Fix off-by-one error in read IO tracing

AsyncReadBuffer()'s no-IO needed path passed
TRACE_POSTGRESQL_BUFFER_READ_DONE the wrong block number because it had
already incremented operation->nblocks_done. Fix by folding the
nblocks_done offset into the blocknum local variable at initialization.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/u73un3xeljr4fiidzwi4ikcr6vm7oqugn4fo5vqpstjio6anl2%40hph6fvdiiria
Backpatch-through: 18

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 00bc609529a5bb94c589a63599b814e517d11ff4..10afae1990ba78b9c273766182082c8aa0188a23 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -1875,10 +1875,10 @@ AsyncReadBuffers(ReadBuffersOperation *operation, int *nblocks_progress)
 {
        Buffer     *buffers = &operation->buffers[0];
        int                     flags = operation->flags;
-       BlockNumber blocknum = operation->blocknum;
        ForkNumber      forknum = operation->forknum;
        char            persistence = operation->persistence;
        int16           nblocks_done = operation->nblocks_done;
+       BlockNumber blocknum = operation->blocknum + nblocks_done;
        Buffer     *io_buffers = &operation->buffers[nblocks_done];
        int                     io_buffers_len = 0;
        PgAioHandle *ioh;
@@ -1990,7 +1990,7 @@ AsyncReadBuffers(ReadBuffersOperation *operation, int *nblocks_progress)
                 * must have started out as a miss in PinBufferForBlock(). The other
                 * backend will track this as a 'read'.
                 */
-               TRACE_POSTGRESQL_BUFFER_READ_DONE(forknum, blocknum + operation->nblocks_done,
+               TRACE_POSTGRESQL_BUFFER_READ_DONE(forknum, blocknum,
                                                                                  operation->smgr->smgr_rlocator.locator.spcOid,
                                                                                  operation->smgr->smgr_rlocator.locator.dbOid,
                                                                                  operation->smgr->smgr_rlocator.locator.relNumber,
@@ -2062,7 +2062,7 @@ AsyncReadBuffers(ReadBuffersOperation *operation, int *nblocks_progress)
                 */
                io_start = pgstat_prepare_io_time(track_io_timing);
                smgrstartreadv(ioh, operation->smgr, forknum,
-                                          blocknum + nblocks_done,
+                                          blocknum,
                                           io_pages, io_buffers_len);
                pgstat_count_io_op_time(io_object, io_context, IOOP_READ,
                                                                io_start, 1, io_buffers_len * BLCKSZ);