Many changes to the Fuchsia platform involve changing FIDL APIs that 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 that 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 that 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 that 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 | -- | -- | -- | -- | ❌ | -- | ✅ |
alias | type | -- | -- | -- | ⚠️ | ⚠️ | -- | -- |
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.
Examples: adding an event, a method.
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.
Examples: removing an event, a method.
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.
Example: adding a table member.
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.
Example: removing a table member.
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
For strict
unions, care must be taken to transition switches on the union tag.
Example: adding a union variant.
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
For strict
unions, care must be taken to transition switches on the union tag.
Example: removing a union variant.
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.
Example: adding an enum member.
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.
Example: removing an enum member.
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. For strict bits, 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.
Example: adding a bits member.
ABI
It is binary-compatible to remove a bits member. For strict bits, 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.
Example: removing a bits member.
ABI
It is binary-compatible to rename a bits member.
API
It is not source-compatible to rename a bits member.
ABI
It is sometimes binary-compatible to update the value of a const
declaration. If a constant value affects the public interface semantics (for example, by representing a runtime invariant in the interface), changing the constant value is binary-incompatible due to mismatched expectations between peers on different versions.
API
It is usually source-compatible 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 that would fail with the updated value.
ABI
It is ABI compatible to rename a type alias.
API
It is not source-compatible to rename a type alias.
ABI
Typically, it is not ABI compatible to change the underlying type of a type alias. However, if the original underlying type and the replacement underlying type are ABI compatible, then a change is ABI compatible.
API
It is not source-compatible to change the underlying type of a type alias.
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.
strict
to flexible
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).Example: changing a bits declaration from strict
to flexible
.
flexible
to strict
Changing a bits declaration from flexible
to strict
is:
flexible
to strict
will require removing usages of flexible
-only APIs.-Dwarning
or #![deny(warnings)]
.const
in the top level library namespace for strict bits, but a static const
member of the generated class for flexible bits.Example: changing a bits declaration from flexible
to strict
.
strict
to flexible
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.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.
Example: changing an enum declaration from strict
to flexible
.
flexible
to strict
Changing an enum declaration from flexible
to strict
is:
flexible
-only APIs, such as uses of the unknown placeholder, must be removed first.Example: changing an enum declaration from flexible
to strict
.
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
.
Example: changing a union declaration from strict
to flexible
, or flexible
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 RFC-0057 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
@max_bytes
@max_handles
@unknown
For more information on what a constraint is in FIDL, see Type, layout, constraint.
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, 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 that would then be rejected at runtime.
For instance:
vector<T>:128
to vector<T>:256
relaxes a constraints, i.e. move values will be allowed. As a result, readers must be transitioned ahead of writers.handle?
to be required handle
tightens a constraint, optional handles that were accepted before will no longer be. As a result, writers must be transitioned ahead of readers.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 that do not support ADTs like C++.