io/fs: do not replace files with unchanged contents

Do not update file timestamps when contents have not actually changed.

src/io/fs.rs

@@ -1,6 +1,6 @@
 use std::{
 	fs::{self, File},
-	io::{BufWriter, Write},
+	io::{BufReader, BufWriter, Read, Write},
 	path::{Path, PathBuf},
 };
 
@@ -41,19 +41,44 @@ where
 	.with_context(|| format!("Failed to write file {}", path.display()))
 }
 
+pub fn equal(path1: &Path, path2: &Path) -> Result<bool> {
+	let mut file1 = BufReader::new(
+		fs::File::open(path1)
+			.with_context(|| format!("Failed to open file {}", path1.display()))?,
+	)
+	.bytes();
+	let mut file2 = BufReader::new(
+		fs::File::open(path2)
+			.with_context(|| format!("Failed to open file {}", path2.display()))?,
+	)
+	.bytes();
+
+	Ok(loop {
+		match (file1.next().transpose()?, file2.next().transpose()?) {
+			(Some(b1), Some(b2)) if b1 == b2 => continue,
+			(None, None) => break true,
+			_ => break false,
+		};
+	})
+}
+
 pub fn create_with_tmpfile<T, F>(path: &Path, f: F) -> Result<T>
 where
 	F: FnOnce(&mut BufWriter<File>) -> Result<T>,
 {
 	let tmp_path = tmpfile_name(path);
+	let mut cleanup = true;
 
 	let ret = (|| {
 		let ret = create(&tmp_path, f)?;
-		rename(&tmp_path, path)?;
+		if !matches!(equal(path, &tmp_path), Result::Ok(true)) {
+			rename(&tmp_path, path)?;
+			cleanup = false;
+		}
 		Ok(ret)
 	})();
 
-	if ret.is_err() {
+	if cleanup {
 		let _ = fs::remove_file(&tmp_path);
 	}