parent: synchronize get_core_source()

Single task 100 SSH target run, before:

        3533181 function calls (3533083 primitive calls) in 616.688 seconds
        User time (seconds): 32.52
        System time (seconds): 2.71
        Percent of CPU this job got: 64%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 0:54.88

After:

        451602 function calls (451504 primitive calls) in 570.746 seconds
        User time (seconds): 29.48
        System time (seconds): 2.81
        Percent of CPU this job got: 67%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 0:48.20

mitogen/parent.py

@@ -155,7 +155,11 @@ def get_sys_executable():
     return '/usr/bin/python'
 
 
-def get_core_source():
+_core_source_cache = None
+_core_source_lock = threading.Lock()
+
+
+def _get_core_source():
     """
     In non-masters, simply fetch the cached mitogen.core source code via the
     import mechanism. In masters, this function is replaced with a version that
@@ -164,6 +168,24 @@ def get_core_source():
     return inspect.getsource(mitogen.core)
 
 
+def get_core_source():
+    """
+    _get_core_source() is expensive, even with @lru_cache in minify.py, threads
+    can enter it simultaneously causing severe slowdowns.
+    """
+    global _core_source_cache
+    if _core_source_cache is not None:
+        return _core_source_cache
+
+    _core_source_lock.acquire()
+    try:
+        if _core_source_cache is None:
+            _core_source_cache = _get_core_source()
+        return _core_source_cache
+    finally:
+        _core_source_lock.release()
+
+
 def get_default_remote_name():
     """
     Return the default name appearing in argv[0] of remote machines.
@@ -430,11 +452,11 @@ def tty_create_child(args):
         `(pid, tty_fd, None)`
     """
     master_fd, slave_fd = openpty()
+    try:
         mitogen.core.set_block(slave_fd)
         disable_echo(master_fd)
         disable_echo(slave_fd)
 
-    try:
         pid = detach_popen(
             args=args,
             stdin=slave_fd,
@@ -467,14 +489,15 @@ def hybrid_tty_create_child(args):
         `(pid, socketpair_fd, tty_fd)`
     """
     master_fd, slave_fd = openpty()
-    parentfp, childfp = create_socketpair()
 
-    mitogen.core.set_block(slave_fd)
+    try:
-    mitogen.core.set_block(childfp)
         disable_echo(master_fd)
         disable_echo(slave_fd)
+        mitogen.core.set_block(slave_fd)
 
+        parentfp, childfp = create_socketpair()
         try:
+            mitogen.core.set_block(childfp)
             pid = detach_popen(
                 args=args,
                 stdin=childfp,
@@ -484,11 +507,13 @@ def hybrid_tty_create_child(args):
                 close_fds=True,
             )
         except Exception:
-        os.close(master_fd)
-        os.close(slave_fd)
             parentfp.close()
             childfp.close()
             raise
+    except Exception:
+        os.close(master_fd)
+        os.close(slave_fd)
+        raise
 
     os.close(slave_fd)
     childfp.close()