bisync: speed up integration tests on non-local remotes

Before this change, the integration tests checked whether each remote could
set modtimes on every `bisync` step (`checkPreReqs`, called from
`runBisync`), not once per remote. On a remote that isn't `IsLocal`, each
check sleeps for 1s twice (the Google Cloud Storage per-object rate limit
workaround from be73a10a9), for each side. That added 2-4s to each of the
~110 `bisync` steps in each test, which on a fast remote was most of the
run: on the integration test server, the `:memory:` tests took about 14
minutes.

After this change, the result of the check is remembered for each remote,
so it runs, and sleeps, once per remote per test run. The `:memory:` tests
now take about 7s locally.

cmd/bisync/bisync_test.go

@@ -19,6 +19,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	mutex "sync" // renamed as "sync" already in use
 	"testing"
 	"time"
 	"unicode/utf8"
@@ -61,6 +62,12 @@ const (
 
 var initDate = time.Date(2000, time.January, 1, 0, 0, 0, 0, bisync.TZ)
 
+// modtimeProbes holds the SetModTime error for each remote already probed by checkPreReqs
+var (
+	modtimeProbesMu mutex.Mutex
+	modtimeProbes   = map[string]error{}
+)
+
 /* Useful Command Shortcuts */
 // go test ./cmd/bisync -remote local -race
 // go test ./cmd/bisync -remote local -golden
@@ -1055,7 +1062,7 @@ func (b *bisyncTest) checkPreReqs(ctx context.Context, opt *bisync.Options) (con
 		b.ignoreModtime = true
 	}
 	// test if modtimes are writeable
-	testSetModtime := func(f fs.Fs) {
+	probeSetModtime := func(f fs.Fs) error {
 		ctx := accounting.WithStatsGroup(ctx, random.String(8)) // keep stats separate
 		in := bytes.NewBufferString("modtime_write_test")
 		objinfo := object.NewStaticObjectInfo("modtime_write_test", initDate, int64(len("modtime_write_test")), true, nil, nil)
@@ -1066,7 +1073,7 @@ func (b *bisyncTest) checkPreReqs(ctx context.Context, opt *bisync.Options) (con
 		}
 		err = obj.SetModTime(ctx, initDate)
 		if err == fs.ErrorCantSetModTime {
-			b.t.Skip("skipping test as at least one remote does not support setting modtime")
+			return err
 		}
 		if err == fs.ErrorCantSetModTimeWithoutDelete { // transfers stats expected to differ on this backend
 			logReplacements = append(logReplacements, `^.*There was nothing to transfer.*$`, dropMe)
@@ -1076,8 +1083,21 @@ func (b *bisyncTest) checkPreReqs(ctx context.Context, opt *bisync.Options) (con
 		if !f.Features().IsLocal {
 			time.Sleep(time.Second) // avoid GoogleCloudStorage Error 429 rateLimitExceeded
 		}
-		err = obj.Remove(ctx)
-		require.NoError(b.t, err)
+		require.NoError(b.t, obj.Remove(ctx))
+		return err
+	}
+	// probing on every bisync step would sleep for minutes on non-local remotes
+	testSetModtime := func(f fs.Fs) {
+		modtimeProbesMu.Lock()
+		defer modtimeProbesMu.Unlock()
+		err, probed := modtimeProbes[f.Name()]
+		if !probed {
+			err = probeSetModtime(f)
+			modtimeProbes[f.Name()] = err
+		}
+		if err == fs.ErrorCantSetModTime {
+			b.t.Skip("skipping test as at least one remote does not support setting modtime")
+		}
 	}
 	if b.testCase != "nomodtime" {
 		testSetModtime(b.fs1)