pager: preserve Hg's exit code (and fix Windows support) (issue3225)

This changes how the pager extension invokes the pager. Prior to this change,
the extension would fork Hg and exec the pager in the parent process. This
loses Hg exit code, and it doesn't work on Windows.

Now the pager is invoked using the subprocess library, and an atexit handler is
registered that makes Hg wait for the pager to exit before it exits itself.

Note that if you exit the pager before Hg is done running, you'll get an exit
code of 255, which is caused by Python blowing up due to a broken pipe. If you
set pager.quiet=True, you'll get the OS-level return code of 141.

hgext/pager.py

@@ -53,37 +53,27 b' used. Use a boolean value like yes, no, '
 normal behavior.
 '''
 
-import sys, os, signal, shlex, errno
+import atexit, sys, os, signal, subprocess
 from mercurial import commands, dispatch, util, extensions
 from mercurial.i18n import _
 
 def _runpager(p):
-    if not util.safehasattr(os, 'fork'):
-        sys.stdout = util.popen(p, 'wb')
-        if util.isatty(sys.stderr):
-            sys.stderr = sys.stdout
-        return
-    fdin, fdout = os.pipe()
-    pid = os.fork()
-    if pid == 0:
-        os.close(fdin)
-        os.dup2(fdout, sys.stdout.fileno())
-        if util.isatty(sys.stderr):
-            os.dup2(fdout, sys.stderr.fileno())
-        os.close(fdout)
-        return
-    os.dup2(fdin, sys.stdin.fileno())
-    os.close(fdin)
-    os.close(fdout)
-    try:
-        os.execvp('/bin/sh', ['/bin/sh', '-c', p])
-    except OSError, e:
-        if e.errno == errno.ENOENT:
-            # no /bin/sh, try executing the pager directly
-            args = shlex.split(p)
-            os.execvp(args[0], args)
-        else:
-            raise
+    pager = subprocess.Popen(p, shell=True, bufsize=-1,
+                             close_fds=util.closefds, stdin=subprocess.PIPE,
+                             stdout=sys.stdout, stderr=sys.stderr)
+
+    stdout = os.dup(sys.stdout.fileno())
+    stderr = os.dup(sys.stderr.fileno())
+    os.dup2(pager.stdin.fileno(), sys.stdout.fileno())
+    if util.isatty(sys.stderr):
+        os.dup2(pager.stdin.fileno(), sys.stderr.fileno())
+
+    @atexit.register
+    def killpager():
+        pager.stdin.close()
+        os.dup2(stdout, sys.stdout.fileno())
+        os.dup2(stderr, sys.stderr.fileno())
+        pager.wait()
 
 def uisetup(ui):
     if ui.plain() or '--debugger' in sys.argv or not util.isatty(sys.stdout):