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Expand C++ SDK documentation (proxy-wasm#158)
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Add docs/api_overview.md page, move current README.md to
docs/building.md, and add a short top-level README.md that links to both
pages.

Add inline comments to header files to document individual classes and
methods.

Signed-off-by: Michael Warres <[email protected]>
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mpwarres authored Aug 5, 2024
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[![Build Status][build-badge]][build-link]
[![Apache 2.0 License][license-badge]][license-link]

The SDK has dependencies on specific versions of the C++ WebAssembly toolchain Emscripten (https://emscripten.org) and the protobuf library, therefor use of a Docker image is recommended.

## Docker

A Dockerfile for the C++ SDK is provided in Dockerfile-sdk.

It can built in this directory by:

```bash
docker build -t wasmsdk:v2 -f Dockerfile-sdk .
```

The docker image can be used for compiling wasm files.

### Creating a project for use with the Docker build image

Create a directory with your source files and a Makefile:

```makefile
PROXY_WASM_CPP_SDK=/sdk

all: myproject.wasm

include ${PROXY_WASM_CPP_SDK}/Makefile.base_lite
```

Source file (myproject.cc):

```c++
#include <string>
#include <unordered_map>

#include "proxy_wasm_intrinsics.h"

class ExampleContext : public Context {
public:
explicit ExampleContext(uint32_t id, RootContext* root) : Context(id, root) {}

FilterHeadersStatus onRequestHeaders(uint32_t headers, bool end_of_stream) override;
void onDone() override;
};
static RegisterContextFactory register_ExampleContext(CONTEXT_FACTORY(ExampleContext));

FilterHeadersStatus ExampleContext::onRequestHeaders(uint32_t headers, bool end_of_stream) {
logInfo(std::string("onRequestHeaders ") + std::to_string(id()));
auto path = getRequestHeader(":path");
logInfo(std::string("header path ") + std::string(path->view()));
return FilterHeadersStatus::Continue;
}

void ExampleContext::onDone() { logInfo("onDone " + std::to_string(id())); }
```

### Compiling with the Docker build image

Run docker:

```bash
docker run -v $PWD:/work -w /work wasmsdk:v2 /build_wasm.sh
```

### Caching the standard libraries

The first time that emscripten runs it will generate the standard libraries. To cache these in the docker image,
after the first successful compilation (e.g myproject.cc above), commit the image with the standard libraries:

```bash
docker commit `docker ps -l | grep wasmsdk:v2 | awk '{print $1}'` wasmsdk:v2
```

This will save time on subsequent compiles.

### Using the SDK from a newer/specific version of Envoy

To use a newer/specific version of the SDK (e.g. from the version of Enovy you are going to deploy the WebAssembly module to) bind that volume and use it in the Makefile.

Here is an example Makefile referencing the SDK at ../envoy/api/wasm/cpp and mounted as 'sdk' in the /work directory:

```makefile
PROXY_WASM_CPP_SDK=/work/sdk

all: myproject.wasm

include ${PROXY_WASM_CPP_SDK}/Makefile.base_lite
```

Run docker pointing to Envoy sources in a directory parallel (at the same level) as your project directory:

```bash
docker run -v $PWD:/work -v $PWD/../envoy/api/wasm/cpp:/work/sdk -w /work wasmsdk:v2 bash /build_wasm.sh
```

### Using abseil form the image

Abseil (optionally) is built in /root/abseil and can be used. Note that the abseil containers (e.g. absl::flat\_hash\_set) exercise many syscalls which are not supported. Consequantally individual files should be pulled in which are relatively self contained (e.g. strings). Example customized Makefile:

```makefile
PROXY_WASM_CPP_SDK=/sdk
CPP_API:=${PROXY_WASM_CPP_SDK}
CPP_CONTEXT_LIB = ${CPP_API}/proxy_wasm_intrinsics.cc
ABSL = /root/abseil-cpp
ABSL_CPP = ${ABSL}/absl/strings/str_cat.cc ${ABSL}/absl/strings/str_split.cc ${ABSL}/absl/strings/numbers.cc ${ABSL}/absl/strings/ascii.cc

all: plugin.wasm

%.wasm %.wat: %.cc ${CPP_API}/proxy_wasm_intrinsics.h ${CPP_API}/proxy_wasm_enums.h ${CPP_API}/proxy_wasm_externs.h ${CPP_API}/proxy_wasm_api.h ${CPP_API}/proxy_wasm_intrinsics.js ${CPP_CONTEXT_LIB}
ls /root
em++ --no-entry -s EXPORTED_FUNCTIONS=['_malloc'] --std=c++17 -O3 -flto -I${CPP_API} -I${CPP_API}/google/protobuf -I/usr/local/include -I${ABSL} --js-library ${CPP_API}/proxy_wasm_intrinsics.js ${ABSL_CPP} $*.cc ${CPP_API}/proxy_wasm_intrinsics.pb.cc ${CPP_CONTEXT_LIB} ${CPP_API}/libprotobuf.a -o $*.wasm
```

