ce1a75c4fe
StartReadBuffers() reports a short read when it finds a cached block that ends a range needing I/O by updating the caller's *nblocks. It doesn't want to have to unpin the trailing hit that it knows the caller wants, so the v17 version used sleight of hand in the name of simplicity: it included it in *nblocks as if it were part of the I/O, but internally tracked the shorter real I/O size in io_buffers_len (now removed). This API change "forwards" the delimiting buffer to the next call. It's still pinned, and still stored in the caller's array, but *nblocks no longer includes stray buffers that are not really part of the operation. The expectation is that the caller still wants the rest of the blocks and will call again starting from that point, and now it can pass the already pinned buffer back in (or choose not to and release it). The change is needed for the coming asynchronous I/O version's larger version of the problem: by definition it must move BM_IO_IN_PROGRESS negotiation from WaitReadBuffers() to StartReadBuffers(), but it might already have many buffers pinned before it discovers a need to split an I/O. (The current synchronous I/O version hides that detail from callers by looping over smaller reads if required to make all covered buffers valid in WaitReadBuffers(), so it looks like one operation but it might occasionally be several under the covers.) Aside from avoiding unnecessary pin traffic, this will also be important for later work on out-of-order streams: you can't prioritize data that is already available right now if that fact is hidden from you. The new API is natural for read_stream.c (see ed0b87ca). After a short read it leaves forwarded buffers where they fell in its circular queue for the continuing call to pick up. Single-block StartReadBuffer() and traditional ReadBuffer() share code but are not affected by the change. They don't do multi-block I/O. Reviewed-by: Andres Freund <andres@anarazel.de> (earlier versions) Discussion: https://postgr.es/m/CA%2BhUKGK_%3D4CVmMHvsHjOVrK6t4F%3DLBpFzsrr3R%2BaJYN8kcTfWg%40mail.gmail.com
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.