CODEWITHSUNDEEP
mysqlgocgomysql-udf
Brian Leishman
Brian Leishman

Reputation: 8555

How to debug/dump Go variable while building with cgo?

I'm trying to write a MySQL UDF in Go with cgo, in which I have a basic one functioning, but there's little bits and pieces that I can't figure out because I have no idea what some of the C variables are in terms of Go.

This is an example that I have written in C that forces the type of one of the MySQL parameters to an int

my_bool unhex_sha3_init(UDF_INIT *initid, UDF_ARGS *args, char *message) {
    if (args->arg_count != 2) {
        strcpy(message, "`unhex_sha3`() requires 2 parameters: the message part, and the bits");
        return 1;
    }

    args->arg_type[1] = INT_RESULT;

    initid->maybe_null = 1; //can return null

    return 0;
}

And that works fine, but then I try to do the same/similar thing with this other function in Go like this

//export get_url_param_init
func get_url_param_init(initid *C.UDF_INIT, args *C.UDF_ARGS, message *C.char) C.my_bool {
    if args.arg_count != 2 {
        message = C.CString("`get_url_param` require 2 parameters: the URL string and the param name")
        return 1
    }

    (*args.arg_type)[0] = C.STRING_RESULT
    (*args.arg_type)[1] = C.STRING_RESULT

    initid.maybe_null = 1

    return 0
}

With this build error

./main.go:24: invalid operation: (*args.arg_type)[0] (type uint32 does not support indexing)

And I'm not totally sure what that means. Shouldn't this be a slice of some sort, not a uint32?

And this is where it'd be super helpful have some way of dumping the args struct somewhere somehow (maybe even in Go syntax as a super plus) so that I can tell what I'm working with.

Well I used spew to dump the variable contents to a tmp file inside the init function (commenting out the lines that made it not compile) and I got this

(string) (len=3) "%#v"
(*main._Ctype_struct_st_udf_args)(0x7ff318006af8)({
 arg_count: (main._Ctype_uint) 2,
 _: ([4]uint8) (len=4 cap=4) {
  00000000  00 00 00 00                                       |....|
 },
 arg_type: (*uint32)(0x7ff318006d18)(0),
 args: (**main._Ctype_char)(0x7ff318006d20->0x7ff3180251b0)(0),
 lengths: (*main._Ctype_ulong)(0x7ff318006d30)(0),
 maybe_null: (*main._Ctype_char)(0x7ff318006d40)(0),
 attributes: (**main._Ctype_char)(0x7ff318006d58->0x7ff318006b88)(39),
 attribute_lengths: (*main._Ctype_ulong)(0x7ff318006d68)(2),
 extension: (unsafe.Pointer) <nil>
})

Upvotes: 1

Views: 590

Answers (1)

Brian Leishman
Brian Leishman

Reputation: 8555

Alright so huge help with @JimB who stuck with me even though I'm clearly less adept with Go (and especially CGO) but I've got a working version of my UDF, which is an easy and straight forward (and fast) function that pulls a single parameter out of a URL string and decodes it correctly and what not (e.g. %20 gets returned as a space, basically how you would expect it to work).

This seemed incredibly tricky with a pure C UDF because I don't really know C (as well as I know other languages), and there's a lot that can go wrong with URL parsing and URL parameter decoding, and native MySQL functions are slow (and there's not really a good, clean way to do the decoding either), so Go seemed like the better-than-perfect candidate for this kind of problem, for strong performance, ease of writing, and wide variety of easy to use built ins & third party libraries.

The full UDF and it's installation/usage instructions are here https://github.com/StirlingMarketingGroup/mysql-get-url-param/blob/master/main.go

First problem was debugging output. And I did that by Fprintfing to a tmp file instead of the standard output, so that I could check the file to see variable dumps.

t, err := ioutil.TempFile(os.TempDir(), "get-url-param")

fmt.Fprintf(t, "%#v\n", args.arg_type)

And then after I got my output (I was expecting args.arg_type to be an array like it is in C, but instead was a number) I needed to convert the data referenced by that number (the pointer to the start of the C array) to a Go array so I could set it's values.

argsTypes := *(*[2]uint32)(unsafe.Pointer(args.arg_type))

argsTypes[0] = C.STRING_RESULT
argsTypes[1] = C.STRING_RESULT

Upvotes: 0

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