Improve version change logic

epoch/flow_operations.go

@@ -2,6 +2,7 @@ package epoch
 
 import (
 	"fmt"
+	"reflect"
 
 	"github.com/bytedance/sonic/ast"
 )
@@ -261,7 +262,7 @@ func (op *ResponseRemoveFieldIfDefault) ApplyToResponse(node *ast.Node) error {
 	}
 
 	// Compare with default - only remove if they match
-	if fieldValue == op.Default {
+	if reflect.DeepEqual(fieldValue, op.Default) {
 		return DeleteNodeField(node, op.Name)
 	}
 

epoch/middleware.go

@@ -401,8 +401,14 @@ func (vah *VersionAwareHandler) handleWithMigration(c *gin.Context, requestedVer
 		body:           make([]byte, 0),
 		statusCode:     200,
 	}
+	originalWriter := c.Writer
 	c.Writer = responseCapture
 
+	// Ensure the original writer is always restored, even if the handler panics.
+	// Without this, Gin's recovery middleware would operate on the capture writer,
+	// causing broken error responses.
+	defer func() { c.Writer = originalWriter }()
+
 	// 3. Call the handler (which expects head version data)
 	vah.handler(c)
 
@@ -510,7 +516,9 @@ func (vah *VersionAwareHandler) migrateRequest(
 		return fmt.Errorf("failed to get raw JSON from migrated request: %w", err)
 	}
 
-	c.Request.Body = io.NopCloser(bytes.NewReader([]byte(migratedJSON)))
+	migratedBytes := []byte(migratedJSON)
+	c.Request.Body = io.NopCloser(bytes.NewReader(migratedBytes))
+	c.Request.ContentLength = int64(len(migratedBytes))
 
 	return nil
 }

epoch/version_change.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"reflect"
+	"sort"
 	"strings"
 
 	"github.com/bytedance/sonic/ast"
@@ -337,8 +338,22 @@ type MigrationChain struct {
 
 // NewMigrationChain creates a new migration chain with cycle detection
 func NewMigrationChain(changes []*VersionChange) (*MigrationChain, error) {
+	// Sort changes by FromVersion (oldest first) so forward migration can iterate sequentially.
+	// This is required because MigrateRequest relies on sequential iteration through sorted changes.
+	sorted := make([]*VersionChange, len(changes))
+	copy(sorted, changes)
+	sort.Slice(sorted, func(i, j int) bool {
+		// Primary sort: by FromVersion (older first)
+		cmp := sorted[i].FromVersion().Compare(sorted[j].FromVersion())
+		if cmp != 0 {
+			return cmp < 0
+		}
+		// Secondary sort: by ToVersion (older first) for changes with the same FromVersion
+		return sorted[i].ToVersion().Compare(sorted[j].ToVersion()) < 0
+	})
+
 	mc := &MigrationChain{
-		changes: changes,
+		changes: sorted,
 	}
 
 	// Detect cycles in the version graph
@@ -385,36 +400,25 @@ func (mc *MigrationChain) MigrateRequest(ctx context.Context, requestInfo *Reque
 		return nil
 	}
 
-	// Find the starting point in the version chain
-	start := -1
-	for i, change := range mc.changes {
-		if change.FromVersion().Equal(from) || change.FromVersion().IsOlderThan(from) {
-			start = i
-			break
+	// Apply changes in sequence (changes are sorted by FromVersion in NewMigrationChain).
+	// A change is included if:
+	// 1. Its FromVersion >= from (don't apply changes from before our starting version)
+	// 2. Its ToVersion <= targetVersion (don't apply changes past our target)
+	for _, change := range mc.changes {
+		// Skip changes that start before our starting version
+		if change.FromVersion().IsOlderThan(from) {
+			continue
 		}
-	}
 
-	if start == -1 {
-		return fmt.Errorf("no migration path found from version %s to %s (available changes: %d)",
-			from.String(), to.String(), len(mc.changes))
-	}
-
-	// Apply changes in sequence until we reach the target version
-	for i := start; i < len(mc.changes); i++ {
-		change := mc.changes[i]
-
-		// Stop if this change would take us past the target
+		// Stop if this change goes past the target version
 		if change.ToVersion().IsNewerThan(targetVersion) {
 			break
 		}
 
-		// Apply this change if it's part of the migration path
-		if (change.ToVersion().Equal(targetVersion) || change.ToVersion().IsOlderThan(targetVersion)) &&
-			(change.FromVersion().IsOlderThan(targetVersion) || change.FromVersion().Equal(targetVersion)) {
-			if err := change.MigrateRequest(ctx, requestInfo); err != nil {
-				return fmt.Errorf("migration failed at %s->%s: %w",
-					change.FromVersion().String(), change.ToVersion().String(), err)
-			}
+		// Apply this change - it's in the migration path [from, targetVersion]
+		if err := change.MigrateRequest(ctx, requestInfo); err != nil {
+			return fmt.Errorf("migration failed at %s->%s: %w",
+				change.FromVersion().String(), change.ToVersion().String(), err)
 		}
 	}
 
@@ -456,7 +460,9 @@ func (mc *MigrationChain) MigrateResponse(ctx context.Context, responseInfo *Res
 	// We need to apply changes at each step in reverse
 
 	iterationCount := 0
-	maxIterations := 10 // Safety limit
+	// Safety limit derived from the number of changes + 1.
+	// Cycles are already detected at construction time, so this is a defensive safeguard.
+	maxIterations := len(mc.changes) + 1
 
 	for !currentVersion.Equal(to) {
 		iterationCount++
@@ -509,9 +515,34 @@ func (mc *MigrationChain) MigrateResponse(ctx context.Context, responseInfo *Res
 	return nil
 }
 
-// AddChange adds a new version change to the chain
-func (mc *MigrationChain) AddChange(change *VersionChange) {
+// AddChange adds a new version change to the chain.
+// The change is inserted in sorted order (by FromVersion, then ToVersion) and
+// cycle detection is re-run to ensure the chain remains valid.
+func (mc *MigrationChain) AddChange(change *VersionChange) error {
 	mc.changes = append(mc.changes, change)
+
+	// Re-sort to maintain the ordering invariant required by MigrateRequest
+	sort.Slice(mc.changes, func(i, j int) bool {
+		cmp := mc.changes[i].FromVersion().Compare(mc.changes[j].FromVersion())
+		if cmp != 0 {
+			return cmp < 0
+		}
+		return mc.changes[i].ToVersion().Compare(mc.changes[j].ToVersion()) < 0
+	})
+
+	// Re-run cycle detection to ensure the new change doesn't introduce a cycle
+	if err := mc.detectCycles(); err != nil {
+		// Rollback: remove the newly added change
+		for i, c := range mc.changes {
+			if c == change {
+				mc.changes = append(mc.changes[:i], mc.changes[i+1:]...)
+				break
+			}
+		}
+		return err
+	}
+
+	return nil
 }
 
 // detectCycles uses depth-first search to find cycles in the version graph