Re: zsh 5.2 build with --enable-stack-allocation crashes on large environments

[Bart Schaefer <schaefer@brasslantern.com>]

On Oct 18,  1:27pm, Charles Daffern wrote:
}
} The large environment variable causes zsh to crash on launch. Equally, a
} large number of environment variables has the same effect.

[...]

} The above breakpoint is on the munmap in popheap. The entire "heaps"
} variable is being freed at one point (because "h" is freed and they
} became equal for some reason). I'm not familiar enough with the code to
} figure out how this is happening though.

This would not be unusual in the event of a popheap() -- the whole point
is to be able to discard in one go at pop everything that was allocated
since the push.

With --enable-stack-allocation, the only uses of the heap up to the
point of your segfault are for the split_env_string() function, which
means that pushheap() in createparamtable() is a no-op because no heap
space has yet been allocated to need pushing.

This breaks an assumption in popheap() that there will always be at
least one ->next in the chain that points to memory belonging to a
pushheap() or zhalloc() from somewhere farther back in the call stack.
This is harmless if only a single "arena" has been used by the time
that popheap() is called, but with a large enough chunk needed by
split_env_string() there will be a second arena with no predecessor.

This is handled in freeheap() by leaving the munmap/free for popheap.
I'm not thrilled from the standpoint of all of our work to optimize
mem.c about having to special-case this in popheap() too, but the other
option is to special-case it in pushheap() which is worse, or to find
somewhere to allocate an unnecessary fragment of memory to be an arena
that's never released.

diff --git a/Src/mem.c b/Src/mem.c
index a1744c6..8c7eb80 100644
--- a/Src/mem.c
+++ b/Src/mem.c
@@ -497,7 +497,8 @@ popheap(void)
 		    continue;
 		}
 		h->next = NULL;
-	    }
+	    } else if (hl == h)	/* This is the last arena of all */
+		hl = NULL;
 #ifdef USE_MMAP
 	    munmap((void *) h, h->size);
 #else