Test file copying preserves mtime

Adds a test to ensure that a file copied via the AppCache feature preserves the file's original mtime. 


In #302 I determined that as long as the files copied into the cache dir preserve the original mtime then the FIFO cache cleaning logic that we rely on is valid. This test closes out this comment that asserted we need to verify whether or not the buildpack preserved mtime when copying files #302 (comment).


[#close 302]

buildpacks/ruby/Cargo.toml

@@ -33,3 +33,4 @@ toml = "0.8"
 
 [dev-dependencies]
 libcnb-test = "=0.23.0"
+filetime = "0.2.25"

buildpacks/ruby/src/main.rs

@@ -29,6 +29,9 @@ mod user_errors;
 #[cfg(test)]
 use libcnb_test as _;
 
+#[cfg(test)]
+use filetime as _;
+
 use clap as _;
 
 struct RubyBuildpack;

commons/src/cache/app_cache.rs

@@ -448,4 +448,35 @@ mod tests {
         assert!(store.cache.join("lol.txt").exists());
         assert!(!store.path.join("lol.txt").exists());
     }
+
+    #[test]
+    fn mtime_preserved() {
+        let tmpdir = tempfile::tempdir().unwrap();
+        let cache_path = tmpdir.path().join("cache");
+        let app_path = tmpdir.path().join("app");
+
+        fs_err::create_dir_all(&cache_path).unwrap();
+        fs_err::create_dir_all(&app_path).unwrap();
+
+        let store = AppCache {
+            path: app_path.clone(),
+            cache: cache_path,
+            limit: Byte::from_u64(512),
+            keep_path: KeepPath::BuildOnly,
+            cache_state: CacheState::NewEmpty,
+        };
+
+        let mtime = filetime::FileTime::from_unix_time(1000, 0);
+
+        let path = app_path.join("lol.txt");
+        fs_err::write(&path, "hahaha").unwrap();
+        filetime::set_file_mtime(&path, mtime).unwrap();
+
+        store.save().unwrap();
+
+        let metadata = fs_err::metadata(store.cache.join("lol.txt")).unwrap();
+        let actual = filetime::FileTime::from_last_modification_time(&metadata);
+
+        assert_eq!(mtime, actual);
+    }
 }