fix: Only touch include file if contents is changed (#1058)

Most generated files are untouched when the contents doesn't change. Use the same mechanism for include file as well.

Author: caspermeijn

prost-build/src/config.rs

@@ -806,30 +806,22 @@ impl Config {
                 .expect("every module should have a filename");
             let output_path = target.join(file_name);
 
-            let previous_content = fs::read(&output_path);
-
-            if previous_content
-                .map(|previous_content| previous_content == content.as_bytes())
-                .unwrap_or(false)
-            {
-                trace!("unchanged: {:?}", file_name);
-            } else {
-                trace!("writing: {:?}", file_name);
-                fs::write(output_path, content)?;
-            }
+            write_file_if_changed(&output_path, content.as_bytes())?;
         }
 
         if let Some(ref include_file) = self.include_file {
-            trace!("Writing include file: {:?}", target.join(include_file));
-            let mut file = fs::File::create(target.join(include_file))?;
-            self.write_line(&mut file, 0, "// This file is @generated by prost-build.")?;
+            let path = target.join(include_file);
+            trace!("Writing include file: {:?}", path);
+            let mut buffer = Vec::new();
+            self.write_line(&mut buffer, 0, "// This file is @generated by prost-build.")?;
             self.write_includes(
                 modules.keys().collect(),
-                &mut file,
+                &mut buffer,
                 if target_is_env { None } else { Some(&target) },
                 &file_names,
             )?;
-            file.flush()?;
+
+            write_file_if_changed(&path, &buffer)?;
         }
 
         Ok(())
@@ -1072,6 +1064,26 @@ impl Config {
     }
 }
 
+/// Write a slice as the entire contents of a file.
+///
+/// This function will create a file if it does not exist,
+/// and will entirely replace its contents if it does. When
+/// the contents is already correct, it doesn't touch to the file.
+fn write_file_if_changed(path: &Path, content: &[u8]) -> std::io::Result<()> {
+    let previous_content = fs::read(path);
+
+    if previous_content
+        .map(|previous_content| previous_content == content)
+        .unwrap_or(false)
+    {
+        trace!("unchanged: {:?}", path);
+        Ok(())
+    } else {
+        trace!("writing: {:?}", path);
+        fs::write(path, content)
+    }
+}
+
 impl default::Default for Config {
     fn default() -> Config {
         Config {