Repository: mitsuhiko/minijinja (rust)

Issue:
Calling `{% break %}` on an iterator results in a lost value
## Description
The `{% break %}` statement consumes an item of the iterator that you can't have access to anymore. If you start consuming the iterator again, that value doesn't appear, but instead it continues with the next value.

This wasn't the case in the Python implementation of Jinja2.

## Reproduction steps

You can see in the example below that even though the iterator was `0..10` the item `6` is swallowed after calling `{% break %}`.

```rs
use std::fs::File;

use minijinja::{Environment, Value};

fn main() {
    let mut env = Environment::new();
    env.add_template("template", include_str!("template.jinja"))
        .unwrap();
    let template = env.get_template("template").unwrap();

    let out = File::create("test.txt").unwrap();

    template
        .render_to_write(
            minijinja::context! { items => Value::make_one_shot_iterator(0..10)},
            out,
        )
        .unwrap();
}
```

```jinja
{%- for item in items -%}
    {%- if item == 5 -%}
        {{ item }}
        {% break %}
    {%- endif -%}

    {{ item }}
{% endfor %}
newloop

{% for item in items -%}
    {{ item }}
{% endfor %}
```

output:
```
0
1
2
3
4
5

newloop

7
8
9
```

Additional helpful information:

- Version of minijinja: `2.6.0` with the `loop_controls` feature
- Version of rustc: `v1.85.0-nightly`
- Operating system and version: `Ubuntu 24.04`

## What did you expect
I would expect the value `6` in the example above to be part of the second loop.
In other words, the `break` statement shouldn't have swallowed it.




Pull request:
loop.nextitem is now a lazy operation
Previously we always pulled the next item which meant that the iteration ran one in the past.  It also caused lost items when combined with `{% break %}`.


Fixes #676 

Patch:
diff --git a/.cargo/config.toml b/.cargo/config.toml
index ff68877..cd86710 100644
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -1,4 +1,4 @@
-[target.wasm32-wasi]
+[target.wasm32-wasip1]
 runner = ["./scripts/wasmtime-wrapper.sh"]
 
 [target.aarch64-unknown-linux-gnu]
diff --git a/CHANGELOG.md b/CHANGELOG.md
index a3d29a6..28fe1b9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ All notable changes to MiniJinja are documented here.
 ## 2.7.0
 
 - Removed string interning.  #675
+- `loop.nextitem` is now a lazy operation.  This prevents issues when
+  iterating over one-shot iterators combined with `{% break %}` and
+  it now ensures that the iterator is not running "one item ahead".  #677
 
 ## 2.6.0
 
diff --git a/Makefile b/Makefile
index 7eac203..62a0fcf 100644
--- a/Makefile
+++ b/Makefile
@@ -26,7 +26,7 @@ test: test-msrv test-cli
 
 .PHONY: wasi-test
 wasi-test:
-	@cd minijinja; cargo test --all-features --target=wasm32-wasi -- --nocapture
+	@cd minijinja; cargo test --all-features --target=wasm32-wasip1 -- --nocapture
 
 .PHONY: python-test
 python-test:
diff --git a/minijinja/src/syntax.rs b/minijinja/src/syntax.rs
index 0f47e73..7415ca4 100644
--- a/minijinja/src/syntax.rs
+++ b/minijinja/src/syntax.rs
@@ -746,6 +746,11 @@
 //! {%- endfor %}
 //! ```
 //!
+//! **Note on one-shot iterators:** if you break from a loop but you have
+//! accessed the `loop.nextitem` special variable, then you will lose one item.
+//! This is because accessing that attribute will peak into the iterator and
+//! there is no support for "putting values back".
+//!
 #![cfg_attr(
     feature = "custom_syntax",
     doc = r###"
diff --git a/minijinja/src/vm/context.rs b/minijinja/src/vm/context.rs
index 1145f4d..e1157dc 100644
--- a/minijinja/src/vm/context.rs
+++ b/minijinja/src/vm/context.rs
@@ -1,29 +1,20 @@
 use std::borrow::Cow;
 use std::collections::{BTreeMap, HashSet};
 use std::fmt;
+
+#[cfg(any(feature = "macros", feature = "multi_template"))]
 use std::sync::Arc;
 
 use crate::environment::Environment;
 use crate::error::{Error, ErrorKind};
-use crate::value::{Value, ValueIter};
-use crate::vm::loop_object::Loop;
+use crate::value::Value;
+use crate::vm::loop_object::LoopState;
 
