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

[DavidSpickett]

Summary

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

[DavidSpickett]

Explain why I use the gdb quit command.

[DavidSpickett]

Remove stray newline.

[ldionne]

So IIUC, the test was being run before 8.3, but it would fail silently? That's pretty bad.

libcxx/utils/libcxx/test/features.py

13

Please just import subprocess and then qualify names below.

[ldionne]

LGTM with my fixes and green CI.

[DavidSpickett]

So IIUC, the test was being run before 8.3, but it would fail silently? That's pretty bad.

The good news is that on Focal at least, only two tests in the u32string section fail. I'm going to look into those.

[DavidSpickett]

Just import subprocess.

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

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-// REQUIRES: host-has-gdb
+// REQUIRES: host-has-gdb-with-python
 // UNSUPPORTED: libcpp-has-no-localization
 // UNSUPPORTED: c++03
 
 // RUN: %{cxx} %{flags} %s -o %t.exe %{compile_flags} -g %{link_flags}
 // Ensure locale-independence for unicode tests.
 // RUN: %{gdb} -nx -batch -iex "set autoload off" -ex "source %S/../../../utils/gdb/libcxx/printers.py" -ex "python register_libcxx_printer_loader()" -ex "source %S/gdb_pretty_printer_test.py" %t.exe
 
 #include <bitset>

libcxx/utils/libcxx/test/features.py

 #===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===----------------------------------------------------------------------===##
 
 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)
 _isGCC        = lambda cfg: '__GNUC__' in compilerMacros(cfg) and '__clang__' not in compilerMacros(cfg)
 _isMSVC       = lambda cfg: '_MSC_VER' in compilerMacros(cfg)
 _msvcVersion  = lambda cfg: (int(compilerMacros(cfg)['_MSC_VER']) // 100, int(compilerMacros(cfg)['_MSC_VER']) % 100)
 
 DEFAULT_FEATURES = [

 # The build host could differ from the target platform for cross-compilation.
 DEFAULT_FEATURES += [
   Feature(name='buildhost={}'.format(sys.platform.lower().strip())),
   # sys.platform can be represented by "sub-system" on Windows host, such as 'win32', 'cygwin', 'mingw' & etc.
   # Here is a consolidated feature for the build host plaform name on Windows.
   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'))]
   )
 ]