sweet: make tile38 and go-build with -short even shorter

-short is not meant to be used for benchmarking; just for testing and
debugging.

Change tile38 to run with a bare (empty) database, since a substantial
amount of time is spent on loading the database from disk. The lack of a
database also means that the tile38 server uses substantially less
memory, so we can let it run in parallel with more tasks in the
end-to-end test.

Change go-build to not do a first build of the target package during
Sweet's build step: it means the results won't be as useful since
they'll include downloading packages, but in -short mode it doesn't
matter.

Change-Id: I9e3559c4077fff696a374b97f91491783a26e5ac
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/506559
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

sweet/cmd/sweet/integration_test.go

@@ -192,7 +192,7 @@ func TestSweetEndToEnd(t *testing.T) {
 	sema := semaphore.NewWeighted(8)
 	var wg sync.WaitGroup
 	for i, shard := range []shard{
-		{"tile38", 4},
+		{"tile38", 2},
 		{"go-build", 4},
 		{"biogo-igor", 1},
 		{"biogo-krishna", 1},

sweet/harnesses/go-build.go

@@ -109,7 +109,11 @@ func (h GoBuild) Build(pcfg *common.Config, bcfg *common.BuildConfig) error {
 		if err != nil {
 			return err
 		}
-
+		if bcfg.Short {
+			// Short mode isn't intended to produce good benchmark results;
+			// it's meant for testing and debugging. Skip the additional build step.
+			continue
+		}
 		// Build the benchmark once, pulling in any requisite packages.
 		//
 		// Run the go tool with ExecEnv, as that is what we will use

sweet/harnesses/tile38.go

@@ -57,11 +57,17 @@ func (h Tile38) Build(cfg *common.Config, bcfg *common.BuildConfig) error {
 }
 
 func (h Tile38) Run(cfg *common.Config, rcfg *common.RunConfig) error {
-	// Make sure all the data passed to the server is writable.
-	// The server needs to be able to open its persistent storage as read-write.
-	dataPath := filepath.Join(rcfg.AssetsDir, "data")
-	if err := makeWriteable(dataPath); err != nil {
-		return err
+	var dataPath string
+	if rcfg.Short {
+		// Don't load the real data for short mode. It takes a long time.
+		dataPath = filepath.Join(rcfg.TmpDir, "data-empty-fake")
+	} else {
+		// Make sure all the data passed to the server is writable.
+		// The server needs to be able to open its persistent storage as read-write.
+		dataPath = filepath.Join(rcfg.AssetsDir, "data")
+		if err := makeWriteable(dataPath); err != nil {
+			return err
+		}
 	}
 	args := append(rcfg.Args, []string{
 		"-host", "127.0.0.1",