(ITS#7853) Improve safety of mdb_env_close

[dw@botanicus.net]

Full_Name: David Wilson
Version: LMDB 0.9.11
OS: 
URL: https://raw.githubusercontent.com/dw/py-lmdb/master/misc/readers_mrb_env.patch
Submission from: (NULL) (178.238.153.20)


Currently if a user (wilfully or accidentally, say, through composition of
third party libs) opens the same LMDB environment twice within a process, and
maintains active read transactions at the time one mdb_env_close() is called,
all reader slots will be deallocated in all environments due to the logic
around line 4253 that unconditionally clears reader slots based on PID.

I'd like to avoid this in py-lmdb, and seems there are a few alternatives to
fix it:


1) Tell users not to do that (doc fix, nobody reads docs)

2) Externally maintain a set of (st_dev, st_ino) keys representing already
opened environments, and check any attempt to open an environment against this
list, failing if data.mdb's key already appears.

3) Disable the mdb_env_close() cleanup loop when MDB_NOTLS is enabled. Since no
reader slot will ever exist in TLS in this mode, the existing mdb_env_close()
doc "All transactions, databases, and cursors must already be closed before
calling this function." ensures no readers exist at close time, and the loop
need not run.

4) Modify lock.mdb to include MDB_env* address within the process, and update
mdb_env_close() to invalidate only readers associated with the environment
being closed. I dislike using the MDB_env* as an opaque publicly visible
cookie, but perhaps storing this field might be reused for other things in
future.


Option 3 lets the user wilfully mix py-lmdb Environment objects within the
process, since py-lmdb always enables NOTLS, but it does not fix the case where
the user is integrating a Python application with a C library with some opaque
interface, and both the application and the library independently have need to
access the environment.

This kind of scenario regularly crops up in huge software projects, the user
may not even be aware the library uses LMDB internally. In that case, the user
is again exposed to having their reader slots silently become deallocated
through no fault of their own.


Option 4 is the least aesthetic, but has all the benefits of option 3 in
addition to preventing the "accidental integration" scenario. I've attached a
patch for it (against 0.9.11), although I completely understand if it does not
get applied. :)

One alternative would be instead of using MDB_env* address, using a
monotonically increasing counter based on a static global, say, "mrb_token",
but this might break in the case two copies of LMDB were somehow linked in the
application. (It's easy to argue the user is an idiot in this case ;)


David