Pad XLogReaderState's main_data buffer more aggressively.

Originally, we palloc'd this buffer just barely big enough to hold the
largest xlog record seen so far.  It turns out that that can result in
valgrind complaints, because some compilers will emit code that assumes
it can safely fetch padding bytes at the end of a struct, and those
padding bytes were unallocated so far as aset.c was concerned.  We can
fix that by MAXALIGN'ing the palloc request size, ensuring that it is big
enough to include any possible padding that might've been omitted from
the on-disk record.

An additional objection to the original coding is that it could result in
many repeated palloc cycles, in the worst case where we see a series of
gradually larger xlog records.  We can ameliorate that cheaply by
imposing a minimum buffer size that's large enough for most xlog records.
BLCKSZ/2 was chosen after a bit of discussion.

In passing, remove an obsolete comment in struct xl_heap_new_cid that the
combocid field is free due to alignment considerations.  Perhaps that was
true at some point, but it's not now.

Back-patch to 9.5 where this code came in.

Discussion: https://postgr.es/m/E1eHa4J-0006hI-Q8@gemulon.postgresql.org

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index f2da505892408bcf31d9ab1bb4bb4f63e5a4804c..4499fe0627f7abc2d25f94ed724795677e46b925 100644 (file)
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1268,7 +1268,22 @@ DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg)
                {
                        if (state->main_data)
                                pfree(state->main_data);
-                       state->main_data_bufsz = state->main_data_len;
+
+                       /*
+                        * main_data_bufsz must be MAXALIGN'ed.  In many xlog record
+                        * types, we omit trailing struct padding on-disk to save a few
+                        * bytes; but compilers may generate accesses to the xlog struct
+                        * that assume that padding bytes are present.  If the palloc
+                        * request is not large enough to include such padding bytes then
+                        * we'll get valgrind complaints due to otherwise-harmless fetches
+                        * of the padding bytes.
+                        *
+                        * In addition, force the initial request to be reasonably large
+                        * so that we don't waste time with lots of trips through this
+                        * stanza.  BLCKSZ / 2 seems like a good compromise choice.
+                        */
+                       state->main_data_bufsz = MAXALIGN(Max(state->main_data_len,
+                                                                                                 BLCKSZ / 2));
                        state->main_data = palloc(state->main_data_bufsz);
                }
                memcpy(state->main_data, ptr, state->main_data_len);

diff --git a/src/include/access/heapam_xlog.h b/src/include/access/heapam_xlog.h
index 06a82426bb524787b717523a70ede427a7410ae8..54d3f168279f0abed2e46c6f5b98175433326ed1 100644 (file)
--- a/src/include/access/heapam_xlog.h
+++ b/src/include/access/heapam_xlog.h
@@ -339,13 +339,7 @@ typedef struct xl_heap_new_cid
        TransactionId top_xid;
        CommandId       cmin;
        CommandId       cmax;
-
-       /*
-        * don't really need the combocid since we have the actual values right in
-        * this struct, but the padding makes it free and its useful for
-        * debugging.
-        */
-       CommandId       combocid;
+       CommandId       combocid;               /* just for debugging */
 
        /*
         * Store the relfilenode/ctid pair to facilitate lookups.