Fix FormatAllocateString complexity

[jepio]

Description

FormatAllocateString has multiple issues:

• it always allocates at least 512 bytes even when the input is shorter
• once the length of the formatted output exceeds 512 bytes it starts incrementing the buffer size by 1 byte and goes through a malloc,snprintf,free loop until it finds the right size. This has an O(n^2) complexity and is slow for longer strings.
• it has an unnecessary DuplicateString at the end

Rewrite FormatAllocateString to call snprintf once to calculate the required buffer size, then malloc the right buffer and then format the string.

This change is motivated by hitting this edge case through fuzzing CheckFileSystemMountingOption where a lot of reason strings get added to the reason buffer. I've added a test case for long strings and a fuzzer case whose runtime goes from ~3s to 10ms.

Checklist

• I have read the contribution guidelines.
• I added unit-tests to validate my changes. All unit tests are passing.
• I have merged the latest main branch prior to this PR submission.
• I submitted this PR against the main branch.

[MariusNi]

This is a great idea! Just a nit suggestion, same code, just bit changed to only return once after initial input args validation (for consistence with rest of code), how about this?

char* FormatAllocateString(const char* format, ...)
{
    char* stringToReturn = NULL;
    int formatResult = 0;
    int sizeOfBuffer = 0;

    if (NULL == format)
    {
        return stringToReturn;
    }

    va_list arguments;
    va_start(arguments, format);
    // snprintf returns the required buffer size, excluding the null terminator
    sizeOfBuffer = vsnprintf(NULL, 0, format, arguments);
    va_end(arguments);

    if (sizeOfBuffer > 0)
    {
        if (NULL != (stringToReturn = malloc((size_t)sizeOfBuffer + 1)))
        {
            va_start(arguments, format);
            formatResult = vsnprintf(stringToReturn, sizeOfBuffer + 1, format, arguments);
            va_end(arguments);

            if ((formatResult < 0) || (formatResult > sizeOfBuffer))
            {
                FREE_MEMORY(stringToReturn);
            }
        }
    }

    return stringToReturn;
}

[MariusNi]