fix(monitor): clamp Span.Progress Count to Total

[shaun0927]

Problem

After #1208 removed the divide-by-zero panic in Span.Progress, the "zero-total child is complete" fallback can push parentCount above parentTotal when the parent has already consumed its own count and a running child is reporting zero total.

With a parent total=2, count=2 plus one running child with total=0, Span.Progress() returns Progress{Count: 3, Total: 2}. The dashboard progress bar at /api/dashboard/tasks renders this as 150%.

Root Cause

common/monitor/progress.go:138-169:

parentTotal := s.total * childTotal
parentCount := s.count * childTotal
for _, child := range children {
    if child.Total == 0 {
        parentCount += childTotal   // treat zero-total child as complete
        continue
    }
    parentCount += childTotal * child.Count / child.Total
}

When s.count * childTotal == parentTotal and the zero-total branch fires, parentCount exceeds parentTotal by one childTotal unit. There is no clamp before the Progress{} value is returned.

Solution

Clamp parentCount to parentTotal just before returning:

if parentCount > parentTotal {
    parentCount = parentTotal
}

This is the minimal change that keeps the "zero-total child is complete" semantics from #1208 while restoring the 0 <= Count <= Total invariant.

Testing

New pinning test in common/monitor/progress_overflow_test.go:

func TestProgress_ZeroTotalChildDoesNotExceed100Percent(t *testing.T) {
    _, parent := Start(context.Background(), "parent", 2)
    parent.Add(2)

    ctx := context.WithValue(context.Background(), spanKeyName, parent)
    _, _ = Start(ctx, "child", 0)

    got := parent.Progress()
    if got.Count > got.Total {
        t.Fatalf("Progress.Count (%d) exceeds Progress.Total (%d)", got.Count, got.Total)
    }
}

• Before the clamp, this test fails with Count (3) exceeds Total (2).
• After the clamp, all existing common/monitor tests (including TestProgress/divide_by_zero, TestProgress/child_with_zero_total, TestProgress/all_children_with_zero_total added in Fix divide by zero in progress calculation #1208) still pass.

Scope

Single-hunk fix plus a targeted regression test. No behavioural change for the non-overflowing cases; the clamp only engages on the exact scenario #1208 could not reach.

Fixes #1241

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.09%. Comparing base (d7a249a) to head (7af1a6a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1243      +/-   ##
==========================================
+ Coverage   73.05%   73.09%   +0.03%     
==========================================
  Files          89       89              
  Lines       16651    16653       +2     
==========================================
+ Hits        12165    12173       +8     
+ Misses       3247     3242       -5     
+ Partials     1239     1238       -1     

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