lib/model: Don't set ignore bit when it's already set

This adds a metric for "committed items" to the database instance that I
use in the test code, and a couple of tests that ensure that scans that
don't change anything also don't commit anything.

There was a case in the scanner where we set the invalid bit on files
that are ignored, even though they were already ignored and had the
invalid bit set. I had assumed this would result in an extra database
commit, but it was in fact filtered out by the Set... Anyway, I think we
can save some work on not pushing that change to the Set at all.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3298

lib/model/model_test.go

@@ -1455,6 +1455,58 @@ func TestIssue3164(t *testing.T) {
 	}
 }
 
+func TestScanNoDatabaseWrite(t *testing.T) {
+	// When scanning, nothing should be committed to database unless
+	// something actually changed.
+
+	db := db.OpenMemory()
+	m := NewModel(defaultConfig, protocol.LocalDeviceID, "device", "syncthing", "dev", db, nil)
+	m.AddFolder(defaultFolderConfig)
+	m.StartFolder("default")
+	m.ServeBackground()
+
+	// Start with no ignores, and restore the previous state when the test completes
+
+	curIgn, _, err := m.GetIgnores("default")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer m.SetIgnores("default", curIgn)
+	m.SetIgnores("default", nil)
+
+	// Scan the folder twice. The second scan should be a no-op database wise
+
+	m.ScanFolder("default")
+	c0 := db.Committed()
+
+	m.ScanFolder("default")
+	c1 := db.Committed()
+
+	if c1 != c0 {
+		t.Errorf("scan should not commit data when nothing changed but %d != %d", c1, c0)
+	}
+
+	// Ignore a file we know exists. It'll be updated in the database.
+
+	m.SetIgnores("default", []string{"foo"})
+
+	m.ScanFolder("default")
+	c2 := db.Committed()
+
+	if c2 <= c1 {
+		t.Errorf("scan should commit data when something got ignored but %d <= %d", c2, c1)
+	}
+
+	// Scan again. Nothing should happen.
+
+	m.ScanFolder("default")
+	c3 := db.Committed()
+
+	if c3 != c2 {
+		t.Errorf("scan should not commit data when nothing changed (with ignores) but %d != %d", c3, c2)
+	}
+}
+
 type fakeAddr struct{}
 
 func (fakeAddr) Network() string {