shell: reenable the trace

We now have a couple of protections:
  - if we get close to running out of space in the trace we drop in an
    error
  - if we run out of space in the trace we stop trying to append
  - if there are errors we cancel future evaluations

This is already much nicer. You can't do much on the Mu computer, but at
least it gracefully gives up and shows its limitations. On my computer
the Mu shell tries to run computations for about 20s before giving up.
That seems at the outer limit of what interactivity supports. If things
take too long, test smaller chunks.

1 files changed, 12 insertions, 0 deletions

diff --git a/shell/trace.mu b/shell/trace.mu
index 410429e6..267b3ebe 100644
--- a/shell/trace.mu
+++ b/shell/trace.mu
@@ -89,11 +89,23 @@ fn trace _self: (addr trace), label: (addr array byte), message: (addr stream by
   var data-ah/eax: (addr handle array trace-line) <- get self, data
   var data/eax: (addr array trace-line) <- lookup *data-ah
   var index-addr/edi: (addr int) <- get self, first-free
+  {
+    compare *index-addr, 0x8000/lines
+    break-if-<
+    return
+  }
   var index/ecx: int <- copy *index-addr
   var offset/ecx: (offset trace-line) <- compute-offset data, index
   var dest/eax: (addr trace-line) <- index data, offset
   var depth/ecx: (addr int) <- get self, curr-depth
   rewind-stream message
+  {
+    compare *index-addr, 0x7ffe/lines
+    break-if-<
+    initialize-trace-line 0/depth, "error", message, dest
+    increment *index-addr
+    return
+  }
   initialize-trace-line *depth, label, message, dest
   increment *index-addr
 }