 #[cfg(feature = "macros")]
 use crate::vm::closure_object::Closure;
 
 type Locals<'env> = BTreeMap<&'env str, Value>;
 
-pub(crate) struct LoopState {
-    pub(crate) with_loop_var: bool,
-    pub(crate) recurse_jump_target: Option<usize>,
-    // if we're popping the frame, do we want to jump somewhere?  The
-    // first item is the target jump instruction, the second argument
-    // tells us if we need to end capturing.
-    pub(crate) current_recursion_jump: Option<(usize, bool)>,
-    pub(crate) iterator: ValueIter,
-    pub(crate) object: Arc<Loop>,
-}
-
 pub(crate) struct Frame<'env> {
     pub(crate) locals: Locals<'env>,
     pub(crate) ctx: Value,
diff --git a/minijinja/src/vm/loop_object.rs b/minijinja/src/vm/loop_object.rs
index d3f5783..785d189 100644
--- a/minijinja/src/vm/loop_object.rs
+++ b/minijinja/src/vm/loop_object.rs
@@ -3,15 +3,126 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 
 use crate::error::{Error, ErrorKind};
-use crate::value::{Enumerator, Object, Value};
+use crate::value::{Enumerator, Object, Value, ValueIter};
 use crate::vm::state::State;
 
+pub(crate) struct LoopState {
+    pub(crate) with_loop_var: bool,
+    pub(crate) recurse_jump_target: Option<usize>,
+
+    // if we're popping the frame, do we want to jump somewhere?  The
+    // first item is the target jump instruction, the second argument
+    // tells us if we need to end capturing.
+    pub(crate) current_recursion_jump: Option<(usize, bool)>,
+
+    // Depending on if adjacent_loop_items is enabled or not, the iterator
+    // is stored either on the loop state or in the loop object.  This is
+    // done because when the feature is disabled, we can avoid using a mutex.
+    pub(crate) object: Arc<Loop>,
+    #[cfg(not(feature = "adjacent_loop_items"))]
+    iter: crate::value::ValueIter,
+}
+
+impl LoopState {
+    pub fn new(
+        iter: ValueIter,
+        depth: usize,
+        with_loop_var: bool,
+        recurse_jump_target: Option<usize>,
+        current_recursion_jump: Option<(usize, bool)>,
+    ) -> LoopState {
+        // for an iterator where the lower and upper bound are matching we can
+        // consider them to have ExactSizeIterator semantics.  We do however not
+        // expect ExactSizeIterator bounds themselves to support iteration by
+        // other means.
+        let len = match iter.size_hint() {
+            (lower, Some(upper)) if lower == upper => Some(lower),
+            _ => None,
+        };
+        LoopState {
+            with_loop_var,
+            recurse_jump_target,
+            current_recursion_jump,
+            object: Arc::new(Loop {
+                idx: AtomicUsize::new(!0usize),
+                len,
+                depth,
+                #[cfg(feature = "adjacent_loop_items")]
+                iter: Mutex::new(AdjacentLoopItemIterWrapper::new(iter)),
+                last_changed_value: Mutex::default(),
+            }),
+            #[cfg(not(feature = "adjacent_loop_items"))]
+            iter,
+        }
+    }
+
+    pub fn did_not_iterate(&self) -> bool {
+        self.object.idx.load(Ordering::Relaxed) == 0
+    }
+
+    pub fn next(&mut self) -> Option<Value> {
+        self.object.idx.fetch_add(1, Ordering::Relaxed);
+        #[cfg(feature = "adjacent_loop_items")]
+        {
+            self.object.iter.lock().unwrap().next()
+        }
+        #[cfg(not(feature = "adjacent_loop_items"))]
+        {
+            self.iter.next()
+        }
+    }
+}
+
+#[cfg(feature = "adjacent_loop_items")]
+pub(crate) struct AdjacentLoopItemIterWrapper {
+    prev_item: Option<Value>,
+    current_item: Option<Value>,
+    next_item: Option<Value>,
+    iter: ValueIter,
+}
+
+#[cfg(feature = "adjacent_loop_items")]
+impl AdjacentLoopItemIterWrapper {
+    pub fn new(iterator: ValueIter) -> AdjacentLoopItemIterWrapper {
+        AdjacentLoopItemIterWrapper {
+            prev_item: None,
+            current_item: None,
+            next_item: None,
+            iter: iterator,
+        }
+    }
+
+    pub fn next(&mut self) -> Option<Value> {
+        self.prev_item = self.current_item.take();
+        self.current_item = if let Some(ref next) = self.next_item.take() {
+            Some(next.clone())
+        } else {
+            self.next_item = None;
+            self.iter.next()
+        };
+        self.current_item.clone()
+    }
+
+    pub fn next_item(&mut self) -> Value {
+        if let Some(ref next) = self.next_item {
+            next.clone()
+        } else {
+            self.next_item = self.iter.next();
+            self.next_item.clone().unwrap_or_default()
+        }
+    }
+
+    pub fn prev_item(&self) -> Value {
+        self.prev_item.clone().unwrap_or_default()
+    }
+}
+
 pub(crate) struct Loop {
     pub len: Option<usize>,
     pub idx: AtomicUsize,
     pub depth: usize,
     #[cfg(feature = "adjacent_loop_items")]
-    pub value_triple: Mutex<(Option<Value>, Option<Value>, Option<Value>)>,
+    pub iter: Mutex<AdjacentLoopItemIterWrapper>,
     pub last_changed_value: Mutex<Option<Vec<Value>>>,
 }
 
