cmd/coordinator: find work in dev mode

Start the main findWorkLoop in dev mode, to discover work to run from
the dashboard. In dev mode, was also replace the linux-amd64 builder
config with one using host-linux-amd64-localdev reverse buildlets.

This provides a complete lifecycle to test out builds with a local dev
coordinator and buildlet.

For golang/go#48803

Change-Id: I8ade6c8bccf3bc51437ca9e7d11c232753fe7465
Reviewed-on: https://go-review.googlesource.com/c/build/+/354638
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

cmd/coordinator/README.md

@@ -7,7 +7,7 @@ Building, running tests, running locally is supported on Linux and macOS only.
 Run
 
 ```sh
-go run golang.org/x/build/cmd/coordinator -mode=dev -env=dev
+go run golang.org/x/build/cmd/coordinator -mode=dev
 ```
 
 to start a server on https://localhost:8119. Proceed past the TLS warning and
@@ -15,6 +15,13 @@ you should get the homepage. Some features won't work when running locally,
 but you should be able to navigate between the homepage, the dashboard,
 the builders page, and do limited local development and testing.
 
+To test builds locally, start a `host-linux-amd64-localdev` reverse buildlet,
+which will run `linux-amd64` tests:
+
+```sh
+go run golang.org/x/build/cmd/buildlet -halt=false -reverse-type=host-linux-amd64-localdev
+```
+
 #### Render the "Trybot Status" page locally
 
 To view/modify the "Trybot Status" page locally, you can run the coordinator

cmd/coordinator/coordinator.go

@@ -398,6 +398,16 @@ func main() {
 		defer ec2Pool.Close()
 	}
 
+	if *mode == "dev" {
+		// Replace linux-amd64 with a config using a -localdev reverse
+		// buildlet so it is possible to run local builds by starting a
+		// local reverse buildlet.
+		dashboard.Builders["linux-amd64"] = &dashboard.BuildConfig{
+			Name:       "linux-amd64",
+			HostType:   "host-linux-amd64-localdev",
+		}
+	}
+
 	go updateInstanceRecord()
 
 	switch *mode {
@@ -466,6 +476,7 @@ func main() {
 	if *mode == "dev" {
 		// TODO(crawshaw): do more in dev mode
 		gce.BuildletPool().SetEnabled(*devEnableGCE)
+		go findWorkLoop()
 	} else {
 		go gce.BuildletPool().CleanUpOldVMs()
 		if pool.KubeErr() == nil {