Add DriverError::StmtParamsNumberExceedsLimit

[stephen-hlx]

When comparing the number of params in a statement against the number of params supplied, if they do not match, the error is supposed to provide what these two numbers are for users to understand what might have gone wrong. This is achieved via following code path in ExecRoutine:

    if self.stmt.num_params() as usize != params.len() {
        Err(DriverError::StmtParamsMismatch {
            required: self.stmt.num_params(),
            supplied: params.len() as u16,
        })?
    }

The maximum number of bind variables in a statement supported by MySQL is 65,535 and this is guarded at the server side which means the aforementioned comparison in ExecRoutine is hit before that.

But at this point, both of these number could go over 65,535, which is the u16::MAX. In that case, when the error is constructed, the conversion from params.len()'s usize to u16 would result in a loss of precision, producing a confusing error. Because self.stmt.num_params() returns a u16 while params.len() returns a usize.

For example, if both the number of the required params and the supplied params are 65,539, which is greater than u16::MAX, self.stmt.num_params() would have returned a truncated number 3 and even with as usize would not have recovered the loss precision. Meanwhile, params.len() still returns 65,539 because it is of usize. The comparison then produces an incorrect result, saying that they do not match, even though they do. What makes it even more confusing is that in the error message there are two identical numbers because params.len() as u16 converts the usize to u16, causing the same loss of precision.

Because the self.stmt.num_params() returns a u16, which intrinsically prevents the detection of mismatched param numbers if they go over u16::MAX, and changing that type would be rather intrusive. Therefore, this commit fixes merely the symptom by providing a new error variant StmtParamsNumberExceedsLimit that represents the case when the number of provided params exceeds the limit.

Ref: https://stackoverflow.com/questions/4922345/how-many-bind-variables-can-i-use-in-a-sql-query-in-mysql-5#comment136409462_11131824

[stephen-hlx]

The failed test seems to be a flaky one?
Do you know how to retry it?