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purego: vendor v0.9.0 with concurrent-FFI race revert #13

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53 changes: 53 additions & 0 deletions purego/FIGMA-FORK.md
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# Vendored `purego` for `figma/statsig-server-core`

This directory vendors [`github.com/ebitengine/purego`](https://github.com/ebitengine/purego)
at upstream tag `v0.9.0`, with **one local patch** applied to work around a
concurrent-FFI return-value race in `func.go` and `syscall_sysv.go`.

## The patch

Reverts the package-wide `sync.Pool` of `*syscall15Args` (introduced by
upstream PR [#282](https://github.com/ebitengine/purego/pull/282) and
reaffirmed in [#328](https://github.com/ebitengine/purego/pull/328))
back to a per-call stack-allocated `syscall15Args`. The pool optimization
saved an allocation per call but introduced a race that lets two
concurrently-dispatching goroutines observe each other's return values —
which surfaces in `statsig-go` consumers as SIGSEGVs in
`runtime.memmove`, glibc `double free or corruption`, nil-derefs at
`*gateJson`, and silently-swapped feature-flag results.

The patched lines are marked with `FIGMA PATCH` comments in both
`func.go` (the `RegisterFunc` hot path) and `syscall_sysv.go` (the
`SyscallN` path). Diff against upstream `v0.9.0` is ~8 lines net.

## Why vendored and not a separate fork

Lives here so the existing `figma/statsig-server-core` release workflow
can cut a paired `purego/vX.Y-figmaN` tag alongside `statsig-go/vX.Y-figmaN`
and `binaries-linux-gnu/vX.Y-figmaN`. No new repo to track or sync.

## Consumer wiring

The Go module path is unchanged (`github.com/ebitengine/purego`) so that
internal imports continue to resolve. Consumers redirect via `go.mod`
`replace`:

```go
require github.com/ebitengine/purego v0.9.0

replace github.com/ebitengine/purego =>
github.com/figma/statsig-server-core/purego v0.9.0-figma1
```

## Removing this patch

When upstream lands a fix for the underlying race, drop this directory
and the corresponding `replace` line in any consumer's `go.mod`. The
patch is intentionally minimal (only `thePool.Get`/`Put` reverted in two
files) to make that a one-line revert.

## Upstream tracking

See the open issue in `ebitengine/purego` for the discrimination matrix,
minimal repro, and proposed upstream fix. The repro is ~10 lines of C +
~80 lines of Go, completely independent of statsig.
201 changes: 201 additions & 0 deletions purego/LICENSE
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113 changes: 113 additions & 0 deletions purego/README.md
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# purego
[![Go Reference](https://pkg.go.dev/badge/github.com/ebitengine/purego?GOOS=darwin.svg)](https://pkg.go.dev/github.com/ebitengine/purego?GOOS=darwin)

A library for calling C functions from Go without Cgo.

> This is beta software so expect bugs and potentially API breaking changes
> but each release will be tagged to avoid breaking people's code.
> Bug reports are encouraged.

## Motivation

The [Ebitengine](https://github.com/hajimehoshi/ebiten) game engine was ported to use only Go on Windows. This enabled
cross-compiling to Windows from any other operating system simply by setting `GOOS=windows`. The purego project was
born to bring that same vision to the other platforms supported by Ebitengine.

## Benefits

- **Simple Cross-Compilation**: No C means you can build for other platforms easily without a C compiler.
- **Faster Compilation**: Efficiently cache your entirely Go builds.
- **Smaller Binaries**: Using Cgo generates a C wrapper function for each C function called. Purego doesn't!
- **Dynamic Linking**: Load symbols at runtime and use it as a plugin system.
- **Foreign Function Interface**: Call into other languages that are compiled into shared objects.
- **Cgo Fallback**: Works even with CGO_ENABLED=1 so incremental porting is possible.
This also means unsupported GOARCHs (freebsd/riscv64, linux/mips, etc.) will still work
except for float arguments and return values.

## Supported Platforms

### Tier 1

Tier 1 platforms are the primary targets officially supported by PureGo. When a new version of PureGo is released, any critical bugs found on Tier 1 platforms are treated as release blockers. The release will be postponed until such issues are resolved.

- **Android**: amd64, arm64
- **iOS**: amd64, arm64
- **Linux**: amd64, arm64
- **macOS**: amd64, arm64
- **Windows**: amd64, arm64

### Tier 2

Tier 2 platforms are supported by PureGo on a best-effort basis. Critical bugs on Tier 2 platforms do not block new PureGo releases. However, fixes contributed by external contributors are very welcome and encouraged.

- **Android**: 386, arm
- **FreeBSD**: amd64, arm64
- **Linux**: 386, arm, loong64
- **Windows**: 386*, arm*

`*` These architectures only support `SyscallN` and `NewCallback`

## Example

The example below only showcases purego use for macOS and Linux. The other platforms require special handling which can
be seen in the complete example at [examples/libc](https://github.com/ebitengine/purego/tree/main/examples/libc) which supports FreeBSD and Windows.

```go
package main

import (
"fmt"
"runtime"

"github.com/ebitengine/purego"
)

func getSystemLibrary() string {
switch runtime.GOOS {
case "darwin":
return "/usr/lib/libSystem.B.dylib"
case "linux":
return "libc.so.6"
default:
panic(fmt.Errorf("GOOS=%s is not supported", runtime.GOOS))
}
}

func main() {
libc, err := purego.Dlopen(getSystemLibrary(), purego.RTLD_NOW|purego.RTLD_GLOBAL)
if err != nil {
panic(err)
}
var puts func(string)
purego.RegisterLibFunc(&puts, libc, "puts")
puts("Calling C from Go without Cgo!")
}
```

Then to run: `CGO_ENABLED=0 go run main.go`

## Questions

If you have questions about how to incorporate purego in your project or want to discuss
how it works join the [Discord](https://discord.gg/HzGZVD6BkY)!

### External Code

Purego uses code that originates from the Go runtime. These files are under the BSD-3
License that can be found [in the Go Source](https://github.com/golang/go/blob/master/LICENSE).
This is a list of the copied files:

* `abi_*.h` from package `runtime/cgo`
* `wincallback.go` from package `runtime`
* `zcallback_darwin_*.s` from package `runtime`
* `internal/fakecgo/abi_*.h` from package `runtime/cgo`
* `internal/fakecgo/asm_GOARCH.s` from package `runtime/cgo`
* `internal/fakecgo/callbacks.go` from package `runtime/cgo`
* `internal/fakecgo/go_GOOS_GOARCH.go` from package `runtime/cgo`
* `internal/fakecgo/iscgo.go` from package `runtime/cgo`
* `internal/fakecgo/setenv.go` from package `runtime/cgo`
* `internal/fakecgo/freebsd.go` from package `runtime/cgo`
* `internal/fakecgo/netbsd.go` from package `runtime/cgo`

The files `abi_*.h` and `internal/fakecgo/abi_*.h` are the same because Bazel does not support cross-package use of
`#include` so we need each one once per package. (cf. [issue](https://github.com/bazelbuild/rules_go/issues/3636))
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