gopls/internal/regtest: add a flag to profile didChange handling

Using the -cpuprofile testing flag for profiling didChange handling
causes the profile to capture the IWL. Add a new -didchange_cpuprof
flag that instruments just the change handling.

Also fix a check for empty workspace files that was preventing
This inaccurate check was preventing the didChange benchmark from
working.

Change-Id: Ie9f5402960ddccda5d6b9b36ae3c111aa5b51bb8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/333939
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

gopls/internal/regtest/bench/bench_test.go

@@ -7,6 +7,8 @@ package bench
 import (
 	"flag"
 	"fmt"
+	"os"
+	"runtime/pprof"
 	"testing"
 	"time"
 
@@ -131,8 +133,9 @@ func TestBenchmarkSymbols(t *testing.T) {
 }
 
 var (
-	benchDir  = flag.String("didchange_dir", "", "If set, run benchmarks in this dir. Must also set regtest_bench_file.")
-	benchFile = flag.String("didchange_file", "", "The file to modify")
+	benchDir     = flag.String("didchange_dir", "", "If set, run benchmarks in this dir. Must also set regtest_bench_file.")
+	benchFile    = flag.String("didchange_file", "", "The file to modify")
+	benchProfile = flag.String("didchange_cpuprof", "", "file to write cpu profiling data to")
 )
 
 // TestBenchmarkDidChange benchmarks modifications of a single file by making
@@ -162,6 +165,17 @@ func TestBenchmarkDidChange(t *testing.T) {
 		// Insert the text we'll be modifying at the top of the file.
 		env.EditBuffer(*benchFile, fake.Edit{Text: "// __REGTEST_PLACEHOLDER_0__\n"})
 		result := testing.Benchmark(func(b *testing.B) {
+			if *benchProfile != "" {
+				profile, err := os.Create(*benchProfile)
+				if err != nil {
+					t.Fatal(err)
+				}
+				defer profile.Close()
+				if err := pprof.StartCPUProfile(profile); err != nil {
+					t.Fatal(err)
+				}
+				defer pprof.StopCPUProfile()
+			}
 			b.ResetTimer()
 			for i := 0; i < b.N; i++ {
 				env.EditBuffer(*benchFile, fake.Edit{

internal/lsp/fake/sandbox.go

@@ -163,7 +163,7 @@ func UnpackTxt(txt string) map[string][]byte {
 }
 
 func validateConfig(config SandboxConfig) error {
-	if filepath.IsAbs(config.Workdir) && (config.Files != nil || config.InGoPath) {
+	if filepath.IsAbs(config.Workdir) && (len(config.Files) > 0 || config.InGoPath) {
 		return errors.New("absolute Workdir cannot be set in conjunction with Files or InGoPath")
 	}
 	if config.Workdir != "" && config.InGoPath {