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Fix for trwsync.pp sporadic failure that appeared in trunk.

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The problem was in the source of random related functions,
  which are used in this test by several thread in parallel,
  while this code is known and advertised as non-thread safe.

  The access by multiple threads led to multiple increments of the
  mt_index static variable, which was then never reset to zero and
  led to all successive calls to random function to return zero.

  This fix simply insures that the mt_state array is never
  accessed past its end, and that mt_index is reset correctly even
  if multiple increments do happen.

  It does not make the random function thread-safe.

  * Use local variable l_index to avoid accessing mt_state array past last element.
  * Change mt_index=MTWIST_N+1 into l_index>=MTWIST_N+1
    to insure that mt_init will be called again if index is past end.

git-svn-id: trunk@34320 -
This commit is contained in:
pierre 2016-08-16 11:59:33 +00:00
parent cdda4da2d3
commit b9e336dcbe
1 changed files with 11 additions and 4 deletions
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15
rtl/inc/system.inc
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@ -606,9 +606,13 @@ end;
function mtwist_u32rand: cardinal;
var
l_index :cardinal;
begin
l_index:=mt_index;
inc(mt_index);
if (RandSeed<>OldRandSeed) or
(mt_index=MTWIST_N+1) then
(l_index>=MTWIST_N+1) then
begin
mtwist_init(RandSeed);
{ Detect resets of randseed
@ -619,11 +623,14 @@ begin
}
RandSeed:=not(RandSeed);
OldRandSeed:=RandSeed;
l_index:=MTWIST_N;
end;
if mt_index=MTWIST_N then
mtwist_update_state;
result:=mt_state[mt_index];
inc(mt_index);
begin
mtwist_update_state;
l_index:=0;
end;
result:=mt_state[l_index];
result:=result xor (result shr 11);
result:=result xor ((result shl 7) and cardinal($9D2C5680));
result:=result xor ((result shl 15) and cardinal($EFC60000));