1 files changed, 50 insertions, 4 deletions

diff --git a/main.go b/main.go
index ec081ba..c7fb8ce 100644
--- a/main.go
+++ b/main.go
@@ -17,6 +17,7 @@ import (
 	"slices"
 	"sort"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/yuin/goldmark"
@@ -707,7 +708,7 @@ func (s *site) write(outputs map[string][]byte, manageGitignore bool) error {
 			from := filepath.Join(backup, filepath.FromSlash(movedOld[i]))
 			to := filepath.Join(s.root, filepath.FromSlash(movedOld[i]))
 			_ = os.MkdirAll(filepath.Dir(to), 0o755)
-			_ = os.Rename(from, to)
+			_ = moveFile(from, to)
 		}
 	}
 	moveOld := func(rel string) error {
@@ -721,7 +722,7 @@ func (s *site) write(outputs map[string][]byte, manageGitignore bool) error {
 		if err := os.MkdirAll(filepath.Dir(to), 0o755); err != nil {
 			return err
 		}
-		if err := os.Rename(from, to); err != nil {
+		if err := moveFile(from, to); err != nil {
 			return err
 		}
 		movedOld = append(movedOld, rel)
@@ -740,16 +741,28 @@ func (s *site) write(outputs map[string][]byte, manageGitignore bool) error {
 		install = append(install, ".gitignore")
 	}
 	for _, rel := range install {
+		to := filepath.Join(s.root, filepath.FromSlash(rel))
+		// Leave byte-identical outputs untouched so their mtimes (and thus
+		// Last-Modified/ETag headers) only change when content does.
+		if existing, err := os.ReadFile(to); err == nil {
+			staged, err := os.ReadFile(filepath.Join(stage, filepath.FromSlash(rel)))
+			if err != nil {
+				rollback()
+				return err
+			}
+			if bytes.Equal(existing, staged) {
+				continue
+			}
+		}
 		if err := moveOld(rel); err != nil {
 			rollback()
 			return err
 		}
-		to := filepath.Join(s.root, filepath.FromSlash(rel))
 		if err := os.MkdirAll(filepath.Dir(to), 0o755); err != nil {
 			rollback()
 			return err
 		}
-		if err := os.Rename(filepath.Join(stage, filepath.FromSlash(rel)), to); err != nil {
+		if err := moveFile(filepath.Join(stage, filepath.FromSlash(rel)), to); err != nil {
 			rollback()
 			return err
 		}
@@ -758,6 +771,39 @@ func (s *site) write(outputs map[string][]byte, manageGitignore bool) error {
 	return nil
 }
 
+// moveFile is os.Rename with a fallback for cross-device moves (EXDEV), which
+// happen when part of the site tree is a symlink onto another filesystem: the
+// contents are staged next to the destination and renamed into place there.
+func moveFile(from, to string) error {
+	err := os.Rename(from, to)
+	if err == nil || !errors.Is(err, syscall.EXDEV) {
+		return err
+	}
+	contents, err := os.ReadFile(from)
+	if err != nil {
+		return err
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(to), ".weft-move-")
+	if err != nil {
+		return err
+	}
+	_, werr := tmp.Write(contents)
+	if cerr := tmp.Close(); werr == nil {
+		werr = cerr
+	}
+	if werr == nil {
+		werr = os.Chmod(tmp.Name(), 0o644)
+	}
+	if werr == nil {
+		werr = os.Rename(tmp.Name(), to)
+	}
+	if werr != nil {
+		_ = os.Remove(tmp.Name())
+		return werr
+	}
+	return os.Remove(from)
+}
+
 func (s *site) gitignore(outputs []string) ([]byte, error) {
 	raw, err := os.ReadFile(filepath.Join(s.root, ".gitignore"))
 	if err != nil && !errors.Is(err, os.ErrNotExist) {