Thanks to feedback from Simon Slavin, I now understand how data_version
works and have it working in my code. But in my testing, I tried
another situation to see what would happen with locking in WAL mode. I
have a process that does the following pseudo-code with a table:
sqlite3_prepare("SELECT * FROM table;");
while (sqlite3_step() == SQLITE_ROW) {
x = current_rowid();
sqlite3_exec("UPDATE table SET value=1 WHERE rowid=x;");
sleep(1);
}
Basically it does a SELECT, then for each row found it does an UPDATE on
that row to set some value. I know there are better ways to do this in
SQL - this is just a test of locking in WAL mode. But it is a real
situation that could happen in my application.
If I run this process twice, the first instance will start generating
SQLITE_BUSY errors on the UPDATE when the second instance starts up. I
thought in WAL mode you could intermix readers and writers. Is this not
the case? The 2 read loops seem to work just fine. It's just that the
first process can no longer do UPDATE's when the second process starts.
I tried compiling with -DSQLITE_THREADSAFE=1 but that did not help. I
suspect that in WAL mode you can have multiple readers but only 1 writer.
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