[libcxx][pretty printers] Check GDB Python scripting support

I found this after upgrading from Ubuntu bionic (gdb 8.1.1) to
Focal (gdb 9.2). (where this test fails, but that's for a
different patch)

9.2 allows you to set breakpoint commands from
Python, which was added in 8.3.
(bintutils a913fffbdee21fdd50e8de0596358be425775678
"Allow breakpoint commands to be set from Python")

The reason this test never failed before was because it did so
silently. "source <python file>" doesn't fail even if that script
raises an Exception.

To fix this extend the gdb lit feature to check that:
* gdb exists
* has Python support
* allows you to set breakpoint commands

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D110334

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// REQUIRES: host-has-gdb
+// REQUIRES: host-has-gdb-with-python
 // UNSUPPORTED: libcpp-has-no-localization
 // UNSUPPORTED: c++03
 

libcxx/utils/libcxx/test/features.py

@@ -10,6 +10,7 @@ from libcxx.test.dsl import *
 import re
 import shutil
 import sys
+import subprocess
 
 _isClang      = lambda cfg: '__clang__' in compilerMacros(cfg) and '__apple_build_version__' not in compilerMacros(cfg)
 _isAppleClang = lambda cfg: '__apple_build_version__' in compilerMacros(cfg)
@@ -159,10 +160,37 @@ DEFAULT_FEATURES += [
   Feature(name='buildhost=windows', when=lambda cfg: platform.system().lower().startswith('windows'))
 ]
 
-# Detect whether GDB is on the system, and if so add a substitution to access it.
+# Detect whether GDB is on the system, has Python scripting and supports
+# adding breakpoint commands. If so add a substitution to access it.
+def check_gdb(cfg):
+  gdb_path = shutil.which('gdb')
+  if gdb_path is None:
+    return False
+
+  # Check that we can set breakpoint commands, which was added in 8.3.
+  # Using the quit command here means that gdb itself exits, not just
+  # the "python <...>" command.
+  test_src = """\
+try:
+  gdb.Breakpoint(\"main\").commands=\"foo\"
+except AttributeError:
+  gdb.execute(\"quit 1\")
+gdb.execute(\"quit\")"""
+
+  try:
+    stdout = subprocess.check_output(
+              [gdb_path, "-ex", "python " + test_src, "--batch"],
+              stderr=subprocess.DEVNULL, universal_newlines=True)
+  except subprocess.CalledProcessError:
+    # We can't set breakpoint commands
+    return False
+
+  # Check we actually ran the Python
+  return not "Python scripting is not supported" in stdout
+
 DEFAULT_FEATURES += [
-  Feature(name='host-has-gdb',
-    when=lambda cfg: shutil.which('gdb') is not None,
+  Feature(name='host-has-gdb-with-python',
+    when=check_gdb,
     actions=[AddSubstitution('%{gdb}', lambda cfg: shutil.which('gdb'))]
   )
 ]