Correct flaws in arc_summary[23] and their test.

The change correctly handles BrokenPipeError and improves the
associated tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12037
Closes #12036

cmd/arc_summary/arc_summary2

@@ -102,18 +102,6 @@ show_tunable_descriptions = False
 alternate_tunable_layout = False
 
 
-def handle_Exception(ex_cls, ex, tb):
-    if ex is IOError:
-        if ex.errno == errno.EPIPE:
-            sys.exit()
-
-    if ex is KeyboardInterrupt:
-        sys.exit()
-
-
-sys.excepthook = handle_Exception
-
-
 def get_Kstat():
     """Collect information on the ZFS subsystem from the /proc virtual
     file system. The name "kstat" is a holdover from the Solaris utility
@@ -1136,6 +1124,7 @@ def main():
     global show_tunable_descriptions
     global alternate_tunable_layout
 
+    try:
         try:
             opts, args = getopt.getopt(
                 sys.argv[1:],
@@ -1179,6 +1168,12 @@ def main():
         for page in pages:
             page(Kstat)
             sys.stdout.write("\n")
+    except IOError as ex:
+        if (ex.errno == errno.EPIPE):
+            sys.exit(0)
+        raise
+    except KeyboardInterrupt:
+        sys.exit(0)
 
 
 if __name__ == '__main__':

cmd/arc_summary/arc_summary3

@@ -43,6 +43,12 @@ import subprocess
 import sys
 import time
 
+# We can't use env -S portably, and we need python3 -u to handle pipes in
+# the shell abruptly closing the way we want to, so...
+import io
+if isinstance(sys.__stderr__.buffer, io.BufferedWriter):
+    os.execv(sys.executable, [sys.executable, "-u"] + sys.argv)
+
 DESCRIPTION = 'Print ARC and other statistics for OpenZFS'
 INDENT = ' '*8
 LINE_LENGTH = 72
@@ -221,6 +227,24 @@ elif sys.platform.startswith('linux'):
 
         return descs
 
+def handle_unraisableException(exc_type, exc_value=None, exc_traceback=None,
+                               err_msg=None, object=None):
+   handle_Exception(exc_type, object, exc_traceback)
+
+def handle_Exception(ex_cls, ex, tb):
+    if ex_cls is KeyboardInterrupt:
+        sys.exit()
+
+    if ex_cls is BrokenPipeError:
+        # It turns out that while sys.exit() triggers an exception
+        # not handled message on Python 3.8+, os._exit() does not.
+        os._exit(0)
+    raise ex
+
+if hasattr(sys,'unraisablehook'): # Python 3.8+
+    sys.unraisablehook = handle_unraisableException
+sys.excepthook = handle_Exception
+
 
 def cleanup_line(single_line):
     """Format a raw line of data from /proc and isolate the name value

tests/zfs-tests/tests/functional/cli_user/misc/arc_summary_001_pos.ksh

@@ -48,6 +48,9 @@ else
 	set -A args  "" "-a" "-d" "-p 1"
 fi
 
+# Without this, the below checks aren't going to work the way we hope...
+set -o pipefail
+
 typeset -i i=0
 while [[ $i -lt ${#args[*]} ]]; do
 	log_must eval "arc_summary ${args[i]} > /dev/null"