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fix(functional-pragmatist): replace make+len patterns to avoid CodeQL violations #23685

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pelikhan merged 3 commits into from
Mar 31, 2026
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fix(functional-pragmatist): replace make+len patterns to avoid CodeQL violations #23685

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Copilot Mar 31, 2026
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fix(functional-pragmatist): avoid make with len() to prevent CodeQL v…
Copilot Mar 31, 2026
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fix(functional-pragmatist): add explanation for CodeQL violation guid…
Copilot Mar 31, 2026
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25 changes: 12 additions & 13 deletions .github/workflows/functional-pragmatist.md
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Original file line number Diff line number Diff line change
Expand Up @@ -458,17 +458,18 @@ func TestFilter(t *testing.T) {
package fp

// Map transforms each element in a slice
// Note: uses var+append to avoid CodeQL violations from make([]U, len(slice))
func Map[T, U any](slice []T, fn func(T) U) []U {
result := make([]U, len(slice))
for i, v := range slice {
result[i] = fn(v)
var result []U
for _, v := range slice {
result = append(result, fn(v))
Comment on lines +461 to +465

Copilot AI Mar 31, 2026

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The updated Map example uses a nil slice (var result []U) and appends into it. This changes the empty-input behavior vs the previous make([]U, len(slice)): Map([]T{}, ...) will now return nil instead of a non-nil empty slice, which can affect callers (e.g., JSON null vs [], reflect.DeepEqual, or equality assertions). If you want to keep the prior semantics while still avoiding capacity/length derived from the input, consider initializing to an empty (non-nil) slice without using len(slice) for sizing, or explicitly documenting the nil-vs-empty behavior change in the guidance.

This issue also appears in the following locations of the same file:

  • line 470
  • line 1389

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}
return result
}

// Filter returns elements that match the predicate
func Filter[T any](slice []T, fn func(T) bool) []T {
result := make([]T, 0, len(slice))
var result []T
for _, v := range slice {
if fn(v) {
result = append(result, v)
Expand Down Expand Up @@ -507,10 +508,10 @@ for _, name := range names {
filters = append(filters, Filter{Name: name})
}

// After: Immutable initialization
filters := make([]Filter, len(names))
for i, name := range names {
filters[i] = Filter{Name: name}
// After: Immutable initialization using append
var filters []Filter
for _, name := range names {
filters = append(filters, Filter{Name: name})
}
Comment on lines +511 to 515

Copilot AI Mar 31, 2026

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In this “Example transformations” snippet, the “Before” and “After” blocks are now effectively identical (both declare var filters []Filter and then append in a loop), so it no longer demonstrates a transformation. Also, the label “Immutable initialization using append” is misleading because the slice variable is still being mutated. Consider either restoring a contrasting “Before” example, or making the “After” example the functional helper call (and renaming the label to avoid implying immutability).

See below for a potential fix:

// Before: Mutable slice construction with index-based assignment
filters := make([]Filter, len(names))
for i, name := range names {
    filters[i] = Filter{Name: name}
}

// After: Functional slice initialization

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// Or even better if simple:
filters := sliceutil.Map(names, func(name string) Filter {
Expand Down Expand Up @@ -570,7 +571,7 @@ activeItems := sliceutil.Filter(items, func(item Item) bool { return item.Active
activeNames := sliceutil.Map(activeItems, func(item Item) string { return item.Name })

// Or inline if it's clearer:
activeNames := make([]string, 0, len(items))
var activeNames []string
for _, item := range items {
Comment on lines 571 to 575

Copilot AI Mar 31, 2026

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This single Go code block declares activeNames multiple times (var activeNames []string, then later activeNames := ..., then again var activeNames []string). If someone copies this block into a function, it won’t compile due to redeclaration in the same scope. Consider splitting alternatives into separate code blocks or using distinct variable names per alternative (e.g., activeNamesImperative / activeNamesPipeline).

See below for a potential fix:

var activeNamesImperative []string
for _, item := range items {
    if item.Active {
        activeNamesImperative = append(activeNamesImperative, item.Name)
    }
}

// After: Functional pipeline
activeItems := sliceutil.Filter(items, func(item Item) bool { return item.Active })
activeNamesPipeline := sliceutil.Map(activeItems, func(item Item) string { return item.Name })

// Or inline if it's clearer:
var activeNamesInline []string
for _, item := range items {
    if item.Active {
        activeNamesInline = append(activeNamesInline, item.Name)

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if item.Active {
activeNames = append(activeNames, item.Name)
Expand Down Expand Up @@ -1339,7 +1340,7 @@ func NewService(config *Config, cache *Cache) *Service
- Go doesn't have built-in map/filter/reduce - that's okay!
- Inline loops are often clearer than generic helpers
- Use type parameters (generics) for helpers to avoid reflection
- Preallocate slices when size is known: `make([]T, len(input))`
- Avoid `make([]T, len(input))` and `make([]T, 0, len(input))` — use `var result []T` + `append` instead; CodeQL flags these patterns because the slice length/capacity is derived from user-controlled input, which can trigger incorrect memory allocation analysis

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The new guideline text attributes the CodeQL finding to “incorrect memory allocation analysis”, but the underlying concern is typically uncontrolled or potentially large allocations when sizing from untrusted/user-controlled input (DoS risk). Consider rewording to emphasize validating/capping untrusted sizes (and that preallocation is fine for trusted, bounded lengths) rather than a blanket prohibition, so the guidance doesn’t read like it’s just teaching people to evade CodeQL.

Suggested change
- Avoid `make([]T, len(input))` and `make([]T, 0, len(input))` — use `var result []T` + `append` instead; CodeQL flags these patterns because the slice length/capacity is derived from user-controlled input, which can trigger incorrect memory allocation analysis
- When sizing slices or maps from user-controlled or otherwise untrusted input, validate and cap the size before calling `make` to avoid uncontrolled allocations and potential DoS; preallocating with `make([]T, len(input))` or `make([]T, 0, len(input))` is fine when `len(input)` comes from trusted, bounded, or already-validated data

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- Simple for-loops are idiomatic Go - don't force functional style
- Functional options is a well-established Go pattern - use it confidently
- Pure functions align well with Go's simplicity philosophy
Expand Down Expand Up @@ -1387,9 +1388,7 @@ func NewConfig(host string, port int) (*Config, error) {
```go
// Return copy to prevent mutation of internal state
func (s *Service) GetItems() []Item {
result := make([]Item, len(s.items))
copy(result, s.items)
return result
return slices.Clone(s.items)
}
```

Expand Down
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