Proposal

.gitignore

@@ -1,4 +1,7 @@
 /target
 **/*.rs.bk
+*.iml
+
+# remove these for production
 Cargo.lock
-*.iml
+*.so

Cargo.toml

@@ -1,9 +1,24 @@
 [package]
-name = "go-cosmwasm"
+name = "go-rust-demo"
 version = "0.1.0"
 authors = ["Ethan Frey <ethanfrey@users.noreply.github.com>"]
 edition = "2018"
+description = "Demo binding a Rust library to a Go application"
+repository = "https://github.com/confio/go-rust-demo"
+license = "Apache-2.0"
+readme = "README.md"
+exclude = [".circleci/*", ".gitignore"]
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+[lib]
+crate-type = ["cdylib"]
 
 [dependencies]
+errno = "0.2"
+
+[build-dependencies]
+cbindgen = { version = "0.9.1" }
+
+[profile.release]
+opt-level = 2
+lto = true
+panic = "unwind"

LICENSE

@@ -0,0 +1,204 @@
+Confio/CosmWasm
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Makefile

@@ -0,0 +1,17 @@
+.PHONY: all build build-rust build-go test
+
+all: build test
+
+build: build-rust build-go
+
+build-rust:
+	cargo build --release
+	cp target/release/libgo_rust_demo.so api
+	# this pulls out ELF symbols, 80% size reduction!
+	strip api/libgo_rust_demo.so
+
+build-go:
+	go build .
+
+test:
+	go test -v ./api

README.md

@@ -0,0 +1,74 @@
+# Go-Rust Demo
+
+This is a simple library showing my learnings in how to combine Go and Rust.
+It assumes there is an existing rust library with the desired functionality,
+and the purpose is to produce a nice Go interface for this.
+
+### Structure
+
+The approach taken is to create one join Go and Rust repo that can then be
+imported by the Go project. On the Rust side, it will produce `extern "C"` bindings
+to the library and compile down to a `cdylib`, that is, you get a `*.so, *.dll, etc`
+that exposes a C API to the Rust code. On the Go side, you use `cgo` to create
+a Go bridge to this code. The full build step
+involves compiling rust -> C library, and linking that library to the Go code.
+For ergonomics of the user, we will include pre-compiled libraries to easily
+link with, and Go developers should just be able to import this directly.
+
+### Test it out
+
+Simply run `make` to compile the rust code, cgo, and then run tests in Go. This assumes
+a recent rust stable and Go 1.12+ installed.
+
+In smaller pieces:
+
+* `make build-rust` - compiles the rust code into a cdylib and copies it into the `./api`
+directory, along with the `cbindgen` auto-generated header file.
+* `make build-go` - compiles all the go code, links in the rust library
+* `make test` - runs the go tests (which also builds the go code in debug mode). 
+These ensure a clean end-to-end handling, passing strings, handling errors, etc.
+
+## Gotchas
+
+Beyond learning the intracacies of both cgo and rust ffi, there are two points
+that required a bit more research. So it is nice to look into how they are solved here.
+I don't pretend that these are perfect solutions, but they cover the cases that
+matter to me. I would love feedback on how to improve as well.
+
+### Passing Strings / Bytes
+
+When there is a large chunk of memory that cannot fit on the stack, passing
+over ffi provides a challenge. The solution in C is usually to pass a desciptor
+to the memory `ptr: *mut u8, len: usize` and then reconstruct a slice inside
+the rust library. Passing out memory involves allocating some memory, creating
+a reference to it and "forgetting" it for Rust's deallocator. Hoping that the
+caller will later make another call to the library to free it.
+
+We define some helpers in `memory.rs` and use a `Buffer` type for passing the pointer
+and size together as an argument or a return value.
+
+### Errors and Panics
+
+We happily use `Result<T, E>` all over the rust code, but that cannot cross the FFI
+boundary. However, C does provide us access to `errno` to signal a numerical error
+code, but not a message. We add some logic to set `errno` upon an error and
+store the error message in a singleton, which can be queried from the called
+by `get_last_error()`. `cgo` provides support in that it will set the `_, err`
+return value if `errno` is set. We can detect this, and then load the custom
+message if we detect something went wrong.
+
+An uncaught panic that hits the FFI boundary may also crash the calling process,
+which is very bad behavior. To avoid this, we must `catch_unwind()` on any code
+that may panic, and then convert it to a normal error that we can report as above.
+Note, that this also requires `panic = "unwind"` to be set in `profile.release` in
+`Cargo.toml`.
+
+You can look at the helpers in `errors.go` and an example using this (with panics)
+in `lib.rs:may_panic`.
+
+## References
+
+This example arose from learnings when producing go bindings to the cosmwasm smart
+contract framework. You can look at [go-cosmwasm](https://github.com/confio/go-cosmwasm)
+to see a full API build using these techniques. I will try to port any future
+learnings to this demo module as well.