Break on unsatisfiable constraints and send to wtfautolayout

.lldbinit

command script import ~/.lldbscripts/break_unsatisfiable.py

break_unsatisfiable.py

import lldb

# Called when the Python module is imported into lldb
def __lldb_init_module(debugger, py_context):
    # Create an lldb command, `open_wtfautolayout` that will call the Python
    # function `open_wtfautolayout()`, below.
    debugger.HandleCommand("command script add "
                           "-f break_unsatisfiable.open_wtfautolayout "
                           "open_wtfautolayout")
    # Create a breakpoint on the UIViewAlertForUnsatisfiableConstraints()
    debugger.HandleCommand("breakpoint set "
                           "--name UIViewAlertForUnsatisfiableConstraints "
                           "--language objc "
                           "--breakpoint-name wtfautolayout")
    
    # Either: add the command open_wtfautolayout to the just-created breakpoint

    # Notes: This causes some kind of communication failure between an
    # lldb-rpc-server and Xcode; the running app hangs and there is no output
    # in Xcode's console until the lldb-rpc-server process is killed.
    # It works perfectly in lldb run from the command line, however.
    # The open_wtfautolayout command itself also works if called directly, and 
    # *if added to a breakpoint created in Xcode's GUI*. It's not clear what 
    # the difference is.
    
#    debugger.HandleCommand("breakpoint command add "
#                           "--one-liner open_wtfautolayout")
#    debugger.HandleCommand("breakpoint command add "
#                           "--one-liner continue")

    # Or: add the Python function `break_handler()` to the just-created
    # breakpoint as a "script"; the signature is different than the signature of
    # an lldb command

    # Notes: This works correctly in Xcode, but I haven't figured out how to
    # get the values of the function arguments.

    debugger.HandleCommand("breakpoint command add --script-type python "
                           "--python-function break_unsatisfiable.break_handler")
#MARK:- Script 
    
def break_handler(frame, location, py_context):
    """
    When breaking in a stack frame for
    UIViewAlertForUnsatisfiableConstraints, opens the constraint error log in
    www.wtfautolayout.com
    """
    # The frame argument *should* provide access to in-scope values, but because
    # we are stopped in a function compiled without debug information, there
    # doesn't seem to be a simple way.
    # Various attempts:
    arguments = frame.GetVariables(True, False, False, True)
    print(map(lambda arg: arg.GetValue(), arguments))
    
    # %rsi assumes running on simulator; see `get_arch()` below
    constraint_log = eval_nsstring_description(frame, "%rsi")
    if not constraint_log:
        print("Could not get [%rsi description].")
    else:
        print(constraint_log)
        
    constraint_log_address = get_rsi_value(frame)
    constraint_log = eval_nsstring_description(frame, constraint_log_address)
    if not constraint_log:
        print("Could not evaluate [%rsi] as a string")
    else:
        print(constraint_log)
    
    # N.B.: Return False to continue from the breakpoint automatically
    return False
    
def get_rsi_value(frame):
    """Given a frame, get the value in the rsi register"""
    for registers in frame.GetRegisters():
        if "general purpose" in registers.GetName().lower():
            rsi = registers.GetChildMemberWithName("rsi").GetValue()
    if not rsi:
        print("rsi not found")
        return ""
    # The value seems to be a string like "(unsigned long long)rsi = 0xfeedface"
    return rsi.split("= ")[-1]
    
def eval_nsstring_description(frame, reference):
    expression = "po [(NSString *){} description]".format(reference)
    result = frame.EvaluateExpression(expression)
    return result.GetValue()

# One possibility for getting the function's second argument may be to figure out
# which register (based on architecture) it's in and use that, because the
# non-debug-info `Frame` does seem to provide (some) access to the registers.
def get_arch(frame):
    """
    Given a frame, follow the object graph back to the current Target and get
    its `triple` property.
    """
    # TODO
    return ""

#MARK:- Command option

def open_wtfautolayout(debugger, command, result, py_context):
    """
    When breaking in a stack frame for
    UIViewAlertForUnsatisfiableConstraints, opens the constraint error log in
    www.wtfautolayout.com
    """
    constraint_log = evaluate_arg2(debugger)
    if not constraint_log:
        result.AppendWarning("Could not evaluate $arg2")
        return
    # constraint_log now contains the string we need to construct the URL
    print >> result, constraint_log    # Just for demonstration
    # Returning false to continue does not work here because this is a command

def evaluate_arg2(debugger):
    """Get the debugger's interpreter's value for `[$arg2 description]`"""
    expr_result = lldb.SBCommandReturnObject()
    interpreter = debugger.GetCommandInterpreter()
    interpreter.HandleCommand("expression -lobjc -O -- [$arg2 description]",
                              expr_result)
    if expr_result.GetError():
        return None
    return expr_result.GetOutput()

I tried to accomplish the same thing before coming across your script. Our approaches were somewhat different, but I ran into the same issue with lldb-rpc-server, so I filed a radar: https://openradar.appspot.com/43975320.