Many changes to the Fuchsia platform involve changing FIDL APIs which have already been published. Unless managed carefully, these changes risk breaking existing usages. Failed changes manifest in the following ways:
The Fuchsia project requires that changes to published FIDL libraries are both source-compatible and binary-compatible for partners.
Note: Some changes are binary-compatible, yet require a specific transition path to avoid runtime validation issues. Binary-compatibility indicates that two peers have the same understanding of how to read or write the data, though these two peers may disagree on which values are deemed valid. As an example, a uint32 is binary-compatible with a enum : uint32, even though the enum has runtime validation which restricts the domain to only the specific values identified by the enum members.
For the purpose of describing interface compatibility, FIDL libraries are made up of declarations. Each declaration has a name, type, attributes, and members. Once an API is used outside of fuchsia.git the safest assumption is that all changes to it must be both binary-compatible and source-compatible with current clients. This usually means evolving libraries using soft transitions, where the backwards-incompatible portions of a change are left to the end when they will have no impact because all clients have already been migrated. See safely removing members for more information on the most common soft transition pattern.
Note: Source-compatibility guarantees are only guaranteed under “normal” circumstances. It is possible to write code which causes these guarantees to be violated, e.g. static asserts.
Aside from a declaration‘s name and attributes, all changes to its contract are expressed in terms of changes to the declaration’s members. This relationship also means incompatible changes to a declaration become incompatible changes to all the FIDL libraries which depend on that declaration, not just direct consumers of the original library's generated bindings.
All operations are safe to perform if you are certain that all consumers can be migrated atomically, i.e. they are all in the same source repository as the library definition. Otherwise, these operations must be completed as the final stage in a soft transition after all clients have been migrated away.
The table below summarizes various member changes and their respective safety level when some clients cannot be migrated atomically:
| Parent | Change Target | Reorder Lines | Add | Remove | Rename | Change Type | Change Ordinal | (Default) Value |
|---|---|---|---|---|---|---|---|---|
| library | declaration | ✅ | ✅ | ⚠️ | ❌ | ❌ | -- | -- |
| protocol | method | ✅ | ⚠️ | ⚠️ | ⚠️ | ❌ | ❌ | -- |
| method | parameter | ❌ | ❌ | ❌ | ⚠️ | ❌ | -- | -- |
| struct | field | ❌ | ❌ | ❌ | ❌ | ❌ | -- | ✅ |
| table | field | ✅ | ✅ | ✅️ | ⚠️ | ❌ | ❌ | -- |
| union | variant | ✅ | ⚠️ | ⚠️ | ⚠️ | ❌ | ❌ | -- |
| enum | member | ✅ | ⚠️ | ⚠️ | ⚠️ | ❌ | -- | ✅ |
| bits | member | ✅ | ⚠️ | ⚠️ | ⚠️ | ❌ | -- | ✅ |
| const | value | -- | -- | -- | -- | ❌ | -- | ✅ |
| all | attribute | -- | ⚠️ | ⚠️ | -- | -- | -- | -- |
| type | constraint | -- | ⚠️ | ⚠️ | -- | -- | -- | -- |
| decl | modifier | -- | ⚠️ | ⚠️ | -- | -- | -- | -- |
Legend:
ABI: It is binary-compatible to remove a library declaration.
API: Before removing a library declaration, ensure that no uses of this declaration exists.
ABI: It is binary-compatible to add a method to a protocol.
API: To safely add a method to a protocol, mark the new method with [Transitional]. Once all implementations of the new method are in place, you can remove the [Transitional] attribute.
ABI: It is binary-compatible to remove a method from a protocol.
API: To safely remove a method from a protocol, start by marking the method with [Transitional]. Once this has fully propagated, you can remove all implementations of the method, then remove the method from the FIDL protocol.
Note: When using the Rust bindings, you need to manually add catch-all cases (_) to all the match statements rather than rely on the [Transitional] attribute. Read more about how [Transitional] impacts the Rust bindings.
ABI: Method renames can be made safe with use of the [Selector = "..."] attribute.
API: It is not possible to rename a method in a source-compatible way.
ABI: It is binary-compatible to rename a method parameter.
API: Bindings typically rely on positional arguments, such that renaming a method parameter is source-compatible.
ABI: It is binary-compatible to add a table field.
API: It is source-compatible to add a table field.
ABI: It is binary-compatible to remove a table field.
API: There must not be any use of the field to ensure a source-compatible removal.
ABI: It is binary-compatible to rename a table field.
API: It is not source-compatible to rename a table field.
ABI: It is binary-compatible to add a union variant. To ensure the added union variant is not rejected during runtime validation, it must have propagated to readers ahead of it being used by writers.
API: Care must be taken to transition switches on the union tag.
ABI: It is binary-compatible to remove a union variant. To ensure the removed union variant is not rejected during runtime validation, no writer may use the union variant when it is removed.
API: Care must be taken to transition switches on the union tag.
ABI: It is binary-compatible to rename a union variant.
API: It is not source-compatible to rename a union variant.
ABI: It is binary-compatible to add an enum member. To ensure the added enum member is not rejected during runtime validation, it must have propagated to readers ahead of it being used by writers.
API: Care must be taken to transition switches on the enum.
