Added bootstrap/configure option to force pselect

All modern Linux kernels have ppoll() but sometimes
you might want to compile on something ancient.

This patch adds the possibility to force the use
of pselect() instead by passing --force-pselect
to bootstrap/configure.

The use of ppoll() is still default for Linux
and OpenBSD

bootstrap.py

@@ -36,6 +36,8 @@
 parser.add_option('--platform',
                   help='target platform (' + '/'.join(platform_helper.platforms()) + ')',
                   choices=platform_helper.platforms())
+parser.add_option('--force-pselect', action='store_true',
+                  help="ppoll() is used by default on Linux and OpenBSD, but older versions might need to use pselect instead",)
 (options, conf_args) = parser.parse_args()
 
 
@@ -107,6 +109,10 @@ def run(*args, **kwargs):
         cflags.append('-D_WIN32_WINNT=0x0501')
     if options.x64:
         cflags.append('-m64')
+if (platform.is_linux() or platform.is_openbsd()) and not options.force_pselect:
+    cflags.append('-DUSE_PPOLL')
+if options.force_pselect:
+    conf_args.append("--force-pselect")
 args.extend(cflags)
 args.extend(ldflags)
 binary = 'ninja.bootstrap'

configure.py

@@ -47,6 +47,8 @@
 parser.add_option('--with-python', metavar='EXE',
                   help='use EXE as the Python interpreter',
                   default=os.path.basename(sys.executable))
+parser.add_option('--force-pselect', action='store_true',
+                  help="ppoll() is used by default on Linux and OpenBSD, but older versions might need to use pselect instead",)
 (options, args) = parser.parse_args()
 if args:
     print('ERROR: extra unparsed command-line arguments:', args)
@@ -163,6 +165,9 @@ def binary(name):
         cflags.append('-fno-omit-frame-pointer')
         libs.extend(['-Wl,--no-as-needed', '-lprofiler'])
 
+if (platform.is_linux() or platform.is_openbsd()) and not options.force_pselect:
+    cflags.append('-DUSE_PPOLL')
+
 def shell_escape(str):
     """Escape str such that it's interpreted as a single argument by
     the shell."""

src/subprocess-posix.cc

@@ -40,12 +40,12 @@ bool Subprocess::Start(SubprocessSet* set, const string& command) {
   if (pipe(output_pipe) < 0)
     Fatal("pipe: %s", strerror(errno));
   fd_ = output_pipe[0];
-#if !defined(linux) && !defined(__OpenBSD__)
+#if !defined(USE_PPOLL)
   // On Linux and OpenBSD, we use ppoll in DoWork(); elsewhere we use pselect
   // and so must avoid overly-large FDs.
   if (fd_ >= static_cast<int>(FD_SETSIZE))
     Fatal("pipe: %s", strerror(EMFILE));
-#endif  // !linux && !__OpenBSD__
+#endif  // !USE_PPOLL
   SetCloseOnExec(fd_);
 
   pid_ = fork();
@@ -178,7 +178,7 @@ Subprocess *SubprocessSet::Add(const string& command) {
   return subprocess;
 }
 
-#if defined(linux) || defined(__OpenBSD__)
+#ifdef USE_PPOLL
 bool SubprocessSet::DoWork() {
   vector<pollfd> fds;
   nfds_t nfds = 0;