Adding the skeleton for versioned APIs, both servers and clients

This patch proposes a generic framework to be able to easily define, maintain
and extend versioned API groups going forward.

The whole idea is documented in `api/README.md`; reviewers should read that
file before reviewing the rest of the PR.

Please note that all `*_generated.go` files in this PR are _meant_ to be
auto-generated by a tool based on gengo, but that tool will only be written
after this patch's approach is accepted and this PR merged; in particular
`*_generated.go` files in this PR should be reviewed.

With integration tests with a dummy API group, across 3 versions, with breaking
changes in conversions from one version to the next.

Also some unit tests where relevant.

Signed-off-by: Jean Rouge <[email protected]>

api/README.md

@@ -0,0 +1,204 @@
+# CSI-proxy's API
+
+## Overview
+
+CSI-proxy's API is a GRPC, versioned API.
+
+The server exposes a number of API groups, all independent of each other. Additionally, each API group has one or more versions. Each version in each group listens for GRPC messages on a Windows named pipe of the form `\\.\\pipe\\csi-proxy-<api_group_name>-<version>` (e.g. `\\.\\pipe\\csi-proxy-filesystem-v2alpha1` or `\\.\\pipe\\csi-proxy-iscsi-v1`).
+
+APIs are defined by protobuf files; each API group should live in its own directory under `api/<api_group_name>` in this repo's root (e.g. `api/iscsi`), and then define each of its version in `api/<api_group_name>/<version>/api.proto` files (e.g. `api/iscsi/v1/api.proto`). Each `proto` file should define exactly one RPC service.
+
+Internally, there is only one internal server `struct` per API group, that handles all the versions for that API group. That server is defined in this repo's `internal/server/<api_group_name>` (e.g. `internal/server/iscsi`) go package. This go package should follow the following pattern:
+
+<a name="serverPkgTree"></a>
+```
+internal/server/<api_group_name>
+├── internal
+│   └── types.go
+└── server.go
+```
+where `types.go` should contain the internal types corresponding to the various protobuf types for that API group - these internal structures must be able to represent all the different versions of the API. For example, given a `dummy` API group with two versions defined by the following `proto` files:
+
+`api/dummy/v1alpha1/api.proto`
+```proto
+syntax = "proto3";
+
+package v1alpha1;
+
+service Dummy {
+    // ComputeDouble computes the double of the input. Real smart stuff!
+    rpc ComputeDouble(ComputeDoubleRequest) returns (ComputeDoubleResponse) {}
+}
+
+message ComputeDoubleRequest{
+    int32 input32 = 1;
+}
+
+message ComputeDoubleResponse{
+    int32 response32 = 1;
+}
+```
+
+and
+
+`api/dummy/v1/api.proto`
+```proto
+syntax = "proto3";
+
+package v1;
+
+service Dummy {
+    // ComputeDouble computes the double of the input. Real smart stuff!
+    rpc ComputeDouble(ComputeDoubleRequest) returns (ComputeDoubleResponse) {}
+}
+
+message ComputeDoubleRequest{
+    // we changed in favor of an int64 field here
+    int64 input = 2;
+}
+
+message ComputeDoubleResponse{
+    int64 response = 2;
+    
+    // set to true if the result overflowed
+    bool overflow = 3;
+}
+```
+
+then `internal/server/dummy/internal/types.go` could look something like:
+```go
+type ComputeDoubleRequest struct {
+	Input int64
+}
+
+type ComputeDoubleResponse struct {
+	Response int64
+	Overflow bool
+}
+```
+and then the API group's server (`internal/server/dummy/server.go`) needs to define the callbacks to handle requests for all API versions, e.g.:
+```go
+type Server struct{}
+
+func (s *Server) ComputeDouble(ctx context.Context, request *internal.ComputeDoubleRequest, version apiversion.Version) (*internal.ComputeDoubleResponse, error) {
+	in := request.Input64
+	out := 2 * in
+
+	response := &internal.ComputeDoubleResponse{}
+
+	if sign(in) != sign(out) {
+		// overflow
+		response.Overflow = true
+	} else {
+		response.Response = out
+	}
+
+	return response, nil
+}
+
+func sign(x int64) int {
+	switch {
+	case x > 0:
+		return 1
+	case x < 0:
+		return -1
+	default:
+		return 0
+	}
+}
+```
+All the boilerplate code to:
+ * add a named pipe to the server for each version of the API group, listening for its version's requests, and replying with its version's responses
+ * convert versioned requests to internal representations
+ * and then internal responses back to versioned responses
+ * create clients to talk to that API group & versioned
+is then generated automatically using [gengo](https://github.com/kubernetes/gengo).
+
+The only caveat is that when conversions cannot be made trivially (e.g. when fields from internal and versioned `struct`s have different types), API devs need to define conversion functions. They can do that by creating an (otherwise optional) `internal/server/<api_group_name>/internal/<version>/conversion.go` file, containing functions of the form `func convert_pb_<Type>_To_internal_<Type>(in *pb.<Type>, out *internal.<Type>) error` or `func convert_internal_<Type>_To_pb_<Type>(in *internal.<Type>, out *pb.<Type>) error`; for example, in our `dummy` example above, we need to define a conversion function to account for the different fields in requests and responses from `v1alpha1` to `v1`; so `internal/server/dummy/internal/v1alpha1/conversion.go` could look like:
+```go
+func convert_pb_ComputeDoubleRequest_To_internal_ComputeDoubleRequest(in *pb.ComputeDoubleRequest, out *internal.ComputeDoubleRequest) error {
+	out.Input64 = int64(in.Input32)
+	return nil
+}
+
+func convert_internal_ComputeDoubleResponse_To_pb_ComputeDoubleResponse(in *internal.ComputeDoubleResponse, out *pb.ComputeDoubleResponse) error {
