There was a weird bug in the GC system. The image-writing code causes a

GC to assemble a compacted heap image in newspace which it then writes out
to disk. Then the VM calls ABORT-GC to cancel the GC operation, which scans
the current space, fixing up the "broken hearts" -- restoring word 1 of
each structure that got clobbered with a forwarding pointer.

Unfortunately, someone (possibly myself) had inserted a post_gc_fdports() call
into the VM between the gc and the abort. This procedure updates a C vector of
Scheme values (fdports[]) by following forwarding pointers -- BUT -- in this
instance we didn't really want to break hearts, and the abort-gc code didn't
know about the fdports[] vector, so it couldn't undo the effects. This caused
the fdports[] vec to point into hyperspace after the image dump, and *that*
meant on the next GC, all the live ports were considered dead. Oops.

The fix was to remove this bogus call. The post_gc_fdports() proc is now
called only after a *real* GC.
    -Olin

scheme48vm.c

@@ -3480,7 +3480,14 @@ long Tinterpret()
   RSstackS = SstackS;
   RScode_pointerS = Scode_pointerS;
   RSenvS = SenvS;
-  post_gc_fdports(); /* Hack for scsh's i/o system. Olin. */
+  /* Do NOT put a call to the scsh I/O gc hook post_gc_fdports() here.
+  ** This GC is not for real -- it's for writing images, and will be
+  ** aborted. But the part of post_gc_fdports that updates the fdports[]
+  ** elements to point into new-space *won't* be aborted, and oops, we're
+  ** pointing into hyperspace. This whole thing is a mess; we need to port
+  ** to a newer Scheme 48 release that gives us structured GC hooks.
+  **     -Olin
+  */
   close_untraced_portsB_return_tag = 0;
   goto close_untraced_portsB;
  close_untraced_portsB_return_0:

scsh/fdports1.c

@@ -301,17 +301,16 @@ void post_gc_fdports(void)
 {
     int fd;
 
-#if 0
+#ifdef NOISY_FDGC
     fputs("{GC", stderr); fflush(stderr);
 #endif
     for(fd=0; fd<NUM_FDPORTS; fd++) {
 
 	scheme_value port = fdports[fd];
-
 	if(STOBP(port)) {
 	    long header = STOB_HEADER(port);
 	    if(STOBP(header)) {
-#if 0
+#ifdef NOISY_FDGC
 		fprintf(stderr, "Copying port[fd] %d[%d] header %d\n",
 			port, fd, header);
 		fflush(stderr);
@@ -324,7 +323,7 @@ void post_gc_fdports(void)
 		/* Port wasn't copied -- is garbage.
 		   If fd unrevealed, close it. */
 		int rev = EXTRACT_FIXNUM(*PortRev(port));
-#if 0
+#ifdef NOISY_FDGC
 		fprintf(stderr, "GC'ing %srevealed port[fd] %d[%d]\n",
 			rev == 0 ? "un" : "",
 			port, fd);
@@ -335,7 +334,7 @@ void post_gc_fdports(void)
 		}
 	    }
 	}
-#if 0
+#ifdef NOISY_FDGC
     fputs("}", stderr); fflush(stderr);
 #endif
     }