Simplify code generation logic for a faster and better UX (#12493)

The changes here simplify the string of targets executed as part of
`make gen`. The simplifications include:
* parallelised code generation whenever possible
* re-implementation of documentation generation for a significant
  simplification and improved readability.
* unified mocks generation to avoid multiple slow calls to `go run` per
  package.

Note, the changes introduced here are purely mechanical and do not alter
the Lotus node functionality.

Fixes #8392

gen/docs/main.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sync/errgroup"
+
+	"github.com/filecoin-project/lotus/api/docgen"
+	docgen_openrpc "github.com/filecoin-project/lotus/api/docgen-openrpc"
+)
+
+func main() {
+	var lets errgroup.Group
+	lets.SetLimit(1) // TODO: Investigate why this can't run in parallel.
+	lets.Go(generateApiFull)
+	lets.Go(generateApiV0Methods)
+	lets.Go(generateStorage)
+	lets.Go(generateWorker)
+	lets.Go(generateOpenRpcGateway)
+	if err := lets.Wait(); err != nil {
+		fmt.Println("Error:", err)
+		os.Exit(1)
+	}
+	fmt.Println("All documentations generated successfully.")
+}
+
+func generateWorker() error {
+	if ainfo, err := docgen.ParseApiASTInfo("api/api_worker.go", "Worker", "api", "./api"); err != nil {
+		return err
+	} else if err := generateMarkdown("documentation/en/api-v0-methods-worker.md", "Worker", "api", ainfo); err != nil {
+		return err
+	} else {
+		return generateOpenRpc("build/openrpc/worker.json", "Worker", "api", ainfo)
+	}
+}
+
+func generateStorage() error {
+	if ainfo, err := docgen.ParseApiASTInfo("api/api_storage.go", "StorageMiner", "api", "./api"); err != nil {
+		return err
+	} else if err := generateMarkdown("documentation/en/api-v0-methods-miner.md", "StorageMiner", "api", ainfo); err != nil {
+		return err
+	} else {
+		return generateOpenRpc("build/openrpc/miner.json", "StorageMiner", "api", ainfo)
+	}
+}
+
+func generateApiV0Methods() error {
+	if ainfo, err := docgen.ParseApiASTInfo("api/v0api/full.go", "FullNode", "v0api", "./api/v0api"); err != nil {
+		return err
+	} else {
+		return generateMarkdown("documentation/en/api-v0-methods.md", "FullNode", "v0api", ainfo)
+	}
+}
+
+func generateApiFull() error {
+	if ainfo, err := docgen.ParseApiASTInfo("api/api_full.go", "FullNode", "api", "./api"); err != nil {
+		return err
+	} else if err := generateMarkdown("documentation/en/api-v1-unstable-methods.md", "FullNode", "api", ainfo); err != nil {
+		return err
+	} else {
+		return generateOpenRpc("build/openrpc/full.json", "FullNode", "api", ainfo)
+	}
+}
+
+func generateOpenRpcGateway() error {
+	if ainfo, err := docgen.ParseApiASTInfo("api/api_gateway.go", "Gateway", "api", "./api"); err != nil {
+		return err
+	} else {
+		return generateOpenRpc("build/openrpc/gateway.json", "Gateway", "api", ainfo)
+	}
+}
+
+func generateMarkdown(path, iface, pkg string, info docgen.ApiASTInfo) error {
+	out, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		_ = out.Close()
+	}()
+	return docgen.Generate(out, iface, pkg, info)
+}
+
+func generateOpenRpc(path, iface, pkg string, info docgen.ApiASTInfo) error {
+	out, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		_ = out.Close()
+	}()
+	return docgen_openrpc.Generate(out, iface, pkg, info, false)
+}