Quoth Drake Wilson <dr...@dasyatidae.net>, on 2013-04-04 10:20:44 -0500: > So it is perfectly okay to use unprotected mmap accesses if an I/O > error on the file will already make the entire process uncontinuable. > The question is whether this applies to arbitrary SQLite databases > that an application may open, and I suspect that (a) it probably > doesn't, and (b) this reliability transitivity behavior would be a > significant departure from earlier SQLite versions.
Here is a much more direct and concrete example. Referenced files may be retrieved from: http://dasyatidae.net/files/2013/sqlite3-201304040051/ Here are the steps I used. This is on a modern Debian GNU/Linux AMD64 system. - Compile kvserv.c along with an _earlier_ (probably system) version of SQLite than the snapshot amalgamation mentioned above---I used: gcc -std=c99 -o kvserv kvserv.c -lsqlite3 -lpthread -ldl - Mount a removable disk that you don't care about very much (I used a spare USB flash disk), and copy keyval1.db to it. Unmount, unplug, replug, and remount the disk read-only. The database is deliberately a few megabytes in size to reduce the chance that all of it will be read ahead into cache; I used: echo 1 | sudo tee /proc/sys/vm/drop_caches a bit ad-hoc to help ensure this, though it should not theoretically be necessary. - Symlink the copied file to keyval.db in the current directory (all the other files should be on a reliable local disk), and ensure UDP port 11105 is not in use. Run kvserv. In a separate terminal, run something akin to: socat - udp6-datagram:[::1]:11105 (In retrospect I should have used a Unix-domain socket, but I do not have time to change it right now; I apologize for the inconvenience.) - Issue queries to the simple key-value server by entering keys, one per line, in the socat terminal. In particular, the keys 'a', 'b', and 'c' are defined in the given DB, along with all five-digit decimal numbers. Responses should be returned beginning with "OK" followed by either result data or nothing. - Unplug the removable disk hard, simulating a media failure. Issue additional queries. Responses should be returned beginning with "NG", indicating that there was an error retrieving the requested data. Repeating these steps, but compiling the application with the sqlite3.c from the 201304040051 snapshot amalgamation that uses unprotected mmap, causes the entire kvserv process to die with SIGBUS as soon as a query tries to access the volume while it is unplugged. Unless the design of kvserv.c is relevantly unreasonable, this should help demonstrate the danger of switching SQLite to use unprotected mmap by default. ---> Drake Wilson _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