ABI: It is binary-compatible to remove an enum member. To ensure the removed enum member is not rejected during runtime validation, no writer may use the enum member when it is removed.
API: Care must be taken to transition switches on the enum. Ensure that no uses of this enum member exists.
ABI: It is binary-compatible to rename an enum member.
API: It is not source-compatible to rename an enum member.
ABI: It is binary-compatible to add a bits member. To ensure the added bits member is not rejected during runtime validation, it must have propagated to readers ahead of it being used by writers.
API: It is source-compatible to add a bits member.
ABI: It is binary-compatible to remove a bits member. To ensure the removed bits member is not rejected during runtime validation, no writer may use the bits member when it is removed.
API: It is source-compatible to remove a bits member. Ensure that no uses of this bits member exists.
ABI: It is binary-compatible to rename a bits member.
API: It is not source-compatible to rename a bits member.
It is safe to update the value of a const declaration. In rare circumstances, such a change could cause source-compatibility issues if the constant is used in static asserts which would fail with the updated value.
Changing the strictness modifier of an enum, bits, or union declaration is binary-compatible. Changing from flexible to strict may cause runtime validation errors as unknown data for a previously flexible type will start being rejected.
Generally, changing the strictness on a declaration is source-incompatible, but possible to soft transition. Details for each declaration and binding are provided below.
Changing a bits declaration from strict to flexible is:
enum class and flexible bits are generated as a class (which cannot be used as a non-type template parameter).Changing a bits declaration from flexible to strict is:
flexible to strict will require removing usages of flexible-only APIs.-Dwarning or #![deny(warnings)].Changing an enum declaration from strict to flexible is:
match statements must be updated to handle unknown enum values when using a match statement.enum class whereas flexible enums are generated as a class, which cannot be used as a non-type template parameter.const in the top level library namespace for strict bits, but a static const member of the generated class for flexible bits.After changing from strict to flexible, care must be taken to correctly handle any unknown enums.
strict enums that already have a specific member to represent the unknown case can transition to being flexible by using the [Unknown] attribute.
Changing an enum declaration from flexible to strict is:
flexible-only APIs, such as uses of the unknown placeholder, must be removed first.Changing a union declaration from strict to flexible is source-compatible, and changing from flexible to strict is source-incompatible. To perform the latter, any usages flexible-only APIs for the union must be removed before it can be changed to strict.
Adding or removing the resource modifier on a struct, table, or union is binary-compatible. Removing the resource modifier may cause runtime validation errors: flexible types, such as tables and flexible unions, will now fail to decode any unknown data (i.e. unknown variants for flexible unions and unknown fields for tables) that contains handles. Note that this particular scenario does not apply to LLCPP because LLCPP never stores unknown handles.
Adding or removing the resource modifier is not source-compatible. Furthermore, bindings are encouraged to diverge APIs if they can leverage the value type versus resource type distinction for specific benefits in the target language (see FTP-057 for context).
Most soft transitions follow this basic shape:
In a successful soft transition, only the second step is dangerous.
Note: Safely removing methods is more involved, see removing a method from a protocol.
Renaming declarations themselves is a source-incompatible change. Similarly, renaming declaration members (e.g. a struct field) is source-incompatible.
Often, a source-compatible rename is possible following the long process of adding a duplicate member with the desired name, switching all code to shift from the old member to the new member, then deleting the old member. This approach can be quite direct with table fields for instance.
Renames are binary-compatible, except in the case of libraries, protocols, methods and events. See the [Selector] attribute for binary-compatible renames of these.
Removing [Discoverable] is a source-incompatible change. You first need to ensure that there are no references to the generated protocol name before removing this attribute.
Adding or changing [Selector] is a binary-incompatible change on its own, but can be used in the same change as method renames to preserve binary-compatibility.
Removing [Transitional] is a source-incompatible change. You first need to ensure that all implementations of the method are in place.
Adding or changing [Transport] is a source-incompatible and binary-incompatible change.
Changes to the following attributes have no effect on compatibility, although they often accompany other incompatible changes:
[Deprecated] (although it may in the future if/when implemented)[Doc][MaxBytes][MaxHandles]ABI: Relaxing or tightening constraints is binary-compatible. However, when evolving constraints, care must be taken to transition readers or writers to avoid runtime validation issues.
When relaxing a constraint (e.g. changing a vector's maximum allowable size to grow from vector<T>:128 to vector<T>:256), all readers must transition ahead of writers to avoid values being rejected at runtime.
Conversely, when tightening a constraint, all writers must transition ahead of readers to avoid emitting values which would then be rejected at runtime.
API: Relaxing or tightening constraints is source-compatible.
When adding an enum member (or adding a union variant), any switch on the enum (respectively the union tag) must first evolve to handle the soon to be added member (resp. variant). This is done by adding a default case for instance, or a catch-all _ match. Depending on compiler flags, this may require additional attributes such as #[allow(dead_code)].
Similarly, when removing an enum member (or removing a union variant), any switch on the enum (respectively the union tag) must first evolve to replace the soon to be removed member (resp. variant) by a default case.
Note: A union tag is the discriminator indicating which variant is currently held by the union (see lexicon). This is often an enum in languages which do not support ADTs like C++.