From 6448f0926519fa599351f94949596e664dc92bf7 Mon Sep 17 00:00:00 2001
From: David Leong <116610336+leongdl@users.noreply.github.com>
Date: Fri, 24 Jul 2026 18:36:16 -0700
Subject: [PATCH] fix(cli): Defer zero-duration chunk adjustment

Signed-off-by: David Leong <116610336+leongdl@users.noreply.github.com>
---
 crates/openjd-cli/src/run/execution.rs | 55 +++++++++++++++++++++-----
 specs/cli/run.md                       | 22 ++++++-----
 2 files changed, 59 insertions(+), 18 deletions(-)

diff --git a/crates/openjd-cli/src/run/execution.rs b/crates/openjd-cli/src/run/execution.rs
index fbbadcb5..889a1f19 100644
--- a/crates/openjd-cli/src/run/execution.rs
+++ b/crates/openjd-cli/src/run/execution.rs
@@ -537,6 +537,28 @@ fn adaptive_target_runtime(step: &Step, is_adaptive: bool) -> f64 {
         .unwrap_or(0.0)
 }
 
+fn calculate_adaptive_chunk_size(
+    current_chunk_size: usize,
+    completed_task_count: usize,
+    completed_task_duration: f64,
+    target_runtime_seconds: f64,
+) -> Option<usize> {
+    if completed_task_count == 0
+        || completed_task_duration <= 0.0
+        || !completed_task_duration.is_finite()
+    {
+        return None;
+    }
+
+    let duration_per_task = completed_task_duration / completed_task_count as f64;
+    let mut adaptive_chunk_size = target_runtime_seconds / duration_per_task;
+    if completed_task_count < 10 && adaptive_chunk_size > current_chunk_size as f64 {
+        adaptive_chunk_size = 0.75 * current_chunk_size as f64 + 0.25 * adaptive_chunk_size;
+    }
+
+    Some((adaptive_chunk_size as usize).max(1))
+}
+
 fn adjust_adaptive_chunk_size(
     ctx: &RunContext,
     iter: &mut openjd_model::StepParameterSpaceIterator,
@@ -544,16 +566,16 @@ fn adjust_adaptive_chunk_size(
     completed_task_duration: f64,
     target_runtime_seconds: f64,
 ) {
-    let duration_per_task = completed_task_duration / completed_task_count as f64;
-    let mut adaptive_chunk_size = target_runtime_seconds / duration_per_task;
-    if completed_task_count < 10 {
-        let current = iter.chunks_default_task_count().unwrap_or(1) as f64;
-        if adaptive_chunk_size > current {
-            adaptive_chunk_size = 0.75 * current + 0.25 * adaptive_chunk_size;
-        }
-    }
+    let current_chunk_size = iter.chunks_default_task_count().unwrap_or(1);
+    let Some(adaptive_chunk_size) = calculate_adaptive_chunk_size(
+        current_chunk_size,
+        completed_task_count,
+        completed_task_duration,
+        target_runtime_seconds,
+    ) else {
+        return;
+    };
 
-    let adaptive_chunk_size = (adaptive_chunk_size as usize).max(1);
     if Some(adaptive_chunk_size) != iter.chunks_default_task_count() {
         println!(
             "{}\tAdjusting chunk size to {adaptive_chunk_size}",
@@ -603,3 +625,18 @@ fn report_result(ctx: &mut RunContext, args: &RunArgs, job: &Job, working_dir: &
     };
     crate::common::print_cli_result(&result, &args.output);
 }
+
+#[cfg(test)]
+mod tests {
+    use super::calculate_adaptive_chunk_size;
+
+    #[test]
+    fn zero_duration_defers_adaptive_chunk_adjustment() {
+        assert_eq!(calculate_adaptive_chunk_size(1, 1, 0.0, 60.0), None);
+    }
+
+    #[test]
+    fn measurable_duration_preserves_early_sample_blending() {
+        assert_eq!(calculate_adaptive_chunk_size(1, 1, 1.0, 9.0), Some(3));
+    }
+}
diff --git a/specs/cli/run.md b/specs/cli/run.md
index 0ed6a2c8..4c91e473 100644
--- a/specs/cli/run.md
+++ b/specs/cli/run.md
@@ -199,17 +199,21 @@ dynamically adjust chunk sizes toward a target runtime:
 
 ```
 For each completed task:
-  1. Count items in the chunk (from the RangeExpr value)
-  2. Accumulate total items and total duration
-  3. Compute duration_per_task = total_duration / total_items
-  4. Compute ideal_chunk_size = target_runtime_seconds / duration_per_task
-  5. For the first 10 tasks, blend: 75% current + 25% ideal (conservative ramp)
-  6. Clamp to minimum of 1
-  7. Update iterator's default task count if changed
+1. Count items in the chunk (from the RangeExpr value)
+2. Accumulate total items and total duration
+3. If the cumulative duration is zero or non-finite, keep the current chunk size and wait
+   for a measurable sample
+4. Compute duration_per_task = total_duration / total_items
+5. Compute ideal_chunk_size = target_runtime_seconds / duration_per_task
+6. For the first 10 tasks, blend: 75% current + 25% ideal (conservative ramp)
+7. Clamp to minimum of 1
+8. Update iterator's default task count if changed
 ```
 
-The blending in step 5 prevents wild oscillation when early tasks have atypical durations.
-After 10 tasks, the estimate stabilizes and the ideal size is used directly.
+Deferring adjustment when elapsed time is not measurable prevents a zero duration from
+producing an infinite ideal size that would saturate to `usize::MAX`. The blending in step 6
+prevents wild oscillation when early tasks have atypical durations. After 10 tasks, the
+estimate stabilizes and the ideal size is used directly.
 
 `target_runtime_seconds` comes from the step's CHUNK[INT] parameter definition. If not
 specified or zero, the model layer's `StepParameterSpaceIterator` sets `chunks_adaptive()`