svn.go

     1  // Copyright 2022 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package vcweb
     6  
     7  import (
     8  	"io"
     9  	"log"
    10  	"net"
    11  	"net/http"
    12  	"os/exec"
    13  	"strings"
    14  	"sync"
    15  )
    16  
    17  // An svnHandler serves requests for Subversion repos.
    18  //
    19  // Unlike the other vcweb handlers, svnHandler does not serve the Subversion
    20  // protocol directly over the HTTP connection. Instead, it opens a separate port
    21  // that serves the (non-HTTP) 'svn' protocol. The test binary can retrieve the
    22  // URL for that port by sending an HTTP request with the query parameter
    23  // "vcwebsvn=1".
    24  //
    25  // We take this approach because the 'svn' protocol is implemented by a
    26  // lightweight 'svnserve' binary that is usually packaged along with the 'svn'
    27  // client binary, whereas only known implementation of the Subversion HTTP
    28  // protocol is the mod_dav_svn apache2 module. Apache2 has a lot of dependencies
    29  // and also seems to rely on global configuration via well-known file paths, so
    30  // implementing a hermetic test using apache2 would require the test to run in a
    31  // complicated container environment, which wouldn't be nearly as
    32  // straightforward for Go contributors to set up and test against on their local
    33  // machine.
    34  type svnHandler struct {
    35  	svnRoot string // a directory containing all svn repos to be served
    36  	logger  *log.Logger
    37  
    38  	pathOnce     sync.Once
    39  	svnservePath string // the path to the 'svnserve' executable
    40  	svnserveErr  error
    41  
    42  	listenOnce sync.Once
    43  	s          chan *svnState // 1-buffered
    44  }
    45  
    46  // An svnState describes the state of a port serving the 'svn://' protocol.
    47  type svnState struct {
    48  	listener  net.Listener
    49  	listenErr error
    50  	conns     map[net.Conn]struct{}
    51  	closing   bool
    52  	done      chan struct{}
    53  }
    54  
    55  func (h *svnHandler) Available() bool {
    56  	h.pathOnce.Do(func() {
    57  		h.svnservePath, h.svnserveErr = exec.LookPath("svnserve")
    58  	})
    59  	return h.svnserveErr == nil
    60  }
    61  
    62  // Handler returns an http.Handler that checks for the "vcwebsvn" query
    63  // parameter and then serves the 'svn://' URL for the repository at the
    64  // requested path.
    65  // The HTTP client is expected to read that URL and pass it to the 'svn' client.
    66  func (h *svnHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
    67  	if !h.Available() {
    68  		return nil, ServerNotInstalledError{name: "svn"}
    69  	}
    70  
    71  	// Go ahead and start the listener now, so that if it fails (for example, due
    72  	// to port exhaustion) we can return an error from the Handler method instead
    73  	// of serving an error for each individual HTTP request.
    74  	h.listenOnce.Do(func() {
    75  		h.s = make(chan *svnState, 1)
    76  		l, err := net.Listen("tcp", "localhost:0")
    77  		done := make(chan struct{})
    78  
    79  		h.s <- &svnState{
    80  			listener:  l,
    81  			listenErr: err,
    82  			conns:     map[net.Conn]struct{}{},
    83  			done:      done,
    84  		}
    85  		if err != nil {
    86  			close(done)
    87  			return
    88  		}
    89  
    90  		h.logger.Printf("serving svn on svn://%v", l.Addr())
    91  
    92  		go func() {
    93  			for {
    94  				c, err := l.Accept()
    95  
    96  				s := <-h.s
    97  				if err != nil {
    98  					s.listenErr = err
    99  					if len(s.conns) == 0 {
   100  						close(s.done)
   101  					}
   102  					h.s <- s
   103  					return
   104  				}
   105  				if s.closing {
   106  					c.Close()
   107  				} else {
   108  					s.conns[c] = struct{}{}
   109  					go h.serve(c)
   110  				}
   111  				h.s <- s
   112  			}
   113  		}()
   114  	})
   115  
   116  	s := <-h.s
   117  	addr := ""
   118  	if s.listener != nil {
   119  		addr = s.listener.Addr().String()
   120  	}
   121  	err := s.listenErr
   122  	h.s <- s
   123  	if err != nil {
   124  		return nil, err
   125  	}
   126  
   127  	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
   128  		if req.FormValue("vcwebsvn") != "" {
   129  			w.Header().Add("Content-Type", "text/plain; charset=UTF-8")
   130  			io.WriteString(w, "svn://"+addr+"\n")
   131  			return
   132  		}
   133  		http.NotFound(w, req)
   134  	})
   135  
   136  	return handler, nil
   137  }
   138  
   139  // serve serves a single 'svn://' connection on c.
   140  func (h *svnHandler) serve(c net.Conn) {
   141  	defer func() {
   142  		c.Close()
   143  
   144  		s := <-h.s
   145  		delete(s.conns, c)
   146  		if len(s.conns) == 0 && s.listenErr != nil {
   147  			close(s.done)
   148  		}
   149  		h.s <- s
   150  	}()
   151  
   152  	// The "--inetd" flag causes svnserve to speak the 'svn' protocol over its
   153  	// stdin and stdout streams as if invoked by the Unix "inetd" service.
   154  	// We aren't using inetd, but we are implementing essentially the same
   155  	// approach: using a host process to listen for connections and spawn
   156  	// subprocesses to serve them.
   157  	cmd := exec.Command(h.svnservePath, "--read-only", "--root="+h.svnRoot, "--inetd")
   158  	cmd.Stdin = c
   159  	cmd.Stdout = c
   160  	stderr := new(strings.Builder)
   161  	cmd.Stderr = stderr
   162  	err := cmd.Run()
   163  
   164  	var errFrag any = "ok"
   165  	if err != nil {
   166  		errFrag = err
   167  	}
   168  	stderrFrag := ""
   169  	if stderr.Len() > 0 {
   170  		stderrFrag = "\n" + stderr.String()
   171  	}
   172  	h.logger.Printf("%v: %s%s", cmd, errFrag, stderrFrag)
   173  }
   174  
   175  // Close stops accepting new svn:// connections and terminates the existing
   176  // ones, then waits for the 'svnserve' subprocesses to complete.
   177  func (h *svnHandler) Close() error {
   178  	h.listenOnce.Do(func() {})
   179  	if h.s == nil {
   180  		return nil
   181  	}
   182  
   183  	var err error
   184  	s := <-h.s
   185  	s.closing = true
   186  	if s.listener == nil {
   187  		err = s.listenErr
   188  	} else {
   189  		err = s.listener.Close()
   190  	}
   191  	for c := range s.conns {
   192  		c.Close()
   193  	}
   194  	done := s.done
   195  	h.s <- s
   196  
   197  	<-done
   198  	return err
   199  }
   200
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