Makefile: Add target for building proto docs

The old `gitaly-proto` published a GitLab Pages site with documentation
for the Gitaly protobufs. This is useful for users outside of the Gitaly
team to better understand RPCs without digging into the repo.

To start migrating this functionality over to the Gitaly repo, add a
target to the Makefile to build HTML documentation for our protos.

Changelog: added

4 files changed, 197 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 14ce614e4..cb8e54140 100644
--- a/Makefile
+++ b/Makefile
@@ -53,6 +53,7 @@ PROTOC_GEN_GO               := ${TOOLS_DIR}/protoc-gen-go
 PROTOC_GEN_GO_GRPC          := ${TOOLS_DIR}/protoc-gen-go-grpc
 PROTOC_GEN_GITALY_LINT      := ${TOOLS_DIR}/protoc-gen-gitaly-lint
 PROTOC_GEN_GITALY_PROTOLIST := ${TOOLS_DIR}/protoc-gen-gitaly-protolist
+PROTOC_GEN_DOC              := ${TOOLS_DIR}/protoc-gen-doc
 GOTESTSUM                   := ${TOOLS_DIR}/gotestsum
 GOCOVER_COBERTURA           := ${TOOLS_DIR}/gocover-cobertura
 DELVE                       := ${TOOLS_DIR}/dlv
@@ -499,6 +500,12 @@ build-proto-gem:
 publish-proto-gem: build-proto-gem
 	${Q}gem push "${BUILD_DIR}/gitaly.gem"
 
+.PHONY: build-proto-docs
+## Build HTML documentation for Gitaly Protobuf definitions.
+build-proto-docs: ${PROTOC} ${PROTOC_GEN_DOC}
+	${Q}rm -rf ${BUILD_DIR}/proto-docs && mkdir -p ${BUILD_DIR}/proto-docs
+	${Q}${PROTOC} -I ${SOURCE_DIR}/proto -I ${PROTOC_INSTALL_DIR}/include --doc_out=${BUILD_DIR}/proto-docs --doc_opt=html,index.html --plugin=protoc-gen-doc=${PROTOC_GEN_DOC} ${SOURCE_DIR}/proto/*.proto
+
 .PHONY: no-changes
 no-changes:
 	${Q}${GIT} diff --exit-code
@@ -687,6 +694,7 @@ ${GOTESTSUM}:         TOOL_PACKAGE = gotest.tools/gotestsum
 ${GO_LICENSES}:       TOOL_PACKAGE = github.com/google/go-licenses
 ${PROTOC_GEN_GO}:     TOOL_PACKAGE = google.golang.org/protobuf/cmd/protoc-gen-go
 ${PROTOC_GEN_GO_GRPC}:TOOL_PACKAGE = google.golang.org/grpc/cmd/protoc-gen-go-grpc
+${PROTOC_GEN_DOC}:    TOOL_PACKAGE = github.com/pseudomuto/protoc-gen-doc/cmd/protoc-gen-doc
 ${DELVE}:             TOOL_PACKAGE = github.com/go-delve/delve/cmd/dlv
 
 ${TEST_REPO}:
diff --git a/tools/protoc-gen-doc/go.mod b/tools/protoc-gen-doc/go.mod
new file mode 100644
index 000000000..d44fff918
--- /dev/null
+++ b/tools/protoc-gen-doc/go.mod
@@ -0,0 +1,22 @@
+module proto_gen_doc
+
+go 1.17
+
+require github.com/pseudomuto/protoc-gen-doc v1.5.1
+
+require (
+	github.com/Masterminds/semver v1.4.2 // indirect
+	github.com/Masterminds/sprig v2.15.0+incompatible // indirect
+	github.com/aokoli/goutils v1.0.1 // indirect
+	github.com/envoyproxy/protoc-gen-validate v0.3.0-java // indirect
+	github.com/gogo/protobuf v1.1.1 // indirect
+	github.com/golang/protobuf v1.5.2 // indirect
+	github.com/google/uuid v1.1.2 // indirect
+	github.com/huandu/xstrings v1.0.0 // indirect
+	github.com/imdario/mergo v0.3.4 // indirect
+	github.com/mwitkow/go-proto-validators v0.0.0-20180403085117-0950a7990007 // indirect
+	github.com/pseudomuto/protokit v0.2.0 // indirect
+	golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9 // indirect
+	google.golang.org/genproto v0.0.0-20210917145530-b395a37504d4 // indirect
+	google.golang.org/protobuf v1.27.1 // indirect
+)
diff --git a/tools/protoc-gen-doc/go.sum b/tools/protoc-gen-doc/go.sum
new file mode 100644
index 000000000..77e6c2f95
--- /dev/null
+++ b/tools/protoc-gen-doc/go.sum
@@ -0,0 +1,162 @@
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diff --git a/tools/protoc-gen-doc/tool.go b/tools/protoc-gen-doc/tool.go
new file mode 100644
index 000000000..926408822
--- /dev/null
+++ b/tools/protoc-gen-doc/tool.go
@@ -0,0 +1,5 @@
+package protoc_gen_doc
+
+import (
+	_ "github.com/pseudomuto/protoc-gen-doc/cmd/protoc-gen-doc"
+)