test: Make the "Trapped" signal handler signal-safe

test/openjd/sessions_v0/support_files/app_20s_run.py

@@ -7,14 +7,21 @@
 # Hook SIGTERM (posix) or CTRL_BREAK_EVENT (windows) and print "Trapped"
 # and exit if we get the signal
 
+import os
 import signal
 import sys
 import time
 
 
 def hook(handle, frame):
-    print("Trapped")
-    sys.stdout.flush()
+    # os.write(2) rather than print() + flush(): a signal handler can interrupt
+    # the main loop *inside* sys.stdout.flush(), and re-entering the same
+    # BufferedWriter raises
+    #   RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>
+    # which loses this message and exits non-zero. write(2) is unbuffered, so it
+    # cannot re-enter. Every print here is flushed immediately, so nothing is
+    # sitting in the buffer for this to appear out of order with.
+    os.write(1, b"Trapped\n")
     sys.exit(1)
 
 

test/openjd/sessions_v0/support_files/app_20s_run_ignore_signal.py

@@ -2,14 +2,21 @@
 
 # As app_20s_run.py except it does not exit when it gets a SIGTERM/SIGBREAK
 
+import os
 import signal
 import sys
 import time
 
 
 def hook(handle, frame):
-    print("Trapped")
-    sys.stdout.flush()
+    # os.write(2) rather than print() + flush(): a signal handler can interrupt
+    # the main loop *inside* sys.stdout.flush(), and re-entering the same
+    # BufferedWriter raises
+    #   RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>
+    # which loses this message and exits non-zero. write(2) is unbuffered, so it
+    # cannot re-enter. Every print here is flushed immediately, so nothing is
+    # sitting in the buffer for this to appear out of order with.
+    os.write(1, b"Trapped\n")
 
 
 if sys.platform.startswith("win"):

test/openjd/sessions_v1/support_files/app_20s_run.py

@@ -7,14 +7,21 @@
 # Hook SIGTERM (posix) or CTRL_BREAK_EVENT (windows) and print "Trapped"
 # and exit if we get the signal
 
+import os
 import signal
 import sys
 import time
 
 
 def hook(handle, frame):
-    print("Trapped")
-    sys.stdout.flush()
+    # os.write(2) rather than print() + flush(): a signal handler can interrupt
+    # the main loop *inside* sys.stdout.flush(), and re-entering the same
+    # BufferedWriter raises
+    #   RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>
+    # which loses this message and exits non-zero. write(2) is unbuffered, so it
+    # cannot re-enter. Every print here is flushed immediately, so nothing is
+    # sitting in the buffer for this to appear out of order with.
+    os.write(1, b"Trapped\n")
     sys.exit(1)
 
 

test/openjd/sessions_v1/support_files/app_20s_run_ignore_signal.py

@@ -2,14 +2,21 @@
 
 # As app_20s_run.py except it does not exit when it gets a SIGTERM/SIGBREAK
 
+import os
 import signal
 import sys
 import time
 
 
 def hook(handle, frame):
-    print("Trapped")
-    sys.stdout.flush()
+    # os.write(2) rather than print() + flush(): a signal handler can interrupt
+    # the main loop *inside* sys.stdout.flush(), and re-entering the same
+    # BufferedWriter raises
+    #   RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>
+    # which loses this message and exits non-zero. write(2) is unbuffered, so it
+    # cannot re-enter. Every print here is flushed immediately, so nothing is
+    # sitting in the buffer for this to appear out of order with.
+    os.write(1, b"Trapped\n")
 
 
 if sys.platform.startswith("win"):