sleep between cp and exec in test scripts

Every now and then, I get test failures in the goenv test like:

	> [!windows] cp $EXEC_PATH $NAME/garble$exe
	> [!windows] exec $NAME/garble$exe build
	[fork/exec with"double"quotes/garble: text file busy]
	FAIL: testdata/scripts/goenv.txt:21: unexpected command failure

The root cause is https://go.dev/issue/22315, which isn't going to be
fixed anytime soon, as it is a race condition in Linux itself, triggered
by how heavily concurrent Go tends to be.

For now, try to make the race much less likely to happen.

main_test.go

@@ -18,6 +18,7 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/rogpeppe/go-internal/goproxytest"
@@ -116,6 +117,7 @@ func TestScripts(t *testing.T) {
 			return false, fmt.Errorf("unknown condition")
 		},
 		Cmds: map[string]func(ts *testscript.TestScript, neg bool, args []string){
+			"sleep":             sleep,
 			"binsubstr":         binsubstr,
 			"bincmp":            bincmp,
 			"generate-literals": generateLiterals,
@@ -138,6 +140,20 @@ func createFile(ts *testscript.TestScript, path string) *os.File {
 	return file
 }
 
+// sleep is akin to a shell's sleep builtin.
+// Note that tests should almost never use this; it's currently only used to
+// work around a low-level Go syscall race on Linux.
+func sleep(ts *testscript.TestScript, neg bool, args []string) {
+	if len(args) != 1 {
+		ts.Fatalf("usage: sleep duration")
+	}
+	d, err := time.ParseDuration(args[0])
+	if err != nil {
+		ts.Fatalf("%v", err)
+	}
+	time.Sleep(d)
+}
+
 func binsubstr(ts *testscript.TestScript, neg bool, args []string) {
 	if len(args) < 2 {
 		ts.Fatalf("usage: binsubstr file substr...")

testdata/scripts/goenv.txt

@@ -2,6 +2,16 @@
 env TMPDIR=${WORK}/'.temp ''quotes'' and spaces'
 mkdir ${TMPDIR}
 
+# Unfortunately, due to https://go.dev/issue/22315, cp + exec is racy.
+# Since we run multiple test scripts in parallel as goroutines,
+# if one thread performs a cp while another is forking, the other may keep the
+# file open slightly longer than we think, causing the fork to fail due to the
+# file still being open for writing.
+# Until the root problem is fixed, add a sleep to try to make that narrow window
+# of time less likely to cause problems.
+# TODO(mvdan): remove the sleeps once cp + exec isn't racy anymore.
+env CP_EXEC_SLEEP=10ms
+
 # We need to properly quote the path to garble for toolexec.
 # If we don't, characters like spaces or quotes will result in errors.
 # EXEC_PATH is the test binary's os.Executable.
@@ -10,6 +20,7 @@ mkdir ${TMPDIR}
 env NAME='with spaces'
 mkdir $NAME
 cp $EXEC_PATH $NAME/garble$exe
+sleep $CP_EXEC_SLEEP
 exec $NAME/garble$exe build
 
 # Ensure that we cleaned up the temporary files.
@@ -18,6 +29,7 @@ exec $NAME/garble$exe build
 [!windows] env NAME='with"double"quotes'
 [!windows] mkdir $NAME
 [!windows] cp $EXEC_PATH $NAME/garble$exe
+[!windows] sleep $CP_EXEC_SLEEP
 [!windows] exec $NAME/garble$exe build
 
 env NAME='with''single''quotes'
@@ -28,6 +40,7 @@ exec $NAME/garble$exe build
 [!windows] env NAME='with"both''quotes'
 [!windows] mkdir $NAME
 [!windows] cp $EXEC_PATH $NAME/garble$exe
+[!windows] sleep $CP_EXEC_SLEEP
 [!windows] ! exec $NAME/garble$exe build
 [!windows] stderr 'cannot be quoted'