmd.Start(); err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		defer func() {
+			if err := cmd.Process.Signal(os.Interrupt); err != nil && !errors.Is(err, os.ErrProcessDone) {
+				cmd.Process.Kill()
+			}
+			err := cmd.Wait()
+			if out := strings.TrimSuffix(stderr.String(), "interrupted!\n"); out != "" {
+				logger.Printf("%v: %v\n%s", cmd, err, out)
+			} else {
+				logger.Printf("%v", cmd)
+			}
+		}()
+
+		select {
+		case <-req.Context().Done():
+			logger.Printf("%v: %v", req.Context().Err(), cmd)
+			http.Error(w, req.Context().Err().Error(), http.StatusBadGateway)
+			return
+		case err := <-hgURLError:
+			logger.Printf("%v: %v", cmd, err)
+			http.Error(w, err.Error(), http.StatusBadGateway)
+			return
+		case url := <-hgURL:
+			logger.Printf("proxying hg request to %s", url)
+			httputil.NewSingleHostReverseProxy(url).ServeHTTP(w, req)
+		}
+	})
+
+	return handler, nil
+}