lib/config, lib/model: Limit concurrent pulls (fixes #5914) (#6290)

Adds a new folder state "Waiting to Sync" in the same vein as the
existing "Waiting to Scan". This vastly improves performances in the
rare cases when there are lots and lots of folders operating.

lib/config/config_test.go

@@ -86,8 +86,13 @@ func TestDefaultValues(t *testing.T) {
 
 func TestDeviceConfig(t *testing.T) {
 	for i := OldestHandledVersion; i <= CurrentVersion; i++ {
+		cfgFile := fmt.Sprintf("testdata/v%d.xml", i)
+		if _, err := os.Stat(cfgFile); os.IsNotExist(err) {
+			continue
+		}
+
 		os.RemoveAll(filepath.Join("testdata", DefaultMarkerName))
-		wr, err := load(fmt.Sprintf("testdata/v%d.xml", i), device1)
+		wr, err := load(cfgFile, device1)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -1127,6 +1132,27 @@ func TestRemoveDeviceWithEmptyID(t *testing.T) {
 	}
 }
 
+func TestMaxConcurrentFolders(t *testing.T) {
+	cases := []struct {
+		input  int
+		output int
+	}{
+		{input: -42, output: 0},
+		{input: -1, output: 0},
+		{input: 0, output: runtime.GOMAXPROCS(-1)},
+		{input: 1, output: 1},
+		{input: 42, output: 42},
+	}
+
+	for _, tc := range cases {
+		opts := OptionsConfiguration{RawMaxFolderConcurrency: tc.input}
+		res := opts.MaxFolderConcurrency()
+		if res != tc.output {
+			t.Errorf("Wrong MaxFolderConcurrency, %d => %d, expected %d", tc.input, res, tc.output)
+		}
+	}
+}
+
 // defaultConfigAsMap returns a valid default config as a JSON-decoded
 // map[string]interface{}. This is useful to override random elements and
 // re-encode into JSON.