test(tvix/nix-compat): add debug assertions for nar reader r/8007

Adds debug assertions to ensure that the reader's variants are upheld.
If any of the following happens, then the currently in use reader must
be abandoned:
  * A directory or file reader encounters an error
  * A directory or file reader is dropped before being fully read from
Additionally, a directory reader must not be read from again after it
has returned None.
These checks are only used when debug_assertions are on, so vanish in
release mode.

Resolves two TODO items added by edef

Change-Id: I27bd9643a632798db5351957506c166b9bd5ca4e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11508
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: edef <edef@edef.eu>
Autosubmit: Aria Shrimpton <me@aria.rip>
Tested-by: BuildkiteCI

1 files changed, 215 insertions, 57 deletions

diff --git a/tvix/nix-compat/src/nar/reader/test.rs b/tvix/nix-compat/src/nar/reader/test.rs
index fd0d6a9f5afd..02dc4767c916 100644
--- a/tvix/nix-compat/src/nar/reader/test.rs
+++ b/tvix/nix-compat/src/nar/reader/test.rs
@@ -46,75 +46,233 @@ fn complicated() {
     match node {
         nar::reader::Node::Directory(mut dir_reader) => {
             // first entry is .keep, an empty regular file.
-            let entry = dir_reader
-                .next()
-                .expect("next must succeed")
-                .expect("must be some");
-
-            assert_eq!(&b".keep"[..], &entry.name);
-
-            match entry.node {
-                nar::reader::Node::File {
-                    executable,
-                    mut reader,
-                } => {
-                    assert!(!executable);
-                    assert_eq!(reader.read(&mut [0]).unwrap(), 0);
+            must_read_file(
+                ".keep",
+                dir_reader
+                    .next()
+                    .expect("next must succeed")
+                    .expect("must be some"),
+            );
+
+            // second entry is aa, a symlink to /nix/store/somewhereelse
+            must_be_symlink(
+                "aa",
+                "/nix/store/somewhereelse",
+                dir_reader
+                    .next()
+                    .expect("next must be some")
+                    .expect("must be some"),
+            );
+
+            {
+                // third entry is a directory called "keep"
+                let entry = dir_reader
+                    .next()
+                    .expect("next must be some")
+                    .expect("must be some");
+
+                assert_eq!(&b"keep"[..], &entry.name);
+
+                match entry.node {
+                    nar::reader::Node::Directory(mut subdir_reader) => {
+                        {
+                            // first entry is .keep, an empty regular file.
+                            let entry = subdir_reader
+                                .next()
+                                .expect("next must succeed")
+                                .expect("must be some");
+
+                            must_read_file(".keep", entry);
+                        }
+
+                        // we must read the None
+                        assert!(
+                            subdir_reader.next().expect("next must succeed").is_none(),
+                            "keep directory contains only .keep"
+                        );
+                    }
+                    _ => panic!("unexpected type for keep/.keep"),
                 }
-                _ => panic!("unexpected type for .keep"),
-            }
+            };
+
+            // reading more entries yields None (and we actually must read until this)
+            assert!(dir_reader.next().expect("must succeed").is_none());
+        }
+        _ => panic!("unexpected type"),
+    }
+}
+
+#[test]
+#[should_panic]
+fn file_read_abandoned() {
+    let mut f = std::io::Cursor::new(include_bytes!("../tests/complicated.nar"));
+    let node = nar::reader::open(&mut f).unwrap();
+
+    match node {
+        nar::reader::Node::Directory(mut dir_reader) => {
+            // first entry is .keep, an empty regular file.
+            {
+                let entry = dir_reader
+                    .next()
+                    .expect("next must succeed")
+                    .expect("must be some");
+
+                assert_eq!(&b".keep"[..], &entry.name);
+                // don't bother to finish reading it.
+            };
+
+            // this should panic (not return an error), because we are meant to abandon the archive reader now.
+            assert!(dir_reader.next().expect("must succeed").is_none());
+        }
+        _ => panic!("unexpected type"),
+    }
+}
+
+#[test]
+#[should_panic]
+fn dir_read_abandoned() {
+    let mut f = std::io::Cursor::new(include_bytes!("../tests/complicated.nar"));
+    let node = nar::reader::open(&mut f).unwrap();
+
+    match node {
+        nar::reader::Node::Directory(mut dir_reader) => {
+            // first entry is .keep, an empty regular file.
+            must_read_file(
+                ".keep",
+                dir_reader
+                    .next()
+                    .expect("next must succeed")
+                    .expect("must be some"),
+            );
 
             // second entry is aa, a symlink to /nix/store/somewhereelse
-            let entry = dir_reader
-                .next()
-                .expect("next must be some")
-                .expect("must be some");
+            must_be_symlink(
+                "aa",
+                "/nix/store/somewhereelse",
+                dir_reader
+                    .next()
+                    .expect("next must be some")
+                    .expect("must be some"),
+            );
 
