I have been working with the mpack library, and I have come across an error. It appears to be related to using the mpack_writer_destroy function when no other mpack_write_* functions have been used. In other words, an error occurs destroying a writer with no data. I understand this is an edge case because why would one create and then ultimately destroy a writer without using it? However, it feels like mpack should exit cleanly/safely as opposed to causing an abort under these conditions.

I have created a simplified example that reproduces the problem on my machine. I was not sure how to convert it to a test to include with the mpack test suite. Instead, I have included the example as an attachment, along with a screen capture of the error dialog that appears (Error-Dialog.png). Below is the source code for the main function that reproduces the problem, which is located in issue.c of the attached archive:

#include "mpack.h"

int 
main(int argc, char* argv[])
{
    char* data = NULL;
    size_t size = 0;
    mpack_writer_t* writer = malloc(sizeof(mpack_writer_t));
    if (!writer) {
        fprintf(stderr, "A memory-related error occurred!\n");
        return 1;
    }
    mpack_writer_init_growable(writer, &data, &size);
    if (mpack_writer_error(writer) != mpack_ok) {
        fprintf(stderr, "An initialization-related error occurred!\n");
        return 1;
    }
    //mpack_write_int(writer, 56); <-- Uncomment this line, i.e. write some data, to avoid the issue
    if (mpack_writer_destroy(writer) != mpack_ok) {
        fprintf(stderr, "A destruction-related error occurred!\n");
        return 1;
    }
    fprintf(stdout, "Data:");
    for (size_t i = 0; i < size; i++) {
        fprintf(stdout, " %X", data[i]);
    }
    fprintf(stdout, "\n");
    free(data);
    free(writer);
    return 0;
}

The error does not appear to occur on macOS High Sierra. It only occurs on Windows 10, where I am using the MSVC compiler included with Microsoft C++ Build Tools 2017. I have not tried it on Linux. I have created a CMakeLists.txt file to help with building the example and included it in the attached archive file. I am using mpack v0.8.2 with the default configuration (mpack-config.h) in the example (also included in the attached archive). The following instructions can be used to build the example demonstrating the issue:

1. Download the attached archive.
2. Extract the archive.
3. Start the x64 Native Build Tools command prompt that should have been included with Microsoft C++ Build Tools 2017.
4. Navigate to the extracted archive folder.
5. Run the following commands to build and run the example:

mkdir build
cd build
cmake ..
cmake --build .
Debug\run_issue.exe

I believe I have narrowed down the problem to the mpack_growable_writer_teardown function and its use of the mpack_realloc function, but I am not sure on how to resolve the issue, or if this is even an error. Maybe it is just a configuration I need to change? It is very possible I have missed something.

issue.zip