[lldb] make it easier to find LLDB's python

It is surprisingly difficult to write a simple python script that
can reliably `import lldb` without failing, or crashing. I'm
currently resorting to co

[lldb] make it easier to find LLDB's python

It is surprisingly difficult to write a simple python script that
can reliably `import lldb` without failing, or crashing. I'm
currently resorting to convolutions like this:

def find_lldb(may_reexec=False):
if prefix := os.environ.get('LLDB_PYTHON_PREFIX'):
if os.path.realpath(prefix) != os.path.realpath(sys.prefix):
raise Exception("cannot import lldb.\n"
f" sys.prefix should be: {prefix}\n"
f" but it is: {sys.prefix}")
else:
line1, line2 = subprocess.run(
['lldb', '-x', '-b', '-o', 'script print(sys.prefix)'],
encoding='utf8', stdout=subprocess.PIPE,
check=True).stdout.strip().splitlines()
assert line1.strip() == '(lldb) script print(sys.prefix)'
prefix = line2.strip()
os.environ['LLDB_PYTHON_PREFIX'] = prefix

if sys.prefix != prefix:
if not may_reexec:
raise Exception(
"cannot import lldb.\n" +
f" This python, at {sys.prefix}\n"
f" does not math LLDB's python at {prefix}")
os.environ['LLDB_PYTHON_PREFIX'] = prefix
python_exe = os.path.join(prefix, 'bin', 'python3')
os.execl(python_exe, python_exe, *sys.argv)

lldb_path = subprocess.run(['lldb', '-P'],
check=True, stdout=subprocess.PIPE,
encoding='utf8').stdout.strip()

sys.path = [lldb_path] + sys.path

This patch aims to replace all that with:

#!/usr/bin/env lldb-python
import lldb
...

... by adding the following features:

* new command line option: --print-script-interpreter-info. This
prints language-specific information about the script interpreter
in JSON format.

* new tool (unix only): lldb-python which finds python and exec's it.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D112973

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[lldb] fix --source-quietly

Jim says:

lldb has a -Q or --source-quietly option, which supposedly does:

--source-quietly Tells the debugger to execute this one-line lldb command before any

[lldb] fix --source-quietly

Jim says:

lldb has a -Q or --source-quietly option, which supposedly does:

--source-quietly Tells the debugger to execute this one-line lldb command before any file has been loaded.

That seems like a weird description, since we don't generally use source for one line entries, but anyway, let's try it:

> $LLDB_LLVM/clean-mono/build/Debug/bin/lldb -Q "script print('I should be quiet')" a.out -O "script print('I should be before')" -o "script print('I should be after')"
(lldb) script print('I should be before')
I should be before
(lldb) target create "script print('I should be quiet')"
error: unable to find executable for 'script print('I should be quiet')'

That was weird. The first real -O gets sourced but not quietly, then the argument to the -Q gets treated as the target.

> $LLDB_LLVM/clean-mono/build/Debug/bin/lldb -Q a.out -O "script print('I should be before')" -o "script print('I should be after')"
(lldb) script print('I should be before')
I should be before
(lldb) target create "a.out"
Current executable set to '/tmp/a.out' (x86_64).
(lldb) script print('I should be after')
I should be after

Well, that's a little better, but the -Q option seems to have done nothing.

---

This fixes the description of --source-quietly, as well as causing it
to actually suppress echoing while executing the initialization
commands.

Reviewed By: jingham

Differential Revision: https://reviews.llvm.org/D112988

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