improve build finalization

- fallback from file rename to copying on EXDEV, the signal that we
  tried to rename a file onto another filesystem mount
- also check `bytes.Equal` and skip the final overwrite if nothing would
  change - this preserves file mtime and therefore http caching

1 files changed, 34 insertions, 0 deletions

diff --git a/main_test.go b/main_test.go
index c4b0ec7..f2aae0a 100644
--- a/main_test.go
+++ b/main_test.go
@@ -361,3 +361,37 @@ func TestSortingFilteringSlicing(t *testing.T) {
 		t.Fatal("slice")
 	}
 }
+
+func TestUnchangedOutputsKeepMtime(t *testing.T) {
+	root := t.TempDir()
+	writeTestFile(t, root, "layout.tmpl", testLayouts())
+	writeTestFile(t, root, "a.md", "# A\n\nbody\n")
+	writeTestFile(t, root, "b.md", "# B\n\nbody\n")
+	if err := build(root); err != nil {
+		t.Fatal(err)
+	}
+	old := time.Now().Add(-time.Hour)
+	for _, name := range []string{"a.html", "b.html"} {
+		if err := os.Chtimes(filepath.Join(root, name), old, old); err != nil {
+			t.Fatal(err)
+		}
+	}
+	writeTestFile(t, root, "b.md", "# B\n\nchanged\n")
+	if err := build(root); err != nil {
+		t.Fatal(err)
+	}
+	unchanged, err := os.Stat(filepath.Join(root, "a.html"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !unchanged.ModTime().Equal(old) {
+		t.Errorf("unchanged output mtime bumped: %v", unchanged.ModTime())
+	}
+	changed, err := os.Stat(filepath.Join(root, "b.html"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if changed.ModTime().Equal(old) {
+		t.Error("changed output mtime not updated")
+	}
+}