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Commit8735978

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Pad XLogReaderState's main_data buffer more aggressively.
Originally, we palloc'd this buffer just barely big enough to hold thelargest xlog record seen so far. It turns out that that can result invalgrind complaints, because some compilers will emit code that assumesit can safely fetch padding bytes at the end of a struct, and thosepadding bytes were unallocated so far as aset.c was concerned. We canfix that by MAXALIGN'ing the palloc request size, ensuring that it is bigenough to include any possible padding that might've been omitted fromthe on-disk record.An additional objection to the original coding is that it could result inmany repeated palloc cycles, in the worst case where we see a series ofgradually larger xlog records. We can ameliorate that cheaply byimposing 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 thecombocid field is free due to alignment considerations. Perhaps that wastrue 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
1 parent752714d commit8735978

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2 files changed

+17
-8
lines changed

2 files changed

+17
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lines changed

‎src/backend/access/transam/xlogreader.c

Lines changed: 16 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1279,7 +1279,22 @@ DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg)
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{
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if (state->main_data)
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pfree(state->main_data);
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state->main_data_bufsz=state->main_data_len;
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/*
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* main_data_bufsz must be MAXALIGN'ed. In many xlog record
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* types, we omit trailing struct padding on-disk to save a few
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* bytes; but compilers may generate accesses to the xlog struct
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* that assume that padding bytes are present. If the palloc
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* request is not large enough to include such padding bytes then
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* we'll get valgrind complaints due to otherwise-harmless fetches
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* of the padding bytes.
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*
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* In addition, force the initial request to be reasonably large
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* so that we don't waste time with lots of trips through this
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* stanza. BLCKSZ / 2 seems like a good compromise choice.
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*/
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state->main_data_bufsz=MAXALIGN(Max(state->main_data_len,
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BLCKSZ /2));
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state->main_data=palloc(state->main_data_bufsz);
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}
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memcpy(state->main_data,ptr,state->main_data_len);

‎src/include/access/heapam_xlog.h

Lines changed: 1 addition & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -339,13 +339,7 @@ typedef struct xl_heap_new_cid
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TransactionIdtop_xid;
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CommandIdcmin;
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CommandIdcmax;
342-
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/*
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* don't really need the combocid since we have the actual values right in
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* this struct, but the padding makes it free and its useful for
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* debugging.
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*/
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CommandIdcombocid;
342+
CommandIdcombocid;/* just for debugging */
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/*
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* Store the relfilenode/ctid pair to facilitate lookups.

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