misc/dashboard/builder: various cleanups

* allow commit watcher to be disabled, useful for small slow builders who will never be the first to notice a commit.
* builders always update their local master working copy before cloning a specific revision.
* refactor hg repo operations into a new type, Repo.

R=adg, shanemhansen, luitvd
CC=golang-dev
https://golang.org/cl/7326053

misc/dashboard/builder/vcs.go

+// Copyright 2013 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 main
+
+import (
+	"encoding/xml"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+// Repo represents a mercurial repository.
+type Repo struct {
+	Path string
+	sync.Mutex
+}
+
+// RemoteRepo constructs a *Repo representing a remote repository.
+func RemoteRepo(url string) *Repo {
+	return &Repo{
+		Path: url,
+	}
+}
+
+// Clone clones the current Repo to a new destination
+// returning a new *Repo if successful.
+func (r *Repo) Clone(path, rev string) (*Repo, error) {
+	r.Lock()
+	defer r.Unlock()
+	if err := run(*cmdTimeout, nil, *buildroot, r.hgCmd("clone", "-r", rev, r.Path, path)...); err != nil {
+		return nil, err
+	}
+	return &Repo{
+		Path: path,
+	}, nil
+}
+
+// UpdateTo updates the working copy of this Repo to the
+// supplied revision.
+func (r *Repo) UpdateTo(hash string) error {
+	r.Lock()
+	defer r.Unlock()
+	return run(*cmdTimeout, nil, r.Path, r.hgCmd("update", hash)...)
+}
+
+// Exists reports whether this Repo represents a valid Mecurial repository.
+func (r *Repo) Exists() bool {
+	fi, err := os.Stat(filepath.Join(r.Path, ".hg"))
+	if err != nil {
+		return false
+	}
+	return fi.IsDir()
+}
+
+// Pull pulls changes from the default path, that is, the path
+// this Repo was cloned from.
+func (r *Repo) Pull() error {
+	r.Lock()
+	defer r.Unlock()
+	return run(*cmdTimeout, nil, r.Path, r.hgCmd("pull")...)
+}
+
+// Log returns the changelog for this repository.
+func (r *Repo) Log() ([]HgLog, error) {
+	if err := r.Pull(); err != nil {
+		return nil, err
+	}
+	const N = 50 // how many revisions to grab
+
+	r.Lock()
+	defer r.Unlock()
+	data, _, err := runLog(*cmdTimeout, nil, r.Path, r.hgCmd("log",
+		"--encoding=utf-8",
+		"--limit="+strconv.Itoa(N),
+		"--template="+xmlLogTemplate)...,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var logStruct struct {
+		Log []HgLog
+	}
+	err = xml.Unmarshal([]byte("<Top>"+data+"</Top>"), &logStruct)
+	if err != nil {
+		log.Printf("unmarshal hg log: %v", err)
+		return nil, err
+	}
+	return logStruct.Log, nil
+}
+
+// FullHash returns the full hash for the given Mercurial revision.
+func (r *Repo) FullHash(rev string) (string, error) {
+	r.Lock()
+	defer r.Unlock()
+	s, _, err := runLog(*cmdTimeout, nil, r.Path,
+		r.hgCmd("log",
+			"--encoding=utf-8",
+			"--rev="+rev,
+			"--limit=1",
+			"--template={node}")...,
+	)
+	if err != nil {
+		return "", nil
+	}
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return "", fmt.Errorf("cannot find revision")
+	}
+	if len(s) != 40 {
+		return "", fmt.Errorf("hg returned invalid hash " + s)
+	}
+	return s, nil
+}
+
+func (r *Repo) hgCmd(args ...string) []string {
+	return append([]string{"hg", "--config", "extensions.codereview=!"}, args...)
+}
+
+// HgLog represents a single Mercurial revision.
+type HgLog struct {
+	Hash   string
+	Author string
+	Date   string
+	Desc   string
+	Parent string
+
+	// Internal metadata
+	added bool
+}
+
+// xmlLogTemplate is a template to pass to Mercurial to make
+// hg log print the log in valid XML for parsing with xml.Unmarshal.
+const xmlLogTemplate = `
+        <Log>
+        <Hash>{node|escape}</Hash>
+        <Parent>{parent|escape}</Parent>
+        <Author>{author|escape}</Author>
+        <Date>{date|rfc3339date}</Date>
+        <Desc>{desc|escape}</Desc>
+        </Log>
+`