@@ -107,23 +218,9 @@ impl Object for Loop {
             "depth" => Some(Value::from(self.depth + 1)),
             "depth0" => Some(Value::from(self.depth)),
             #[cfg(feature = "adjacent_loop_items")]
-            "previtem" => Some(
-                self.value_triple
-                    .lock()
-                    .unwrap()
-                    .0
-                    .clone()
-                    .unwrap_or(Value::UNDEFINED),
-            ),
+            "previtem" => Some(self.iter.lock().unwrap().prev_item()),
             #[cfg(feature = "adjacent_loop_items")]
-            "nextitem" => Some(
-                self.value_triple
-                    .lock()
-                    .unwrap()
-                    .2
-                    .clone()
-                    .unwrap_or(Value::UNDEFINED),
-            ),
+            "nextitem" => Some(self.iter.lock().unwrap().next_item()),
             _ => None,
         }
     }
diff --git a/minijinja/src/vm/mod.rs b/minijinja/src/vm/mod.rs
index 4ecb90e..a52ed84 100644
--- a/minijinja/src/vm/mod.rs
+++ b/minijinja/src/vm/mod.rs
@@ -1,7 +1,8 @@
 use std::collections::BTreeMap;
 use std::mem;
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::sync::{Arc, Mutex};
+
+#[cfg(any(feature = "macros", feature = "multi_template"))]
+use std::sync::Arc;
 
 use crate::compiler::instructions::{
     Instruction, Instructions, LOOP_FLAG_RECURSIVE, LOOP_FLAG_WITH_LOOP_VAR, MAX_LOCALS,
@@ -12,8 +13,8 @@ use crate::output::{CaptureMode, Output};
 use crate::utils::{untrusted_size_hint, AutoEscape, UndefinedBehavior};
 use crate::value::namespace_object::Namespace;
 use crate::value::{ops, value_map_with_capacity, Kwargs, ObjectRepr, Value, ValueMap};
-use crate::vm::context::{Frame, LoopState, Stack};
-use crate::vm::loop_object::Loop;
+use crate::vm::context::{Frame, Stack};
+use crate::vm::loop_object::LoopState;
 use crate::vm::state::BlockStack;
 
