test: remove external compilation of db cleaner entirely (coder#20240)

fixes coder/internal#1026

Thru a (perhaps too-) clever hack of `init()` functions, I've managed to remove the need to separately compile the cleaner binary. This should fix the flakes we are seeing were the binary compilation takes 10s of seconds on macOS.  The cleaner is encorporated directly into the test binary and we self-exec as the subprocess.

Author: spikecurtis

Makefile

@@ -1020,19 +1020,11 @@ endif
 
 TEST_PACKAGES ?= ./...
 
-warm-go-cache-db-cleaner:
-	# ensure Go's build cache for the cleanercmd is fresh so that tests don't have to build from scratch. This
-	# could take some time and counts against the test's timeout, which can lead to flakes.
-	# c.f. https://github.com/coder/internal/issues/1026
-	mkdir -p build
-	$(GIT_FLAGS) go build -o ./build/cleaner github.com/coder/coder/v2/coderd/database/dbtestutil/cleanercmd
-.PHONY: warm-go-cache-db-cleaner
-
-test: warm-go-cache-db-cleaner
+test:
 	$(GIT_FLAGS) gotestsum --format standard-quiet $(GOTESTSUM_RETRY_FLAGS) --packages="$(TEST_PACKAGES)" -- $(GOTEST_FLAGS)
 .PHONY: test
 
-test-cli: warm-go-cache-db-cleaner
+test-cli:
 	$(MAKE) test TEST_PACKAGES="./cli..."
 .PHONY: test-cli
 

coderd/database/dbtestutil/cleaner.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"os/exec"
 	"os/signal"
 	"time"
 
@@ -21,36 +22,43 @@ const (
 	cleanerRespOK        = "OK"
 	envCleanerParentUUID = "DB_CLEANER_PARENT_UUID"
 	envCleanerDSN        = "DB_CLEANER_DSN"
-)
-
-var (
-	originalWorkingDir   string
-	errGettingWorkingDir error
+	envCleanerMagic      = "DB_CLEANER_MAGIC"
+	envCleanerMagicValue = "XEHdJqWehWek8AaWwopy" // 20 random characters to make this collision resistant
 )
 
 func init() {
-	// We expect our tests to run from somewhere in the project tree where `go run` below in `startCleaner` will
-	// be able to resolve the command package. However, some of the tests modify the working directory during the run.
-	// So, we grab the working directory during package init, before tests are run, and then set that work dir on the
-	// subcommand process before it starts.
-	originalWorkingDir, errGettingWorkingDir = os.Getwd()
+	// We are hijacking the init() function here to do something very non-standard.
+	//
+	// We want to be able to run the cleaner as a subprocess of the test process so that it can outlive the test binary
+	// and still clean up, even if the test process times out or is killed. So, what we do is in startCleaner() below,
+	// which is called in the parent process, we exec our own binary and set a collision-resistant environment variable.
+	// Then here in the init(), which will run before main() and therefore before executing tests, we check for the
+	// environment variable, and if present we know this is the child process and we exec the cleaner. Instead of
+	// returning normally from init() we call os.Exit(). This prevents tests from being re-run in the child process (and
+	// recursion).
+	//
+	// If the magic value is not present, we know we are the parent process and init() returns normally.
+	magicValue := os.Getenv(envCleanerMagic)
+	if magicValue == envCleanerMagicValue {
+		RunCleaner()
+		os.Exit(0)
+	}
 }
 
 // startCleaner starts the cleaner in a subprocess. holdThis is an opaque reference that needs to be kept from being
 // garbage collected until we are done with all test databases (e.g. the end of the process).
-func startCleaner(ctx context.Context, t TBSubset, parentUUID uuid.UUID, dsn string) (holdThis any, err error) {
-	cmd := cleanerCmd(t)
+func startCleaner(ctx context.Context, _ TBSubset, parentUUID uuid.UUID, dsn string) (holdThis any, err error) {
+	bin, err := os.Executable()
+	if err != nil {
+		return nil, xerrors.Errorf("could not get executable path: %w", err)
+	}
+	cmd := exec.Command(bin)
 	cmd.Env = append(os.Environ(),
 		fmt.Sprintf("%s=%s", envCleanerParentUUID, parentUUID.String()),
 		fmt.Sprintf("%s=%s", envCleanerDSN, dsn),
+		fmt.Sprintf("%s=%s", envCleanerMagic, envCleanerMagicValue),
 	)
 
-	// c.f. comment on `func init()` in this file.
-	if errGettingWorkingDir != nil {
-		return nil, xerrors.Errorf("failed to get working directory during init: %w", errGettingWorkingDir)
-	}
-	cmd.Dir = originalWorkingDir
-
 	// Here we don't actually use the reference to the stdin pipe, because we never write anything to it. When this
 	// process exits, the pipe is closed by the OS and this triggers the cleaner to do its cleaning work. But, we do
 	// need to hang on to a reference to it so that it doesn't get garbage collected and trigger cleanup early.
@@ -177,8 +185,7 @@ func (c *cleaner) waitAndClean() {
 }
 
 // RunCleaner runs the test database cleaning process. It takes no arguments but uses stdio and environment variables
-// for its operation. It is designed to be launched as the only task of a `main()` process, but is included in this
-// package to share constants with the parent code that launches it above.
+// for its operation.
 //
 // The cleaner is designed to run in a separate process from the main test suite, connected over stdio. If the main test
 // process ends (panics, times out, or is killed) without explicitly discarding the databases it clones, the cleaner