Here's a small snippet that gives us OOM, but it doesn't infinitely hang on GC like previously. No ports or JSON-parsing.

[klm@kth arp]$ cat smallerror.scm 

(define state '())

(define (geojson-mmsi geojson)
  (let ((mmsi (alist-ref 'mmsi (or (alist-ref 'properties geojson) '()))))
    (assert (number? mmsi))
    mmsi))

(define (prj-geojson geojson state)
  (assert (<= (length state) 20000))
  ;; set properties for mmsi key:
  (alist-update (geojson-mmsi geojson) ;; key (mmsi)
                geojson ;; value (geojson feature)
                state))

(let loop ()
  (set! state
        (prj-geojson
         `((type . "Feature")
           (geometry . ((type . "Point")
                        (coordinates . ,(vector (/ (random 10000) 100) (/ (random 10000) 100)))))
           (properties . ((mmsi . ,(random 10000))
                          (name . "debug")
                          (sog . 5))))
         state))
  (loop))



$ time csi -s ./smallerror.scm -:g -:hm8M  > /tmp/csilog 2>&1

real	0m39.305s
user	0m39.157s
sys	0m0.087s

[klm@kth arp]$ tail /tmp/csilog  -n 30
[GC] stack	0x00007fffddc9b220	0xffffffffddc9b100	0x00007fffddd9b220
[GC]  from	0x00007fd8f132e010	0x00007fd8f1726790	0x00007fd8f172e010	0x00000000003f8780
[GC]    to	0x00007fd8f0bab010	0x00007fd8f0bab010	0x00007fd8f0fab010 
[GC] 0 locatives (from 32)
[GC] level  1	gcs(minor)  0	gcs(major)  4458
[GC] stack	0x00007fffddc9b220	0xffffffffddc9b120	0x00007fffddd9b220
[GC]  from	0x00007fd8f0bab010	0x00007fd8f0f623e8	0x00007fd8f0fab010	0x00000000003b73d8
[GC]    to	0x00007fd8f132e010	0x00007fd8f132e010	0x00007fd8f172e010 
[GC] 0 locatives (from 32)
[GC] (old) fromspace: 	start=0x00007fd8f0bab010, 	limit=0x00007fd8f0fab010
[GC] (old) tospace:   	start=0x00007fd8f132e010, 	limit=0x00007fd8f172e010
[panic] out of memory - heap full while resizing - execution terminated

...more...
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval>
<eval> 	<--