bindings: add crate and Go module with Service inter-op mechanism

A Service looks like a bidirectional channel, but is one which is
synchronously polled to send and receive framed message code and data
payloads. It's intended as a fundamental building block for building
bidirectional in-process streaming services that interoperate between
Go and Rust -- an interface which is a heck of a lot faster than gRPC.

It achieves zero-copy semantics by pushing Go []byte buffers down
into CGO invocations, and surfacing Rust-owned []byte arenas and
Frame payloads. There are some rules callers need to adhere to,
such as not scribbling "sent" memory until Poll() is called, and
not referencing Rust-owned memory returned by Poll() after _another_
Poll() call is made.

It amortizes the CGO overhead by vectorizing dispatch. On the Rust
side, some common functions are provided to monomorphize appropriate
bindings for an implementation of a Service trait.

Lies, damn lies, and benchmarks:

This shows a variety of stride patterns (number of sent messages
per Poll() call), some of which are deliberately worst case.

It's comparing the actual "upper case" service with a no-op service
that has the same setup & moving parts, but avoids the actual CGO
invocation itself.

Comparative benchmarks with equivalent "naive" CGO and "pure" Go
Services are included. These are also zero-copy / return owned
memory.

    $ go test ./go/bindings/ -bench='.' -benchtime 3s
    goos: linux
    goarch: amd64
    pkg: github.com/estuary/flow/go/bindings
    BenchmarkUpperService/cgo-1-24          43781984                79.1 ns/op
    BenchmarkUpperService/noop-1-24         270172464               13.3 ns/op
    BenchmarkUpperService/cgo-3-24          48714016                72.2 ns/op
    BenchmarkUpperService/noop-3-24         368625391                9.70 ns/op
    BenchmarkUpperService/cgo-4-24          91550871                36.8 ns/op
    BenchmarkUpperService/noop-4-24         363729646               10.1 ns/op
    BenchmarkUpperService/cgo-11-24         77817141                45.2 ns/op
    BenchmarkUpperService/noop-11-24        369495175               9.76 ns/op
    BenchmarkUpperService/cgo-15-24         83527456                42.2 ns/op
    BenchmarkUpperService/noop-15-24        382336641                9.84 ns/op
    BenchmarkUpperService/cgo-17-24         135539218               27.0 ns/op
    BenchmarkUpperService/noop-17-24        423105061                8.66 ns/op
    BenchmarkUpperService/cgo-31-24         100000000               32.6 ns/op
    BenchmarkUpperService/noop-31-24        406740273                9.10 ns/op
    BenchmarkUpperService/cgo-32-24         151226794               23.7 ns/op
    BenchmarkUpperService/noop-32-24        423966495                8.67 ns/op
    BenchmarkUpperService/cgo-63-24         129356660               27.7 ns/op
    BenchmarkUpperService/noop-63-24        426944920                8.82 ns/op
    BenchmarkUpperService/cgo-137-24        150434529               23.9 ns/op
    BenchmarkUpperService/noop-137-24               441497529                8.39 ns/op
    BenchmarkUpperService/cgo-426-24                155104615               23.3 ns/op
    BenchmarkUpperService/noop-426-24               452368488                8.30 ns/op
    BenchmarkUpperServiceNaive-24                   34605169               104 ns/op
    BenchmarkUpperServiceGo-24                      278482276               12.8 ns/op
    PASS
    ok      github.com/estuary/flow/go/bindings     111.130s

crates/bindings/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "bindings"
+version = "0.0.0"
+authors = ["Estuary Technologies, Inc"]
+edition = "2018"
+
+[lib]
+crate_type = ["staticlib"]
+
+[dependencies]
+
+[build-dependencies]
+cbindgen = "*"

crates/bindings/build.rs

@@ -0,0 +1,18 @@
+extern crate cbindgen;
+
+use std::env;
+use std::path::PathBuf;
+
+fn main() {
+    let crate_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
+
+    let config = cbindgen::Config::from_file(PathBuf::from(&crate_dir).join("cbindgen.toml"))
+        .expect("failed to parse cbindgen config");
+
+    cbindgen::Builder::new()
+        .with_crate(crate_dir)
+        .with_config(config)
+        .generate()
+        .expect("Unable to generate bindings")
+        .write_to_file("flow_bindings.h");
+}

crates/bindings/cbindgen.toml

@@ -0,0 +1 @@
+language = "C"

crates/bindings/flow_bindings.h

@@ -0,0 +1,97 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+/**
+ * Opaque pointer for a Service instance in the ABI.
+ */
+typedef struct {
+  uint8_t _private[0];
+} ServiceImpl;
+
+/**
+ * Output frame produced by a Service.
+ */
+typedef struct {
+  /**
+   * Service-defined response code.
+   */
+  uint32_t code;
+  /**
+   * Begin data offset into the Channel arena.
+   */
+  uint32_t begin;
+  /**
+   * End data offset into the Channel arena.
+   */
+  uint32_t end;
+} Out;
+
+/**
+ * Channel is shared between CGO and Rust, and holds details
+ * about the language interconnect.
+ */
+typedef struct {
+  ServiceImpl *svc_impl;
+  uint8_t *arena_ptr;
+  uintptr_t arena_len;
+  uintptr_t arena_cap;
+  Out *out_ptr;
+  uintptr_t out_len;
+  uintptr_t out_cap;
+  uint8_t *err_ptr;
+  uintptr_t err_len;
+  uintptr_t err_cap;
+} Channel;
+
+/**
+ * Input frame produced from CGO, which is a single service invocation.
+ * 16 bytes, or 1/4 of a typical cache line.
+ */
+typedef struct {
+  const uint8_t *data_ptr;
+  uint32_t data_len;
+  uint32_t code;
+} In1;
+
+/**
+ * Four invocations, composed into one struct.
+ * 64 bytes, or one typical cache line.
+ */
+typedef struct {
+  In1 in0;
+  In1 in1;
+  In1 in2;
+  In1 in3;
+} In4;
+
+/**
+ * Sixteen invocations, composed into one struct.
+ * 256 bytes, or four typical cache lines.
+ */
+typedef struct {
+  In4 in0;
+  In4 in1;
+  In4 in2;
+  In4 in3;
+} In16;
+
+Channel *upper_case_create(void);
+
+void upper_case_invoke1(Channel *ch, In1 i);
+
+void upper_case_invoke4(Channel *ch, In4 i);
+
+void upper_case_invoke16(Channel *ch, In16 i);
+
+void upper_case_drop(Channel *ch);
+
+ServiceImpl *create_upper_case_naive(void);
+
+uint32_t upper_case_naive(ServiceImpl *svc,
+                          uint32_t _code,
+                          const uint8_t *in_ptr,
+                          uint32_t in_len,
+                          const uint8_t **out_ptr,
+                          uint32_t *out_len);

crates/bindings/src/lib.rs

@@ -0,0 +1,2 @@
+pub mod service;
+mod upper_case;