Drop support for max_length in mysqli_fetch_fields()

[nikic]

Retain the field, but always populate it with zero. This was
already the case for PS without length updating.

max_length has nothing lost in the field metadata -- it is a
property of the specific result set, and requires scanning the
whole result set to compute. PHP itself never uses max_length
with mysqlnd, it is only exposed in the raw mysqli API.

Keeping it for just that purpose is not worthwhile given the costs
involved. People who actually need this for some unfathomable
reason can easily calculate it themselves, while making it obvious
that the calculation requires a full result set scan.

[nikic]

cc @kamil-tekiela Do you know whether max_length is important for some PHP-level application?

Dropping this will allow simplifying our implementation of buffered result sets, and making them use less memory. Currently this is not possible because max_length requires the whole result set to be decoded up front.

[nikic]

Second commit is the cleanup of the result set implementation. Based on this I think we'll be able to a) support PS cursors + get_result and b) typed values in PDO MySQL with emulated PS.

[kamil-tekiela]

Ok, I have done some research and I think we can drop this field, but this should not be done in a patch version. We can include this change in PHP 8.1.

From my initial analysis, I came to the conclusion that this field probably has no use in PHP at all. I was not able to find any sample usage online. The goal of mysqli was to retain as much backwards compatibility with the C API as possible. MySQL C API exposes this value, and it might have certain applications in C/C++ code. However, PHP is higher level and knowing the length of the columns and/or data in bytes is not very useful. This also applies to the length property although we should leave that one alone.

Taking a look at other values exposed by MySQL C API we can see that not all of them are useful for PHP developers. We can probably also hide catalog since the value is always constant. The most used one seems to be the table/column name field.

In terms of all other changes, you have included in the PR I need to test them out first. On the surface, they look ok, but I don't see how it helps with cursor fetches. Let me dig deeper into it now.

[nikic]

Ok, I have done some research and I think we can drop this field, but this should not be done in a patch version. We can include this change in PHP 8.1.

Yeah, this is targeted at PHP 8.1.

From my initial analysis, I came to the conclusion that this field probably has no use in PHP at all. I was not able to find any sample usage online. The goal of mysqli was to retain as much backwards compatibility with the C API as possible. MySQL C API exposes this value, and it might have certain applications in C/C++ code. However, PHP is higher level and knowing the length of the columns and/or data in bytes is not very useful. This also applies to the length property although we should leave that one alone.

On the C API level, max_length exists because users of the C API need to preallocate a buffer of the correct size to fetch a row. max_length determines the smallest buffer size that can be used for all rows. Of course, PHP developers don't need to allocate any buffers, PHP itself takes care of that, so the value is meaningless to them.

Taking a look at other values exposed by MySQL C API we can see that not all of them are useful for PHP developers. We can probably also hide catalog since the value is always constant. The most used one seems to be the table/column name field.

In terms of all other changes, you have included in the PR I need to test them out first. On the surface, they look ok, but I don't see how it helps with cursor fetches. Let me dig deeper into it now.

I originally thought this will help cursors by removing all the code duplication for reading of result sets, so we could handle cursors in a central location. However, I then realized that we can solve the cursor problem more easily (and more correctly) by instructing the cursor to fetch all rows, and then forgetting about it. This is what I implemented in #6518.