Issue 29355: sqlite3: remove sqlite3_stmt_readonly()

In CPython 3.6.0, sqlite3 module uses sqlite3_stmt_readonly(), however this function is only available on SQLite 3.7.4+ (release on 2010-12-07).
Then CPython 3.6.0 can not be compiled on RHEL6 (with SQLite 3.6.x), complaints: issue29098, issue29325.

sqlite3_stmt_readonly() was introduced in 284676cf2ac8, it was used twice [1][2], but it seems that we can avoid using this function in both of them.

[1] https://hg.python.org/cpython/file/3.6/Modules/_sqlite/cursor.c#l517
With palaviv's patches of issue28518, we can eliminate sqlite3_stmt_readonly() in here.

[2] https://hg.python.org/cpython/file/3.6/Modules/_sqlite/cursor.c#l612
We can use the old way in here, the old way is even better IMO.

This issue depends on issue28518.

Another report of sqlite3 not compiling correctly on RHEL6 (https://github.com/jupyterhub/jupyterhub/issues/991). Is there currently a solution to build Python 3.6 with sqlite3 support?

> Is there currently a solution to build Python 3.6 with sqlite3 support?

I installed SQLite 3.16.2 on a Debian-based system with this way:
http://stackoverflow.com/questions/26261080
No idea whether it will work on RHEL 6.

Thanks for the report, Ma.

> We can use the old way in here, the old way is even better IMO.

What do you mean by the old way? The use of switch statement?

The "old way" is not using sqlite3_stmt_readonly().

Before 3.6.0, we only count rowcount for INSERT, UPDATE, DELETE, REPLACE:
https://hg.python.org/cpython/file/3.5/Modules/_sqlite/cursor.c#l689

These four statements can be representd by is_dml in PR 245:
https://github.com/python/cpython/pull/245/commits/cbeabf4044e9e1563d228e359b0e7952f369b42a#diff-5a6abb9997d51e693f3b24986659c857R88

So only change this line is ok, then restore the behavior before 3.6.0 exactly:

- if (!sqlite3_stmt_readonly(self->statement->st)) {
+ if (self->statement->is_dml) {
    self->rowcount += (long)sqlite3_changes(self->connection->db);
} else {
    self->rowcount= -1L;
}

Why it's better?
sqlite3_changes() only works for INSERT, UPDATE, DELETE. see:
https://sqlite.org/c3ref/changes.html
So using sqlite3_stmt_readonly() to determine statements is not necessary at all.

In addition, we can add a comment for code safe in statement.c.
+    /* is_dml is used in two sites:
+        1, determine statements for implicit BEGIN.
+        2, determine statements for counting rowcount */
self->is_dml = (PyOS_strnicmp(p, "insert ", 7) == 0)
            || (PyOS_strnicmp(p, "update ", 7) == 0)
            || (PyOS_strnicmp(p, "delete ", 7) == 0)
            || (PyOS_strnicmp(p, "replace ", 8) == 0);

> - if (!sqlite3_stmt_readonly(self->statement->st)) {
> + if (self->statement->is_dml) {

Ah, yes now I understand what do you mean! :) I agree and this is already in my local branch to fix issue 28518.

The patch for issue 28518 has been merged and it doesn't contain any use of sqlite3_stmt_readonly(). Closing this as 'out of date'. Thanks again Ma!

stage: resolved