On 01/11/2016 18:06, Rowan Worth wrote: > On 12 January 2016 at 03:00, Felipe Gasper <felipe at felipegasper.com> wrote: > >> On 11 Jan 2016 1:45 PM, Scott Hess wrote: >> >>> As far as preventing the other process from using it before the schema >>> exists, do "SELECT count(*) FROM sqlite_master", and if the result is 0, >>> the schema does not exist. If you create the schema as a transaction, >>> that >>> will be atomic. >>> >> But in order for that SELECT to avert TOCTTOU errors, we?d have to do >> BEGIN EXCLUSIVE LOCK at the beginning of every single DB handle creation. >> > No, only the connection which is creating the schema needs BEGIN EXCLUSIVE. > The other connections can determine the schema state based on a normal > "SELECT count(*) FROM sqlite_master": > > * if it returns SQLITE_OK and at least one row, the schema has been created > and it can proceed > * if it returns SQLITE_OK and zero rows, the schema hasn't been created yet > * if it returns SQLITE_BUSY, the schema is in the process of being created > (or there's some other EXCLUSIVE transaction in progress, or a transaction > is being committed at this very moment, or an in-progress write transaction > has spilled sqlite's memory cache) > > > As Scott also hinted at, hard linking DB files is dangerous because > connections against each of the links will use different -journal files. In > the event that your schema creation process (or the machine its running on) > crashes halfway through COMMIT, connecting to the permanent database will > cause corruption to be observed (because it's not aware of the other > journal file and thus can't rollback the partial transaction). > > This may also be possible if another process simply connects to the > permanent DB at just the wrong time (ie. halfway through the schema > creation COMMIT)? Or maybe not, in my experience POSIX locks are maintained > across hardlinks but I haven't checked if this is specified by the standard > or file-system dependent. > > > Do your different connections actually run different code? Or are you > potentially in the situation where two threads are trying to create the > same DB at the same time (each via their own .tmp file)? > > -Rowan > _______________________________________________ > sqlite-users mailing list > sqlite-users at mailinglists.sqlite.org > http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
TOCTTOU? What is that? -- --------------------- Scott Doctor scott at scottdoctor.com
