fix(pgsql): handle nullability/type/default drift on existing columns

Existing databases that already ran an old migration kept stale NOT
NULL constraints and mismatched column types/defaults, because the
schema writer only emitted idempotent ADD COLUMN IF NOT EXISTS guards
and never altered columns that already existed.

- Emit guarded ALTER COLUMN ... SET/DROP NOT NULL when a column's
  nullability differs from the model.
- Emit guarded ALTER COLUMN ... TYPE, falling back to renaming the old
  column and adding a fresh one when the in-place conversion fails.
- Emit guarded ALTER COLUMN ... SET/DROP DEFAULT for default drift.
- Collapse the previously duplicated plain/guarded templates so
  WriteSchema (full-schema, live-state-checking) and WriteMigration
  (diff-based) share the same guarded SQL templates and Go helpers
  instead of maintaining the logic twice.

pkg/writers/pgsql/templates.go

@@ -89,27 +89,10 @@ type AddColumnData struct {
 	NotNull    bool
 }
 
-// AlterColumnTypeData contains data for alter column type template
-type AlterColumnTypeData struct {
-	SchemaName string
-	TableName  string
-	ColumnName string
-	NewType    string
-	UsingExpr  string
-}
-
-// AlterColumnTypeWithFallbackData contains data for the alter column type
-// template that falls back to renaming the old column and adding a fresh
-// one when the in-place type conversion is not possible.
-type AlterColumnTypeWithFallbackData struct {
-	SchemaName    string
-	TableName     string
-	ColumnName    string
-	NewType       string
-	UsingExpr     string
-	OldColumnName string
-}
-
+// AlterColumnTypeWithCheckData contains data for the guarded alter column
+// type template, which only alters existing columns whose live type
+// differs from the desired one, and falls back to renaming the old column
+// and adding a fresh one when the in-place conversion is not possible.
 type AlterColumnTypeWithCheckData struct {
 	SchemaName      string
 	TableName       string
@@ -119,8 +102,10 @@ type AlterColumnTypeWithCheckData struct {
 	UsingExpr       string
 }
 
-// AlterColumnDefaultData contains data for alter column default template
-type AlterColumnDefaultData struct {
+// AlterColumnDefaultWithCheckData contains data for the guarded alter
+// column default template, which only alters existing columns whose live
+// default differs from the desired one.
+type AlterColumnDefaultWithCheckData struct {
 	SchemaName   string
 	TableName    string
 	ColumnName   string
@@ -128,8 +113,10 @@ type AlterColumnDefaultData struct {
 	DefaultValue string
 }
 
-// AlterColumnNullabilityData contains data for alter column nullability template
-type AlterColumnNullabilityData struct {
+// AlterColumnNullabilityWithCheckData contains data for the guarded alter
+// column nullability template, which only alters existing columns whose
+// live NOT NULL state differs from the desired one.
+type AlterColumnNullabilityWithCheckData struct {
 	SchemaName string
 	TableName  string
 	ColumnName string
@@ -322,27 +309,9 @@ func (te *TemplateExecutor) ExecuteAddColumn(data AddColumnData) (string, error)
 	return buf.String(), nil
 }
 
-// ExecuteAlterColumnType executes the alter column type template
-func (te *TemplateExecutor) ExecuteAlterColumnType(data AlterColumnTypeData) (string, error) {
-	var buf bytes.Buffer
-	err := te.templates.ExecuteTemplate(&buf, "alter_column_type.tmpl", data)
-	if err != nil {
-		return "", fmt.Errorf("failed to execute alter_column_type template: %w", err)
-	}
-	return buf.String(), nil
-}
-
-// ExecuteAlterColumnTypeWithFallback executes the alter column type template
-// that renames the old column and adds a new one when the conversion fails.
-func (te *TemplateExecutor) ExecuteAlterColumnTypeWithFallback(data AlterColumnTypeWithFallbackData) (string, error) {
-	var buf bytes.Buffer
-	err := te.templates.ExecuteTemplate(&buf, "alter_column_type_with_fallback.tmpl", data)
-	if err != nil {
-		return "", fmt.Errorf("failed to execute alter_column_type_with_fallback template: %w", err)
-	}
-	return buf.String(), nil
-}
-
+// ExecuteAlterColumnTypeWithCheck executes the guarded alter column type
+// template shared by the full-schema writer and the diff-based migration
+// writer.
 func (te *TemplateExecutor) ExecuteAlterColumnTypeWithCheck(data AlterColumnTypeWithCheckData) (string, error) {
 	var buf bytes.Buffer
 	err := te.templates.ExecuteTemplate(&buf, "alter_column_type_with_check.tmpl", data)
@@ -352,22 +321,25 @@ func (te *TemplateExecutor) ExecuteAlterColumnTypeWithCheck(data AlterColumnType
 	return buf.String(), nil
 }
 
-// ExecuteAlterColumnDefault executes the alter column default template
-func (te *TemplateExecutor) ExecuteAlterColumnDefault(data AlterColumnDefaultData) (string, error) {
+// ExecuteAlterColumnDefaultWithCheck executes the guarded alter column
+// default template shared by the full-schema writer and the diff-based
+// migration writer.
+func (te *TemplateExecutor) ExecuteAlterColumnDefaultWithCheck(data AlterColumnDefaultWithCheckData) (string, error) {
 	var buf bytes.Buffer
-	err := te.templates.ExecuteTemplate(&buf, "alter_column_default.tmpl", data)
+	err := te.templates.ExecuteTemplate(&buf, "alter_column_default_with_check.tmpl", data)
 	if err != nil {
-		return "", fmt.Errorf("failed to execute alter_column_default template: %w", err)
+		return "", fmt.Errorf("failed to execute alter_column_default_with_check template: %w", err)
 	}
 	return buf.String(), nil
 }
 
-// ExecuteAlterColumnNullability executes the alter column nullability template
-func (te *TemplateExecutor) ExecuteAlterColumnNullability(data AlterColumnNullabilityData) (string, error) {
+// ExecuteAlterColumnNullabilityWithCheck executes the guarded alter column
+// nullability template.
+func (te *TemplateExecutor) ExecuteAlterColumnNullabilityWithCheck(data AlterColumnNullabilityWithCheckData) (string, error) {
 	var buf bytes.Buffer
-	err := te.templates.ExecuteTemplate(&buf, "alter_column_nullability.tmpl", data)
+	err := te.templates.ExecuteTemplate(&buf, "alter_column_nullability_with_check.tmpl", data)
 	if err != nil {
-		return "", fmt.Errorf("failed to execute alter_column_nullability template: %w", err)
+		return "", fmt.Errorf("failed to execute alter_column_nullability_with_check template: %w", err)
 	}
 	return buf.String(), nil
 }