StepIntoMongod

step_into_mongod.py

# Created by Dan Gottlieb

import gdb

class StepIntoMongod(gdb.Command):
    """sim (Step Into MongoDB)
    Step into the next MongoDB source line.

    There are two usages. `sim` and `sim <arg>`.
    sim: Steps into the next MongoDB (or WiredTiger) source line. Consider the source line:
      AutoGetCollectionOrViewForReadCommand ctx(opCtx, nss, std::move(dbSLock));

    Doing a step from here will land inside the source for `std::move`:

   step:
     std::move<mongo::lock::DBLock&> (__t=...) at /opt/mongodbtoolchain/v2/bin/../lib/gcc/x86_64-mongodb-linux/5.4.0/../../../../include/c++/5.4.0/bits/move.h:102

    Oftentimes one assumes the standard library/boost methods are doing the right thing and
    instead want to jump to the next line inside MongoDB. Running with `sim` instead gives the
    move constructor directly:

   sim:
     mongo::lock::DBLock::DBLock(mongo::lock::DBLock&&) (<snip>) at src/mongo/db/concurrency/d_concurrency.cpp:213

    The second usage is running with an argument, `sim <arg>`. Oftentimes one might also want to
    step into a specific MongoDB function. In the above example, one may not care about any of the
    logic dealing with parameter passing and rather get directly to the constructor for
    `AutoGetCollectionOrViewForReadCommand`.

   sim AutoGetCollectionOrViewForReadCommand:
     mongo::AutoGetCollectionOrViewForReadCommand::AutoGetCollectionOrViewForReadCommand (<snip>) at src/mongo/db/db_raii.cpp:244

    `sim <arg>` matches <arg> against a substring of the function name. I.e: `sim AutoGetColl`
    would be a short-hand for the same example.

    Also note that a misspelled <arg> may not exist, causing the script to infinite loop, even as
    other breakpoints get hit. This script has a built-in limit of stepping 20 times before
    returning to the user. Take care when passing in an <arg> to not step too in a hard to hit
    breakpoint.
    """


    def __init__(self):
        gdb.Command.__init__(self, "sim", gdb.COMMAND_RUNNING)

    def invoke(self, skipTilSymbolAsFrame, from_tty):
        steps = 0
        while True:
            output = gdb.execute("step", False, True)
            steps += 1
            fr = gdb.selected_frame()
            if skipTilSymbolAsFrame:
                if skipTilSymbolAsFrame in fr.function().name:
                    gdb.write(output)
                    break

                if steps > 20:
                    gdb.write("Stepped 20 times without finding the target frame. Searched for: %s\n" % (skipTilSymbolAsFrame,))
                    return
            else:
                symtabAndLine = fr.find_sal().symtab
                if symtabAndLine.filename.startswith('src/mongo/') or \
                   symtabAndLine.filename.startswith('src/third_party/wiredtiger'):
                    gdb.write(output)
                    break

            output = gdb.execute("finish", False, True)

        return

StepIntoMongod()