fix(tvix/eval): don't print full stack in observer r/5973

Print only the top 6 values of the stack, not the entire stack.
There's very few operations that deal with more values anyways, so the
rest are not likely to be useful.

This gets us one step closer to tracing VERY large executions without
blowing up.

Change-Id: I97472321b0321b25d534d9f53b3aadfacc2318fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8201
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

1 files changed, 8 insertions, 1 deletions

diff --git a/tvix/eval/src/observer.rs b/tvix/eval/src/observer.rs
index 970a644a3d43..29e4fb2d9c06 100644
--- a/tvix/eval/src/observer.rs
+++ b/tvix/eval/src/observer.rs
@@ -166,7 +166,14 @@ impl<W: Write> TracingObserver<W> {
     fn write_stack(&mut self, stack: &[Value]) {
         let _ = write!(&mut self.writer, "[ ");
 
-        for val in stack {
+        // Print out a maximum of 6 values from the top of the stack,
+        // before abbreviating it to `...`.
+        for (i, val) in stack.iter().rev().enumerate() {
+            if i == 6 {
+                let _ = write!(&mut self.writer, "...");
+                break;
+            }
+
             self.write_value(&val);
         }