Extract aliased project mutation primitive

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d7dc302d-e6f2-41e9-a2c8-ed598de47067

Author: zwick

pkg/github/projects_batch_mutation.go

@@ -0,0 +1,132 @@
+package github
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"reflect"
+	"sync"
+
+	"github.com/shurcooL/githubv4"
+)
+
+const batchMutationWireChunkSize = 20
+
+type batchMutationKind int
+
+const (
+	batchMutationUpdate batchMutationKind = iota
+	batchMutationClear
+)
+
+func (k batchMutationKind) fieldName() string {
+	if k == batchMutationClear {
+		return "clearProjectV2ItemFieldValue"
+	}
+	return "updateProjectV2ItemFieldValue"
+}
+
+type projectV2ItemMutationResult struct {
+	ProjectV2Item struct {
+		ID             string
+		FullDatabaseID string `graphql:"fullDatabaseId"`
+	} `graphql:"projectV2Item"`
+}
+
+type reflectedMutationTypeKey struct {
+	kind batchMutationKind
+	size int
+}
+
+var reflectedMutationTypeCache sync.Map
+
+// Reflected types are cached only by operation and chunk size to bound
+// reflect.StructOf's runtime cache; positional names and tags keep request data
+// out of type identities. The pinned Client.Mutate binds its third argument to
+// $input, so item0 uses $input and later aliases use $input1, $input2, ...
+// supplied through the variables map.
+func buildAliasedMutationType(kind batchMutationKind, size int) reflect.Type {
+	key := reflectedMutationTypeKey{kind: kind, size: size}
+	if cached, ok := reflectedMutationTypeCache.Load(key); ok {
+		return cached.(reflect.Type)
+	}
+
+	resultType := reflect.TypeFor[projectV2ItemMutationResult]()
+	fields := make([]reflect.StructField, size)
+	for i := range size {
+		varName := "input"
+		if i > 0 {
+			varName = fmt.Sprintf("input%d", i)
+		}
+		fields[i] = reflect.StructField{
+			Name: fmt.Sprintf("Item%d", i),
+			Type: resultType,
+			Tag:  reflect.StructTag(fmt.Sprintf(`graphql:"item%d: %s(input: $%s)"`, i, kind.fieldName(), varName)),
+		}
+	}
+
+	t := reflect.StructOf(fields)
+	actual, _ := reflectedMutationTypeCache.LoadOrStore(key, t)
+	return actual.(reflect.Type)
+}
+
+type mutationAliasOutcome struct {
+	// Populated confirms this alias returned a project item, even when the
+	// response also contains GraphQL errors.
+	Populated      bool
+	NodeID         string
+	FullDatabaseID string
+}
+
+// The pinned client decodes partial data before returning GraphQL errors but
+// discards errors[].path. Populated aliases confirm writes; unpopulated aliases
+// remain unknown and must not be retried individually.
+func executeAliasedMutation(ctx context.Context, gqlClient *githubv4.Client, kind batchMutationKind, inputs []githubv4.Input) ([]mutationAliasOutcome, error) {
+	if len(inputs) == 0 {
+		return nil, nil
+	}
+	if len(inputs) > batchMutationWireChunkSize {
+		return nil, fmt.Errorf("internal error: chunk of %d exceeds wire chunk size %d", len(inputs), batchMutationWireChunkSize)
+	}
+
+	mutationType := buildAliasedMutationType(kind, len(inputs))
+	mutationPtr := reflect.New(mutationType)
+
+	var variables map[string]any
+	if len(inputs) > 1 {
+		variables = make(map[string]any, len(inputs)-1)
+		for i := 1; i < len(inputs); i++ {
+			variables[fmt.Sprintf("input%d", i)] = inputs[i]
+		}
+	}
+
+	mutateErr := gqlClient.Mutate(ctx, mutationPtr.Interface(), inputs[0], variables)
+
+	outcomes := make([]mutationAliasOutcome, len(inputs))
+	elem := mutationPtr.Elem()
+	for i := range inputs {
+		result, ok := elem.Field(i).Interface().(projectV2ItemMutationResult)
+		if !ok || result.ProjectV2Item.ID == "" {
+			continue
+		}
+		outcomes[i] = mutationAliasOutcome{
+			Populated:      true,
+			NodeID:         result.ProjectV2Item.ID,
+			FullDatabaseID: result.ProjectV2Item.FullDatabaseID,
+		}
+	}
+	return outcomes, mutateErr
+}
+
+// The pinned client's GraphQL response error type is unexported; transport and
+// decoding failures must remain distinguishable.
+func isGraphQLResponseError(err error) bool {
+	for err != nil {
+		errType := reflect.TypeOf(err)
+		if errType.PkgPath() == "github.com/shurcooL/graphql" && errType.Name() == "errors" {
+			return true
+		}
+		err = errors.Unwrap(err)
+	}
+	return false
+}