Precompiled abseil libraries are also available, so the above can also be done as:

```makefile
PROXY_WASM_CPP_SDK=/sdk
CPP_API:=${PROXY_WASM_CPP_SDK}
CPP_CONTEXT_LIB = ${CPP_API}/proxy_wasm_intrinsics.cc
ABSL = /root/abseil-cpp
ABSL_LIBS = ${ABSL}/absl/strings/libabsl_strings.a ${ABSL}/absl/strings/libabsl_strings_internal.a ${ABSL}/absl/strings/libabsl_str_format_internal.a

all: plugin.wasm

%.wasm %.wat: %.cc ${CPP_API}/proxy_wasm_intrinsics.h ${CPP_API}/proxy_wasm_enums.h ${CPP_API}/proxy_wasm_externs.h ${CPP_API}/proxy_wasm_api.h ${CPP_API}/proxy_wasm_intrinsics.js ${CPP_CONTEXT_LIB}
ls /root
em++ --no-entry -s EXPORTED_FUNCTIONS=['_malloc'] --std=c++17 -O3 -flto -I${CPP_API} -I${CPP_API}/google/protobuf -I/usr/local/include -I${ABSL} --js-library ${CPP_API}/proxy_wasm_intrinsics.js $*.cc ${CPP_API}/proxy_wasm_intrinsics.pb.cc ${CPP_CONTEXT_LIB} ${CPP_API}/libprotobuf.a ${ABSL_LIBS} -o $*.wasm
```

### Ownership of the resulting .wasm files

The compiled files may be owned by root. To chown them add the follow lines to the Makefile and docker invocation:

```makefile
PROXY_WASM_CPP_SDK=/sdk

all: myproject.wasm
chown ${uid}.${gid} $^

include ${PROXY_WASM_CPP_SDK}/Makefile.base_lite
```

Invocation file (e.g. build.sh):

```bash
#!/bin/bash
docker run -e uid="$(id -u)" -e gid="$(id -g)" -v $PWD:/work -w /work wasmsdk:v2 /build_wasm.sh
```

## Dependencies for building WASM modules:

If you do not wish to use the Docker file, the dependencies can be installed by script (sdk\_container.sh), or by hand.

### protobuf v3.9.1

You must install the version of protobuf on your build system that matches the libprotobuf.a files (without any patches) so that the generated code matches the .a library. Currently this is based on tag v3.9.1 of https://github.com/protocolbuffers/protobuf.

```bash
git clone https://github.com/protocolbuffers/protobuf
cd protobuf
git checkout v3.9.1
git submodule update --init --recursive
./autogen.sh
./configure
make
make check
sudo make install
```

### emscripten

```bash
git clone https://github.com/emscripten-core/emsdk.git
cd emsdk
./emsdk update-tags
./emsdk install 3.1.7
./emsdk activate 3.1.7

source ./emsdk\_env.sh
```

It is possible later versions will work, e.g.

```bash
./emsdk update-tags
./emsdk install latest
./emsdk activate latest
```

However 3.1.7 is known to work.

### Rebuilding the libprotobuf.a files

If want to rebuild the libprotobuf.a files or use a different version see the instructions at https://github.com/kwonoj/protobuf-wasm. Commit 4bba8b2f38b5004f87489642b6ca4525ae72fe7f works for protobuf v3.9.x.

```bash
git clone https://github.com/protocolbuffers/protobuf protobuf-wasm
cd protobuf-wasm
git checkout v3.9.1
git clone https://github.com/kwonoj/protobuf-wasm wasm-patches
cd wasm-patches && git checkout 4bba8b2f38b5004f87489642b6ca4525ae72fe7f && cd ..
git apply wasm-patches/*.patch
./autogen.sh
emconfigure ./configure --disable-shared CXXFLAGS="-O3 -flto"
emmake make
cd ..
cp protobuf-wasm/src/.libs/libprotobuf-lite.a ${CPP_API}/libprotobuf-lite.a
cp protobuf-wasm/src/.libs/libprotobuf.a ${CPP_API}/libprotobuf.a
```

### WAVM binaries

```bash
git clone [email protected]:WAVM/WAVM.git
cd WAVM
cmake "."
make
sudo make install
```

Note: ensure /usr/local/bin is in your path

Proxy-Wasm is a specification and supporting framework for using
[WebAssembly](https://webassembly.org) (Wasm) to extend the functionality of
network proxies. It enables developers to write custom logic (plugins) that are
compiled to Wasm modules and then loaded and executed by the proxy.

Proxy-Wasm consists of multiple parts:

* An [ABI](https://github.com/proxy-wasm/spec) that specifies the low-level
interface between network proxies and Wasm virtual machines that run the
plugins.
* [Host implementations](https://github.com/proxy-wasm/spec#host-environments)
of the ABI, provided by network proxies.
* [Language-specific SDKs](https://github.com/proxy-wasm/spec#sdks) that layer
on top of the ABI, providing a more natural and programmer-friendly API for
invoking and implementing Proxy-Wasm functions and callbacks.

This repository provides the C++ SDK.

## Getting started

* Read the [API overview](docs/api_overview.md) to learn about [Proxy-Wasm
concepts] and how they are represented in the C++ SDK.
* View an [example plugin](example/http_wasm_example.cc).
* Refer to [API documentation](docs/api_overview.md#codemap).
* [Build](docs/building.md) plugin code.

[build-badge]: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/workflows/C++/badge.svg?branch=master
[build-link]: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/actions?query=workflow%3AC%2B%2B+branch%3Amaster
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