+	i := in.Response
+	if i > math.MaxInt32 || i < math.MinInt64 {
+		// overflow
+		out.Overflow = true
+	} else {
+     		out.Response32 = int32(i)
+    }
+	return nil
+}
+```
+
+## How to change the API
+
+### How to add a new API group
+
+Simply create a new `api/<api_group_name>/<version>/api.proto` file, defining your new service; then generate the Go protobuf code, and run the CSI-proxy generator to generate all the boilerplate code.
+
+FIXME: add more details on which commands to run, and which files to edit when done generating.
+
+### How to make changes to an existing API group & version
+
+Existing APIs changes are only acceptable as long as they are backward compatible; where compatibility is understood here much [as k8s API guidelines define it](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api_changes.md#on-compatibility):
+1. Any API call that succeeded before your change must succeed after your change.
+2. Any API call that does not use your change must behave the same as it did before your change.
+3. Any API call that uses your change must not cause problems (e.g. crash or degrade behavior) when issued against an API servers that do not include your change.
+4. Existing clients need not be aware of your change in order for them to continue to function as they did previously, even when your change is in use.
+
+The main difference with the k8s API's definition of compatibility here is that it is not required to be able round-trip a change (convert to different API versions and back) with no loss of information, since contrary to k8s' API we don't need to store API objects here.
+
+As long as a change abides by the compatibility rules outlined above, one can simply amend the `proto` file defining that API version, and then regenerate all the Go boilerplate code.
+
+### How to add a new version to an existing API group
+
+Any change that cannot be done in a compatible way requires creating a new API version.
+
+Steps to add a new API version:
+1. define it its own new `api.proto` file
+2. generate the Go protobuf code
+3. update the API group's internal representations (in its `types.go` file) to be able to represent all of the group's version (the new and the old ones)
+3. add any needed conversion functions for all existing versions of this API group to account for the changes made at the previous step
+4. re-generate all of the Go boilerplate code
+5. now you can change the API group's server to add your new feature!
+
+### Detailed breakdown of generated files
+
+This section details how `csi-proxy-gen` works, and what files it generates; `csi-proxy-gen` is built on top of [gengo](https://github.com/kubernetes/gengo), and re-uses part of [k8s' code-generator](https://github.com/kubernetes/code-generator), notably to generate conversion functions.
+
+First, it looks for all API group definitions, which are either subdirectories of `api/`, or any go package that contains a `doc.go` file containing a `// +csi-proxy-gen` comment.
+
+Then for each API group it finds:
+1. it iterates through each version subpackage, and in each looks for the `<ApiGroupName>Server` interface, and compiles the list of callbacks that the group's `Server` needs to implement as well as the list of top-level `struct`s (`*Request`s and `*Response`s)
+2. it looks for an existing `internal/server/<api_group_name>/internal/types.go` file:
+    * if it exists, it checks that it contains all the expected top-level `struct`s from the previous step
+    * if it doesn't exist, _and_ the API group only defines one version, it auto-generates one that simply copies the protobuf `struct`s (from the previous step) - this is meant to make it easy to bootstrap a new API group
+3. it generates the `internal/server/<api_group_name>/internal/types_generated.go` file, using the list of callbacks from the first step above
+4. if `internal/server/<api_group_name>/server.go` doesn't exist, it generates a skeleton for it - this, too, is meant to make it easy to bootstrap new API groups
+5. then for each version of the API:
+    1. it looks for an existing `internal/server/<api_group_name>/internal/<version>/conversion.go`, generates an empty one if it doesn't exist; then looks for existing conversion functions
+    2. it generates missing conversion functions to `internal/server/<api_group_name>/internal/<version>/conversion_generated.go`
+    3. it generates `internal/server/<api_group_name>/internal/<version>/server_generated.go`
+6. it generates `internal/server/<api_group_name>/internal/api_group_generated.go` to list all the versioned servers it's just created
+7. and finally, it generates `client/<api_group_name>/<version>/client_generated.go`
+
+When `csi-proxy-gen` has successfully run to completion, [our example API group's go package from earlier](#serverPkgTree) will look something like:
+```
+internal/server/<api_group_name>
+├── api_group_generated.go
+├── internal
+│   ├── types.go
+│   ├── types_generated.go
+│   ├── v1
+│   │   ├── conversion.go
+│   │   ├── conversion_generated.go
+│   │   └── server_generated.go
+│   └── v1alpha1
+│       ├── conversion.go
+│       ├── conversion_generated.go
+│       └── server_generated.go
+└── server.go
+```

api/errors.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+
+package api;
+
+// CommandError details errors yielded by cmdlet calls.
+message CmdletError {
+    // Name of the cmdlet that errored out.
+    string cmdlet_name = 1;
+
+    // Error code that got returned.
+    uint32 code = 2;
+
+    // Human-readable error message - can be empty.
+    string message = 3;
+}