cp: make --b=simple protective of source

When `--backup` is supplied, `cp` will take a backup of *destination* before *source* is copied. When `--backup=simple` is supplied, it is possible for the backup path for *destination* to equal the path for *source*, destroying source before the copy is made. This change prevents this by returning an error instead.

This fixes uutils#3629

src/uu/cp/src/cp.rs

@@ -1256,7 +1256,16 @@ fn handle_existing_dest(source: &Path, dest: &Path, options: &Options) -> CopyRe
 
     let backup_path = backup_control::get_backup_path(options.backup, dest, &options.backup_suffix);
     if let Some(backup_path) = backup_path {
-        backup_dest(dest, &backup_path)?;
+        if paths_refer_to_same_file(source, &backup_path)? {
+            return Err(format!(
+                "backing up {} might destroy source;  {} not copied",
+                dest.quote(),
+                source.quote()
+            )
+            .into());
+        } else {
+            backup_dest(dest, &backup_path)?;
+        }
     }
 
     match options.overwrite {

tests/by-util/test_cp.rs

@@ -38,6 +38,8 @@ static TEST_COPY_FROM_FOLDER: &str = "hello_dir_with_file/";
 static TEST_COPY_FROM_FOLDER_FILE: &str = "hello_dir_with_file/hello_world.txt";
 static TEST_COPY_TO_FOLDER_NEW: &str = "hello_dir_new";
 static TEST_COPY_TO_FOLDER_NEW_FILE: &str = "hello_dir_new/hello_world.txt";
+static TEST_PROTECT_BACKUP_SRC: &str = "protected.txt.bak";
+static TEST_PROTECT_BACKUP_DEST: &str = "protected.txt";
 #[cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd"))]
 static TEST_MOUNT_COPY_FROM_FOLDER: &str = "dir_with_mount";
 #[cfg(any(target_os = "linux", target_os = "android", target_os = "freebsd"))]
@@ -558,6 +560,25 @@ fn test_cp_backup_simple() {
     );
 }
 
+#[test]
+fn test_cp_backup_simple_protect_source() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.arg("--backup=simple")
+        .arg("--suffix")
+        .arg(".bak")
+        .arg(TEST_PROTECT_BACKUP_SRC)
+        .arg(TEST_PROTECT_BACKUP_DEST)
+        .fails()
+        .stderr_only(format!(
+            "cp: backing up '{}' might destroy source;  '{}' not copied",
+            TEST_PROTECT_BACKUP_DEST,
+            TEST_PROTECT_BACKUP_SRC,
+        ));
+
+    assert_eq!(at.read(TEST_PROTECT_BACKUP_SRC), "original text\n");
+    assert_eq!(at.read(TEST_PROTECT_BACKUP_DEST), "new text\n");
+}
+
 #[test]
 fn test_cp_backup_never() {
     let (at, mut ucmd) = at_and_ucmd!();

tests/fixtures/cp/protected.txt

@@ -0,0 +1 @@
+new text

tests/fixtures/cp/protected.txt.bak

@@ -0,0 +1 @@
+original text