🤖 chore: abstract parent lookup pattern in cgroupv2 (#29)

This PR abstracts a repeating parent lookup pattern in cgroupv2 using a
generic helper function. Relates to
#28 (comment)

## Changes

- **Added generic helper** - Handles parent cgroup lookups with
customizable fallback behavior
- **Applied to 5 methods**:
  - `cpuQuota()`
  - `cpuPeriod()` 
  - `memoryMaxBytes()`
  - `memoryCurrentBytes()`
  - `memoryInactiveFileBytes()`

---

🤖 PR was written by Claude Sonnet 4.5 Thinking using [Coder
Mux](https://github.com/coder/cmux) and reviewed by a human 👩

Author: DanielleMaywood

cgroupv2.go

@@ -69,6 +69,25 @@ func newCgroupV2Statter(fs afero.Fs, path string, depth int) (*cgroupV2Statter,
 	}, nil
 }
 
+// getFromParentOrFallback attempts to get a value from the parent cgroup
+// when the current cgroup has no value set. If there is no parent, it
+// calls the fallback function to determine the return value.
+func getFromParentOrFallback[T any](
+	s *cgroupV2Statter,
+	getter func(*cgroupV2Statter) (T, error),
+	fallback func() (T, error),
+) (T, error) {
+	if s.parent != nil {
+		result, err := getter(s.parent)
+		if err != nil {
+			var zero T
+			return zero, xerrors.Errorf("read parent: %w", err)
+		}
+		return result, nil
+	}
+	return fallback()
+}
+
 func (s cgroupV2Statter) cpuUsed() (used float64, err error) {
 	cpuStatPath := filepath.Join(s.path, cgroupV2CPUStat)
 
@@ -94,17 +113,10 @@ func (s cgroupV2Statter) cpuQuota() (float64, error) {
 		}
 
 		// If the value is not a valid integer, assume it is the string
-		// 'max' and that there is no limit set. In this scenario, we call
-		// the parent to find its quota.
-		if s.parent != nil {
-			total, err := s.parent.cpuQuota()
-			if err != nil {
-				return 0, xerrors.Errorf("get parent cpu quota: %w", err)
-			}
-			return total, nil
-		}
-
-		return -1, nil
+		// 'max' and that there is no limit set. Try the parent or return -1.
+		return getFromParentOrFallback(&s, (*cgroupV2Statter).cpuQuota, func() (float64, error) {
+			return -1.0, nil
+		})
 	}
 
 	return float64(quotaUs), nil
@@ -120,18 +132,11 @@ func (s cgroupV2Statter) cpuPeriod() (float64, error) {
 		}
 
 		// If the value is not a valid integer or the cpu.max file does
-		// not exist, we call the parent to find its period. This can happen
-		// in system-level cgroups like init.scope where cpu.max may not exist.
-		if s.parent != nil {
-			period, err := s.parent.cpuPeriod()
-			if err != nil {
-				return 0, xerrors.Errorf("get parent cpu period: %w", err)
-			}
-			return period, nil
-		}
-
-		// No parent and no period found in the cgroup hierarchy.
-		return cgroupV2DefaultPeriodUs, nil
+		// not exist, try the parent or return the default period. This can
+		// happen in system-level cgroups like init.scope where cpu.max may not exist.
+		return getFromParentOrFallback(&s, (*cgroupV2Statter).cpuPeriod, func() (float64, error) {
+			return cgroupV2DefaultPeriodUs, nil
+		})
 	}
 
 	return float64(periodUs), nil
@@ -161,19 +166,11 @@ func (s cgroupV2Statter) memoryMaxBytes() (*float64, error) {
 		}
 
 		// If the value is not a valid integer _or_ the memory max file
-		// does not exist, than we can assume that the limit is 'max'.
-		// If the memory limit is max, and we have a parent, we shall call
-		// the parent to find its maximum memory value.
-		if s.parent != nil {
-			result, err := s.parent.memoryMaxBytes()
-			if err != nil {
-				return nil, xerrors.Errorf("read parent memory max: %w", err)
-			}
-			return result, nil
-		}
-
-		// We have no parent, and no max memory limit, so there is no memory limit.
-		return nil, nil
+		// does not exist, we can assume that the limit is 'max'.
+		// Try the parent or return nil (no limit).
+		return getFromParentOrFallback(&s, (*cgroupV2Statter).memoryMaxBytes, func() (*float64, error) {
+			return nil, nil
+		})
 	}
 
 	return ptr.To(float64(maxUsageBytes)), nil
@@ -188,18 +185,10 @@ func (s cgroupV2Statter) memoryCurrentBytes() (int64, error) {
 			return 0, xerrors.Errorf("read memory current: %w", err)
 		}
 
-		// If the memory current file does not exist, and we have a parent,
-		// we shall call the parent to find its current memory value.
-		if s.parent != nil {
-			result, err := s.parent.memoryCurrentBytes()
-			if err != nil {
-				return 0, xerrors.Errorf("read parent memory current: %w", err)
-			}
-			return result, nil
-		}
-
-		// We have no parent, and no memory current file, so return the error.
-		return 0, xerrors.Errorf("read memory current: %w", err)
+		// If the memory current file does not exist, try the parent or return error.
+		return getFromParentOrFallback(&s, (*cgroupV2Statter).memoryCurrentBytes, func() (int64, error) {
+			return 0, xerrors.Errorf("read memory current: %w", err)
+		})
 	}
 
 	return currUsageBytes, nil
@@ -214,18 +203,10 @@ func (s cgroupV2Statter) memoryInactiveFileBytes() (int64, error) {
 			return 0, xerrors.Errorf("read memory stat inactive_file: %w", err)
 		}
 
-		// If the memory stat file does not exist, and we have a parent,
-		// we shall call the parent to find its inactive_file value.
-		if s.parent != nil {
-			result, err := s.parent.memoryInactiveFileBytes()
-			if err != nil {
-				return 0, xerrors.Errorf("read parent memory stat inactive_file: %w", err)
-			}
-			return result, nil
-		}
-
-		// We have no parent, and no memory stat file, so return the error.
-		return 0, xerrors.Errorf("read memory stat inactive_file: %w", err)
+		// If the memory stat file does not exist, try the parent or return error.
+		return getFromParentOrFallback(&s, (*cgroupV2Statter).memoryInactiveFileBytes, func() (int64, error) {
+			return 0, xerrors.Errorf("read memory stat inactive_file: %w", err)
+		})
 	}
 
 	return inactiveFileBytes, nil