Fix core dump in ReorderBufferRestoreChange on alignment-picky platforms.

When re-reading an update involving both an old tuple and a new tuple from
disk, reorderbuffer.c was careless about whether the new tuple is suitably
aligned for direct access --- in general, it isn't.  We'd missed seeing
this in the buildfarm because the contrib/test_decoding tests exercise this
code path only a few times, and by chance all of those cases have old
tuples with length a multiple of 4, which is usually enough to make the
access to the new tuple's t_len safe.  For some still-not-entirely-clear
reason, however, Debian's sparc build gets a bus error, as reported by
Christoph Berg; perhaps it's assuming 8-byte alignment of the pointer?

The lack of previous field reports is probably because you need all of
these conditions to trigger a crash: an alignment-picky platform (not
Intel), a transaction large enough to spill to disk, an update within
that xact that changes a primary-key field and has an odd-length old tuple,
and of course logical decoding tracing the transaction.

Avoid the alignment assumption by using memcpy instead of fetching t_len
directly, and add a test case that exposes the crash on picky platforms.
Back-patch to 9.4 where the bug was introduced.

Discussion: <20160413094117.GC21485@msg.credativ.de>

contrib/test_decoding/expected/ddl.out

@@ -233,6 +233,8 @@ SELECT 'tx logical msg' FROM pg_logical_emit_message(true, 'test', 'tx logical m
 
 DELETE FROM tr_etoomuch WHERE id < 5000;
 UPDATE tr_etoomuch SET data = - data WHERE id > 5000;
+CREATE TABLE tr_oddlength (id text primary key, data text);
+INSERT INTO tr_oddlength VALUES('ab', 'foo');
 COMMIT;
 /* display results, but hide most of the output */
 SELECT count(*), min(data), max(data)
@@ -244,13 +246,16 @@ ORDER BY 1,2;
      1 | BEGIN                                                                 | BEGIN
      1 | COMMIT                                                                | COMMIT
      1 | message: transactional: 1 prefix: test, sz: 14 content:tx logical msg | message: transactional: 1 prefix: test, sz: 14 content:tx logical msg
+     1 | table public.tr_oddlength: INSERT: id[text]:'ab' data[text]:'foo'     | table public.tr_oddlength: INSERT: id[text]:'ab' data[text]:'foo'
  20467 | table public.tr_etoomuch: DELETE: id[integer]:1                       | table public.tr_etoomuch: UPDATE: id[integer]:9999 data[integer]:-9999
-(4 rows)
+(5 rows)
 
 -- check updates of primary keys work correctly
 BEGIN;
 CREATE TABLE spoolme AS SELECT g.i FROM generate_series(1, 5000) g(i);
 UPDATE tr_etoomuch SET id = -id WHERE id = 5000;
+UPDATE tr_oddlength SET id = 'x', data = 'quux';
+UPDATE tr_oddlength SET id = 'yy', data = 'a';
 DELETE FROM spoolme;
 DROP TABLE spoolme;
 COMMIT;
@@ -260,7 +265,9 @@ WHERE data ~ 'UPDATE';
                                                     data                                                     
 -------------------------------------------------------------------------------------------------------------
  table public.tr_etoomuch: UPDATE: old-key: id[integer]:5000 new-tuple: id[integer]:-5000 data[integer]:5000
-(1 row)
+ table public.tr_oddlength: UPDATE: old-key: id[text]:'ab' new-tuple: id[text]:'x' data[text]:'quux'
+ table public.tr_oddlength: UPDATE: old-key: id[text]:'x' new-tuple: id[text]:'yy' data[text]:'a'
+(3 rows)
 
 -- check that a large, spooled, upsert works
 INSERT INTO tr_etoomuch (id, data)

contrib/test_decoding/sql/ddl.sql

@@ -116,6 +116,8 @@ INSERT INTO tr_etoomuch(data) SELECT g.i FROM generate_series(1, 10234) g(i);
 SELECT 'tx logical msg' FROM pg_logical_emit_message(true, 'test', 'tx logical msg');
 DELETE FROM tr_etoomuch WHERE id < 5000;
 UPDATE tr_etoomuch SET data = - data WHERE id > 5000;
+CREATE TABLE tr_oddlength (id text primary key, data text);
+INSERT INTO tr_oddlength VALUES('ab', 'foo');
 COMMIT;
 
 /* display results, but hide most of the output */
@@ -128,6 +130,8 @@ ORDER BY 1,2;
 BEGIN;
 CREATE TABLE spoolme AS SELECT g.i FROM generate_series(1, 5000) g(i);
 UPDATE tr_etoomuch SET id = -id WHERE id = 5000;
+UPDATE tr_oddlength SET id = 'x', data = 'quux';
+UPDATE tr_oddlength SET id = 'yy', data = 'a';
 DELETE FROM spoolme;
 DROP TABLE spoolme;
 COMMIT;

src/backend/replication/logical/reorderbuffer.c

@@ -2444,6 +2444,10 @@ ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
 /*
  * Convert change from its on-disk format to in-memory format and queue it onto
  * the TXN's ->changes list.
+ *
+ * Note: although "data" is declared char*, at entry it points to a
+ * maxalign'd buffer, making it safe in most of this function to assume
+ * that the pointed-to data is suitably aligned for direct access.
  */
 static void
 ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
@@ -2471,7 +2475,7 @@ ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:
 			if (change->data.tp.oldtuple)
 			{
-				Size		tuplelen = ((HeapTuple) data)->t_len;
+				uint32		tuplelen = ((HeapTuple) data)->t_len;
 
 				change->data.tp.oldtuple =
 					ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);
@@ -2492,7 +2496,11 @@ ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 			if (change->data.tp.newtuple)
 			{
-				Size		tuplelen = ((HeapTuple) data)->t_len;
+				/* here, data might not be suitably aligned! */
+				uint32		tuplelen;
+
+				memcpy(&tuplelen, data + offsetof(HeapTupleData, t_len),
+					   sizeof(uint32));
 
 				change->data.tp.newtuple =
 					ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);