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Fix a few issues with $RANDOM seeding in subshells (#339)

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This commit fixes an issue I found in the subshell $RANDOM
reseeding code.

The main issue is a performance regression in the shbench fibonacci
benchmark, introduced in commit af6a32d1. Performance dropped in
this benchmark because $RANDOM is always reseeded and restored,
even when it's never used in a subshell. Performance results from
before and after this performance fix (results are on Linux with
CC=gcc and CCFLAGS='-O2 -D_std_malloc'):

  $ ./shbench -b bench/fibonacci.ksh -l 100 ./ksh-0f06a2e ./ksh-af6a32d ./ksh-f31e368 ./ksh-randfix

  benchmarking ./ksh-0f06a2e, ./ksh-af6a32d, ./ksh-f31e368, ./ksh-randfix ...
  *** fibonacci.ksh ***
  # ./ksh-0f06a2e  # Recent version of ksh93u+m
  # ./ksh-af6a32d  # Commit that introduced the regression
  # ./ksh-f31e368  # Commit without the regression
  # ./ksh-randfix  # Ksh93u+m with this patch applied

  -------------------------------------------------------------------------------------------------
  name           ./ksh-0f06a2e        ./ksh-af6a32d        ./ksh-f31e368        ./ksh-randfix
  -------------------------------------------------------------------------------------------------
  fibonacci.ksh  0.481 [0.459-0.515]  0.472 [0.455-0.504]  0.396 [0.380-0.442]  0.407 [0.385-0.439]
  -------------------------------------------------------------------------------------------------

src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/sh/{init,subshell}.c:
- Rather than reseed $RANDOM every time a subshell is created, add
  a sh_save_rand_seed() function that does this only when the
  $RANDOM variable is used in a subshell. This function is called
  by the $RANDOM discipline functions nget_rand() and put_rand().
  As a minor optimization, sh_save_rand_seed doesn't reseed if it's
  called from put_rand().
- Because $RANDOM may have a seed of zero (i.e., RANDOM=0),
  sp->rand_seed isn't enough to tell if $RANDOM has been reseeded.
  Add sp->rand_state for this purpose.
- sh_subshell(): Only restore the former $RANDOM seed and state if
  it is necessary to prevent a subshell leak.

src/cmd/ksh93/tests/variables.sh:
- Add two regression tests for bugs I ran into while making this
  patch.
This commit is contained in:
Johnothan King 2021-11-18 23:17:25 -08:00 • committed by Martijn Dekker
parent 745ffd366d
commit 396b388e1f
5 changed files with 59 additions and 12 deletions
Download patch file Download diff file Expand all files Collapse all files

48
src/cmd/ksh93/sh/subshell.c
View file

@ -100,6 +100,9 @@ static struct subshell
int subdup;
char subshare;
char comsub;
unsigned int rand_seed; /* parent shell $RANDOM seed */
int rand_last; /* last random number from $RANDOM in parent shell */
int rand_state; /* 0 means sp->rand_seed hasn't been set, 1 is the opposite */
#if _lib_fchdir
int pwdfd; /* file descriptor for PWD */
char pwdclose;
@ -193,6 +196,17 @@ void sh_subfork(void)
{
/* this is the child part of the fork */
sh_onstate(SH_FORKED);
/*
* $RANDOM is only reseeded when it's used in a subshell, so if $RANDOM hasn't
* been reseeded yet set rp->rand_last to -2. This allows sh_save_rand_seed()
* to reseed $RANDOM later.
*/
if(!sp->rand_state)
{
struct rand *rp;
rp = (struct rand*)RANDNOD->nvfun;
rp->rand_last = -2;
}
subshell_data = 0;
shp->subshell = 0;
shp->comsub = 0;
@ -231,6 +245,24 @@ int nv_subsaved(register Namval_t *np, int flags)
return(0);
}
/*
* Save the current $RANDOM seed and state, then reseed $RANDOM.
*/
void sh_save_rand_seed(struct rand *rp, int reseed)
{
struct subshell *sp = subshell_data;
if(!sh.subshare && sp && !sp->rand_state)
{
sp->rand_seed = rp->rand_seed;
sp->rand_last = rp->rand_last;
sp->rand_state = 1;
if(reseed)
sh_reseed_rand(rp);
}
else if(reseed && rp->rand_last == -2)
sh_reseed_rand(rp);
}
/*
* This routine will make a copy of the given node in the
* layer created by the most recent virtual subshell if the
@ -467,9 +499,6 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
struct sh_scoped savst;
struct dolnod *argsav=0;
int argcnt;
struct rand *rp; /* current $RANDOM discipline function data */
unsigned int save_rand_seed; /* parent shell $RANDOM seed */
int save_rand_last; /* last random number from $RANDOM in parent shell */
memset((char*)sp, 0, sizeof(*sp));
sfsync(shp->outpool);
sh_sigcheck(shp);
@ -580,11 +609,6 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
sp->cpipe = shp->cpipe[1];
shp->cpid = 0;
sh_sigreset(0);
/* save the current $RANDOM seed and state; reseed $RANDOM */
rp = (struct rand*)RANDNOD->nvfun;
save_rand_seed = rp->rand_seed;
save_rand_last = rp->rand_last;
sh_reseed_rand(rp);
}
jmpval = sigsetjmp(buff.buff,0);
if(jmpval==0)
@ -748,6 +772,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
if(!shp->subshare) /* restore environment if saved */
{
int n;
struct rand *rp;
shp->options = sp->options;
/* Clean up subshell hash table. */
if(sp->strack)
@ -841,8 +866,11 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
shp->coutpipe = sp->coutpipe;
/* restore $RANDOM seed and state */
rp = (struct rand*)RANDNOD->nvfun;
srand(rp->rand_seed = save_rand_seed);
rp->rand_last = save_rand_last;
if(sp->rand_state)
{
srand(rp->rand_seed = sp->rand_seed);
rp->rand_last = sp->rand_last;
}
}
shp->subshare = sp->subshare;
shp->subdup = sp->subdup;