go_marshal

This package implements the go_marshal utility.

Overview

go_marshal is a code generation utility similar to go_stateify for marshalling go data structures to and from memory.

go_marshal attempts to improve on binary.Write and the sentry's binary.Marshal by moving the expensive use of reflection from runtime to compile-time.

go_marshal automatically generates implementations for marshal.Marshallable interface. Data structures that require custom serialization can be accomodated through a manual implementation this interface.

Data structures can be flagged for code generation by adding a struct-level comment // +marshal. For additional details and options, see the documentation for the marshal.Marshallable interface.

Usage

See defs.bzl: a new rule is provided, go_marshal.

Under the hood, the go_marshal rule is used to generate a file that will appear in a Go target; the output file should appear explicitly in a srcs list. For example (note that the above is the preferred method):

load("<PKGPATH>/gvisor/tools/go_marshal:defs.bzl", "go_marshal")

go_marshal(
    name = "foo_abi",
    srcs = ["foo.go"],
    out = "foo_abi.go",
    package = "foo",
)

go_library(
    name = "foo",
    srcs = [
        "foo.go",
        "foo_abi.go",
    ],
    ...
)

As part of the interface generation, go_marshal also generates some tests for sanity checking the struct definitions for potential alignment issues, and a simple round-trip test through Marshal/Unmarshal to verify the implementation. These tests use reflection to verify properties of the ABI struct, and should be considered part of the generated interfaces (but are too expensive to execute at runtime). Ensure these tests run at some point.

Restrictions

Not all valid go type definitions can be used with go_marshal. go_marshal is intended for ABI structs, which have these additional restrictions:

• At the moment, go_marshal only supports struct declarations.

• Structs are marshalled as packed types. This means no implicit padding is inserted between fields shorter than the platform register size. For alignment, manually insert padding fields.

• Structs used with go_marshal must have a compile-time static size. This means no dynamically sizes fields like slices or strings. Use statically sized array (byte arrays for strings) instead.

• No pointers, channel, map or function pointer fields, and no fields that are arrays of these types. These don't make sense in an ABI data structure.

• We could support opaque pointers as uintptr, but this is currently not implemented. Implementing this would require handling the architecture dependent native pointer size.

• Fields must either be a primitive integer type (byte, [u]int{8,16,32,64}), or of a type that implements marshal.Marshallable.

• int and uint fields are not allowed. Use an explicitly-sized numeric type.

• float* fields are currently not supported, but could be if necessary.

Appendix

Working with Non-Packed Structs

ABI structs must generally be packed types, meaning they should have no implicit padding between short fields. However, if a field is tagged marshal:"unaligned", go_marshal will fall back to a safer but slower mechanism to deal with potentially unaligned fields.

Note that the non-packed property is inheritted by any other struct that embeds this struct, since the go_marshal tool currently can't reason about alignments for embedded structs that are not aligned.

Because of this, it's generally best to avoid using marshal:"unaligned" and insert explicit padding fields instead.

Working with dynamically sized structs

While go_marshal seamlessly supports statically sized structs (which most ABI structs are), it can also used for other uses cases where marshalling is required. There is some provision to partially support dynamically sized structs that may not be ABI structs. A user can define a dynamic struct and define SizeBytes(), MarshalBytes(dst) and UnmarshalBytes(src) for it. Then user can then add a comment above the struct like // +marshal dynamic while will make go_marshal autogenerate the remaining methods required to complete the Marshallable interface. This feature is currently only available for structs and can not be used alongside the Slice API.

Modifying the go_marshal Tool

The following are some guidelines for modifying the go_marshal tool:

• The go_marshal tool currently does a single pass over all types requesting code generation, in arbitrary order. This means the generated code can't directly obtain information about embedded marshallable types at compile-time. One way to work around this restriction is to add a new Marshallable interface method providing this piece of information, and calling it from the generated code. Use this sparingly, as we want to rely on compile-time information as much as possible for performance.

• No runtime reflection in the code generated for the marshallable interface. The entire point of the tool is to avoid runtime reflection. The generated tests may use reflection.

Debugging

To enable debugging output from the go-marshal tool, use one of the following options, depending on how go-marshal is being invoked:

• Pass --define gomarshal=verbose to the bazel command. Note that this can generate a lot of output depending on what's being compiled, as this will enable debugging for all packages built by the command.

• Set marshal_debug = True on the top-level go_library BUILD rule.

• Set debug = True on the go_marshal BUILD rule.

• Pass -debug to the go-marshal tool invocation.

If bazel complains about stdout output being too large, set a larger value through --experimental_ui_max_stdouterr_bytes, or -1 for unlimited output.