mdb_copy: Does not copy lockfile. Can trigger file growth.
mdb_txn_begin(): Clarify usage restrictions.
mdb_drop(): State what to do rather than what will be done, since
closing the handle could otherwise be read as happening even at failure.
partial revert of d6d2638acc245116b8f091ac425b6700d06c4713 and
26a25df5fcc2fcddae6597a61c1b867fc27c568b
The original code was already tested and working correctly.
Whenever we enter cursor_set() the sub-cursor's flag must be
cleared. If the new cursor position has valid subdata it will
be initialized again, if not then the sub-cursor has nothing
to point to.
(Restructuring for upcoming mdb_page_spill work.)
mdb_freelist_save() can't just Get() the destination, since
mdb_page_spill() may have put the destination in the read-only map.
TODO: Can this new put() modify the freelist, which would break it? The
final iteration's put() can shorten the node, the rest uses MDB_CURRENT.
We could set P_KEEP on dirty freeDB leaves and ovpages, since they are
all about to be modified. But the code in this commit must stay anyway,
if mdb should support dropping a 256G DB. I.e. too big for dirty_list.
When collapsing root, must also move cursor index down,
not just the page pointer.
Also in mtest, break from NEXT loops on error, otherwise it just
prints the previous key/data again, which looks confusing.
If mdb_page_touch() sees a page in txn's dirty_list, that
is the page version txn's cursors should have. Fail if
the user may be seeing and depending on another version.
Restore mc_flags and xcursors, they were tracked but not merged.
Simplify: Track parent txn's original cursors after backing them
up, instead of tracking copies and merging them back at commit.
Page leak, mdb_page_alloc(). On error, don't shorten me_pghead.
Memleak, mdb_ovpage_free(). Free page or keep it in dirty_list.
Bad MIDL, mdb_midl_need(). Fix midl[-1] (allocated size).
Catch I/O errors. Do nothing between OS call failure and ErrCode().
Do not use errno after non-OS-errors like write() >= 0, which could
give a failure return of success (errno 0) or some irrelevant error
code. Drop seek calls, use pwrite/pread/Windows OVERLAPPED offset.