-            assert_eq!(&b"aa"[..], &entry.name);
+            {
+                // third entry is a directory called "keep"
+                let entry = dir_reader
+                    .next()
+                    .expect("next must be some")
+                    .expect("must be some");
 
-            match entry.node {
-                nar::reader::Node::Symlink { target } => {
-                    assert_eq!(&b"/nix/store/somewhereelse"[..], &target);
+                assert_eq!(&b"keep"[..], &entry.name);
+
+                match entry.node {
+                    nar::reader::Node::Directory(_) => {
+                        // don't finish using it, which poisons the archive reader
+                    }
+                    _ => panic!("unexpected type for keep/.keep"),
                 }
-                _ => panic!("unexpected type for aa"),
-            }
-
-            // third entry is a directory called "keep"
-            let entry = dir_reader
-                .next()
-                .expect("next must be some")
-                .expect("must be some");
-
-            assert_eq!(&b"keep"[..], &entry.name);
-
-            match entry.node {
-                nar::reader::Node::Directory(mut subdir_reader) => {
-                    // first entry is .keep, an empty regular file.
-                    let entry = subdir_reader
-                        .next()
-                        .expect("next must succeed")
-                        .expect("must be some");
-
-                    // … it contains a single .keep, an empty regular file.
-                    assert_eq!(&b".keep"[..], &entry.name);
-
-                    match entry.node {
-                        nar::reader::Node::File {
-                            executable,
-                            mut reader,
-                        } => {
-                            assert!(!executable);
-                            assert_eq!(reader.read(&mut [0]).unwrap(), 0);
-                        }
-                        _ => panic!("unexpected type for keep/.keep"),
+            };
+
+            // this should panic, because we didn't finish reading the child subdirectory
+            assert!(dir_reader.next().expect("must succeed").is_none());
+        }
+        _ => panic!("unexpected type"),
+    }
+}
+
+#[test]
+#[should_panic]
+fn dir_read_after_none() {
+    let mut f = std::io::Cursor::new(include_bytes!("../tests/complicated.nar"));
+    let node = nar::reader::open(&mut f).unwrap();
+
+    match node {
+        nar::reader::Node::Directory(mut dir_reader) => {
+            // first entry is .keep, an empty regular file.
+            must_read_file(
+                ".keep",
+                dir_reader
+                    .next()
+                    .expect("next must succeed")
+                    .expect("must be some"),
+            );
+
+            // second entry is aa, a symlink to /nix/store/somewhereelse
+            must_be_symlink(
+                "aa",
+                "/nix/store/somewhereelse",
+                dir_reader
+                    .next()
+                    .expect("next must be some")
+                    .expect("must be some"),
+            );
+
+            {
+                // third entry is a directory called "keep"
+                let entry = dir_reader
+                    .next()
+                    .expect("next must be some")
+                    .expect("must be some");
+
+                assert_eq!(&b"keep"[..], &entry.name);
+
+                match entry.node {
+                    nar::reader::Node::Directory(mut subdir_reader) => {
+                        // first entry is .keep, an empty regular file.
+                        must_read_file(
+                            ".keep",
+                            subdir_reader
+                                .next()
+                                .expect("next must succeed")
+                                .expect("must be some"),
+                        );
+
+                        // we must read the None
+                        assert!(
+                            subdir_reader.next().expect("next must succeed").is_none(),
+                            "keep directory contains only .keep"
+                        );
                     }
+                    _ => panic!("unexpected type for keep/.keep"),
                 }
-                _ => panic!("unexpected type for keep/.keep"),
-            }
+            };
 
             // reading more entries yields None (and we actually must read until this)
             assert!(dir_reader.next().expect("must succeed").is_none());
+
+            // this should panic, because we already got a none so we're meant to stop.
+            dir_reader.next().unwrap();
+            unreachable!()
         }
         _ => panic!("unexpected type"),
     }
 }
+
+fn must_read_file(name: &'static str, entry: nar::reader::Entry<'_, '_>) {
+    assert_eq!(name.as_bytes(), &entry.name);
+
+    match entry.node {
+        nar::reader::Node::File {
+            executable,
+            mut reader,
+        } => {
+            assert!(!executable);
+            assert_eq!(reader.read(&mut [0]).unwrap(), 0);
+        }
+        _ => panic!("unexpected type for {}", name),
+    }
+}
+
+fn must_be_symlink(
+    name: &'static str,
+    exp_target: &'static str,
+    entry: nar::reader::Entry<'_, '_>,
+) {
+    assert_eq!(name.as_bytes(), &entry.name);
+
+    match entry.node {
+        nar::reader::Node::Symlink { target } => {
+            assert_eq!(exp_target.as_bytes(), &target);
+        }
+        _ => panic!("unexpected type for {}", name),
+    }
+}