sqlite: handling multiple concurrent database read-modify-write operations - sqlite

Question

For the sake of argument, assume that I have a very simple database:
CREATE TABLE test(idx integer primary key autoincrement, count integer);
This has one row. The database is accessed by a CGI script, which is called by Apache. The script reads the current value of count, increments it, and writes it back. I can run the script as
curl http://localhost/cgi-bin/test
and it tells me what the new value of count is. The script is actually C++; the basic stripped-down code looks like this:
// 'callback' sets 'count' to the current value of count
sqlite3_exec(con, "select count from test where idx=1", callback, &count, 0);
++count;
command << "update test set count=" << count << " where idx=1";
sqlite3_exec(con, command.str().c_str(), 0, 0, 0);
If I write a bash script that runs 20 instances of curl in the background, then I get lots of messages that the database is locked, and the counter is only incremented to 2 or 3, instead of 20. Ok, that's not very surprising, but how do I fix this?
After some experimenting, I've put both sqlite3_exec statements inside an exclusive transaction:
while(true) {
rc = sqlite3_exec(con, "begin exclusive transaction", 0, 0, 0);
if(rc != SQLITE_BUSY)
break;
std::this_thread::sleep_for(std::chrono::milliseconds(100));
}
if(rc != SQLITE_OK)
error();
...the select and update code shown above, followed by:
sqlite3_exec(con, "end transaction", 0, 0, 0);
This appears to be rock-solid, but I can't make much sense of the relevant bits of the sqlite docs, and I'm not convinced. Is there anything else I need to think about? Note that I don't have any rollbacks, or any other sqlite3 calls, apart from sqlite_open_v2, sqlite3_errmsg, and sqlite_close, no WAL, and I only test for SQLITE_BUSY. For testing, I run the bash script below, with $1 set to 1000 (ie. 1000 curl instances all running the CGI code). This completes in 10 or 11 seconds, and every time I run it it shows the final value of count as 1000, so it appears to be working.
Test script:
#!/bin/bash
sqlite3 /var/www/cgi-bin/test.db <<EOF
update test set count=0 where idx=1;
EOF
for ((c=0; c<$1; c++ ))
do
curl http://localhost/cgi-bin/test > /dev/null 2>&1 &
done
wait
sqlite3 /var/www/cgi-bin/test.db <<EOF
select count from test where idx=1;
EOF