Bug#1024882: scm: FTBFS on armel, armhf and mipsel

[Göran Weinholt <weinholt@debian.org>]

Package: scm
Version: 5f3-1+b1
Severity: serious
Tags: ftbfs, patch
Justification: fails to build from source (but built successfully in the past)

There is a build failure affecting scm on armel, armhf and mipsel:

| ;ERROR: "r4rstest.scm": integer-length: Wrong type in arg1 (0 . 0)
| ; in expression: (#@string->number (#@number->string #@number))
| ; in scope:
| ;   (number)  procedure testit
| ;   (testit . #@define)
| ;   (x)  procedure float-print-test
| ;   (log2 slow-frexp float-precision float-print-test mult-float-print-test float-rw-range-test float-powers-of-2-test . #@define)
| ;   (f0.0 f0.5 f1.0 f2.0 . #@let)
| ;   ()  procedure test-inexact-printing
| ; defined by load: "r4rstest.scm"
| 
| ;STACK TRACE
| 1; ((#@eqv? #@number (#@string->number (#@number->string #@number ...
| 2; (#@let* ((xx (#@+ #@x (#@* #@j #@eps))) (ok? (#@testit #@xx))) ...
| 3; (#@do ((j -100 (#@+ #@j 1))) ((#@or (#@not #@all-ok?) (#@> #@j ...
| 4; (#@define ((log2 (#@let ((l2 (#@log 2)))  (#@lambda (x) (/ (lo ...
| 5; (#@let ((have-inexacts? (#@and (#@string->number "0.0") (#@ine ...
| ...
| 
| ; program args: ("./scm" "-fr4rstest.scm" "-e(test-sc4)(test-cont)(test-delay)(gc)" "-e" "(or (null? errs) (quit 1))")
| make[1]: *** [Makefile:559: check] Error 1

<https://buildd.debian.org/status/fetch.php?pkg=scm&arch=armhf&ver=5f3-1&stamp=1651073592&raw=0>

I've tried to debug it and it's some sort of Heisenbug. The bug is
triggered by the buildds because they run with stdout redirected to a
file and stdin redirected from /dev/null. I think that this changes
the allocation patterns of the program slightly and exposes the bug.

There is a recent change in scm that adds the bigrec() function for
recycling bignums, i.e. freeing them without waiting for the gc to do
it. I believe that some use of this function is the source of the bug.
Note that the assertion error says "Wrong type in arg1 (0 . 0)", which
is the object after bigrecy() has modified it. I tried changing
"CAR(bgnm) = INUM0;" to "CAR(bgnm) = MAKINUM(42);" and the error then
changed to "Wrong type in arg1 (42 . 0)", which suggests that
bigrecy() was run on a reachable bignum.

I haven't managed to figure out exactly which call to bigrecy() is
causing the problem, so here's a patch to just disable it altogether.
The bigrecy() function is a performance hack, so disabling it
shouldn't cause any problems.


diff --git a/scl.c b/scl.c
index a572557..8d8026c 100644
--- a/scl.c
+++ b/scl.c
@@ -133,12 +133,14 @@ void strrecy(str)
 void bigrecy(bgnm)
      SCM bgnm;
 {
+#if 0
   if (IMP(bgnm) || !BIGP(bgnm)) return;
   DEFER_INTS;
   must_free(CHARS(bgnm), (sizet)NUMDIGS(bgnm)*sizeof(BIGDIG));
   CAR(bgnm) = INUM0;
   CDR(bgnm) = INUM0;
   ALLOW_INTS;
+#endif
 }
 
 /* can convert to string accurately with bignums */