Cache scons subprocess calls

SConstruct

@@ -32,6 +32,44 @@ def get_Werror_string(l):
 		return '-W'
 	return '-Werror='
 
+class StaticSubprocess:
+	class CachedCall:
+		def __init__(self,out,err,returncode):
+			self.out = out
+			self.err = err
+			self.returncode = returncode
+	__call_cache = {}
+	@classmethod
+	def pcall(cls,args,stdout,stderr=None):
+		a = repr(args)
+		try:
+			return cls.__call_cache[a]
+		except KeyError:
+			pass
+		p = subprocess.Popen(args, executable=args[0], stdout=stdout, stderr=stderr, close_fds=True)
+		(o, e) = p.communicate()
+		cls.__call_cache[a] = c = cls.CachedCall(o, e, p.wait())
+		return c
+
+class Git(StaticSubprocess):
+	__missing_git = StaticSubprocess.CachedCall(None, None, 1)
+	__path_git = None
+	@classmethod
+	def pcall(cls,args,stdout,stderr=None):
+		git = cls.__path_git
+		if git is None:
+			cls.__path_git = git = (os.environ.get('GIT', 'git').split(),)
+		git = git[0]
+		if not git:
+			return cls.__missing_git
+		return StaticSubprocess.pcall(git + args, stdout=stdout, stderr=stderr)
+	@classmethod
+	def spcall(cls,args,stdout,stderr=None):
+		g = cls.pcall(args, stdout, stderr)
+		if g.returncode:
+			return None
+		return g.out
+
 class ConfigureTests:
 	class Collector:
 		def __init__(self):
@@ -1369,8 +1407,7 @@ class DXXCommon(LazyObjectConstructor):
 		check_header_includes = os.path.join(builddir, 'check_header_includes.cpp')
 		if not self.__shared_header_file_list:
 			open(check_header_includes, 'wt')
-			git = subprocess.Popen(['git', 'ls-files', '-z', '--', '*.h'], executable='/usr/bin/git', stdout=subprocess.PIPE, close_fds=True)
-			headers = git.communicate(None)[0]
+			headers = Git.pcall(['ls-files', '-z', '--', '*.h'], stdout=subprocess.PIPE).out
 			excluded_directories = (
 				'common/arch/cocoa/',
 				'common/arch/carbon/',
@@ -1985,38 +2022,27 @@ class DXXProgram(DXXCommon):
 	def compute_extra_version(cls):
 		c = cls._computed_extra_version
 		if c is None:
-			git = (os.environ.get('GIT', 'git')).split()
-			v = s = ds = None
-			if git:
-				v = cls._compute_extra_version(git)
-				if v:
-					g = subprocess.Popen(git + ['status', '--short', '--branch'], executable=git[0], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-					(go, ge) = g.communicate(None)
-					if not g.wait():
-						s = go
-					g = subprocess.Popen(git + ['diff', '--stat', 'HEAD'], executable=git[0], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-					(go, ge) = g.communicate(None)
-					if not g.wait():
-						ds = go
+			s = ds = None
+			v = cls._compute_extra_version()
+			if v:
+				s = Git.spcall(['status', '--short', '--branch'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+				ds = Git.spcall(['diff', '--stat', 'HEAD'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
 			cls._computed_extra_version = c = (v or '', s, ds)
 		return c
 
 	@classmethod
-	def _compute_extra_version(cls, git):
+	def _compute_extra_version(cls):
 		try:
-			g = subprocess.Popen(git + ['describe', '--tags', '--abbrev=8'], executable=git[0], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+			g = Git.pcall(['describe', '--tags', '--abbrev=8'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
 		except OSError as e:
 			if e.errno == errno.ENOENT:
 				return None
 			raise
-		(go, ge) = g.communicate(None)
-		if g.wait():
+		if g.returncode:
 			return None
-		g = subprocess.Popen(git + ['diff', '--quiet', '--cached'], executable=git[0], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-		c = g.wait()
-		g = subprocess.Popen(git + ['diff', '--quiet'], executable=git[0], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-		d = g.wait()
-		return go.split('\n')[0] + ('+' if c else '') + ('*' if d else '')
+		c = Git.pcall(['diff', '--quiet', '--cached'], stdout=subprocess.PIPE, stderr=subprocess.PIPE).returncode
+		d = Git.pcall(['diff', '--quiet'], stdout=subprocess.PIPE, stderr=subprocess.PIPE).returncode
+		return g.out.split('\n')[0] + ('+' if c else '') + ('*' if d else '')
 
 	def _register_program(self,dxxstr,program_specific_objects=[]):
 		env = self.env
@@ -2045,10 +2071,9 @@ class DXXProgram(DXXCommon):
 		versid_build_environ = ['CXX', 'CPPFLAGS', 'CXXFLAGS', 'LINKFLAGS']
 		versid_cppdefines = env['CPPDEFINES'][:]
 		versid_cppdefines.extend([('DESCENT_%s' % k, self._quote_cppdefine(env.get(k, ''))) for k in versid_build_environ])
-		v = subprocess.Popen(env['CXX'].split(' ') + ['--version'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-		(so,se) = v.communicate(None)
-		if not v.returncode and (so or se):
-			v = (so or se).split('\n')[0]
+		v = StaticSubprocess.pcall(env['CXX'].split(' ') + ['--version'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+		if not v.returncode and (v.out or v.err):
+			v = (v.out or v.err).split('\n')[0]
 			versid_cppdefines.append(('DESCENT_%s' % 'CXX_version', self._quote_cppdefine(v)))
 			versid_build_environ.append('CXX_version')
 		extra_version = self.user_settings.extra_version