 #[cfg(feature = "macros")]
@@ -498,24 +499,7 @@ impl<'env> Vm<'env> {
                     ctx_ok!(self.push_loop(state, a, *flags, pc, next_loop_recursion_jump.take()));
                 }
                 Instruction::Iterate(jump_target) => {
-                    let l = state.ctx.current_loop().unwrap();
-                    l.object.idx.fetch_add(1, Ordering::Relaxed);
-
-                    let next = {
-                        #[cfg(feature = "adjacent_loop_items")]
-                        {
-                            let mut triple = l.object.value_triple.lock().unwrap();
-                            triple.0 = triple.1.take();
-                            triple.1 = triple.2.take();
-                            triple.2 = l.iterator.next();
-                            triple.1.clone()
-                        }
-                        #[cfg(not(feature = "adjacent_loop_items"))]
-                        {
-                            l.iterator.next()
-                        }
-                    };
-                    match next {
+                    match state.ctx.current_loop().unwrap().next() {
                         Some(item) => stack.push(assert_valid!(item)),
                         None => {
                             pc = *jump_target;
@@ -525,7 +509,7 @@ impl<'env> Vm<'env> {
                 }
                 Instruction::PushDidNotIterate => {
                     let l = state.ctx.current_loop().unwrap();
-                    stack.push(Value::from(l.object.idx.load(Ordering::Relaxed) == 0));
+                    stack.push(Value::from(l.did_not_iterate()));
                 }
                 Instruction::Jump(jump_target) => {
                     pc = *jump_target;
@@ -984,15 +968,7 @@ impl<'env> Vm<'env> {
         current_recursion_jump: Option<(usize, bool)>,
     ) -> Result<(), Error> {
         #[allow(unused_mut)]
-        let mut iterator = ok!(state.undefined_behavior().try_iter(iterable));
-        // for an iterator where the lower and upper bound are matching we can
-        // consider them to have ExactSizeIterator semantics.  We do however not
-        // expect ExactSizeIterator bounds themselves to support iteration by
-        // other means.
-        let len = match iterator.size_hint() {
-            (lower, Some(upper)) if lower == upper => Some(lower),
-            _ => None,
-        };
+        let mut iter = ok!(state.undefined_behavior().try_iter(iterable));
         let depth = state
             .ctx
             .current_loop()
@@ -1001,20 +977,13 @@ impl<'env> Vm<'env> {
         let recursive = flags & LOOP_FLAG_RECURSIVE != 0;
         let with_loop_var = flags & LOOP_FLAG_WITH_LOOP_VAR != 0;
         ok!(state.ctx.push_frame(Frame {
-            current_loop: Some(LoopState {
+            current_loop: Some(LoopState::new(
+                iter,
+                depth,
                 with_loop_var,
-                recurse_jump_target: if recursive { Some(pc) } else { None },
-                current_recursion_jump,
-                object: Arc::new(Loop {
-                    idx: AtomicUsize::new(!0usize),
-                    len,
-                    depth,
-                    #[cfg(feature = "adjacent_loop_items")]
-                    value_triple: Mutex::new((None, None, iterator.next())),
-                    last_changed_value: Mutex::default(),
-                }),
-                iterator,
-            }),
+                if recursive { Some(pc) } else { None },
+                current_recursion_jump
+            )),
             ..Frame::default()
         }));
         Ok(())


Test patch:
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 5c9fb6c..a61be6d 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -109,7 +109,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: stable
-          targets: wasm32-wasi
+          targets: wasm32-wasip1
       - uses: Swatinem/rust-cache@v2
       - name: Install WasmTime
         run: |
diff --git a/minijinja/tests/inputs/loop_break_one_shot_iter.txt b/minijinja/tests/inputs/loop_break_one_shot_iter.txt
new file mode 100644
index 0000000..070bb93
--- /dev/null
+++ b/minijinja/tests/inputs/loop_break_one_shot_iter.txt
@@ -0,0 +1,5 @@
+{}
+---
+normal iteration does not lose items:
+{% for item in one_shot_iterator %}{{ item }}{% if item == 1 %}{% break %}{% endif %}{% endfor %}
+{%- for item in one_shot_iterator %}{{ item }}{% endfor %}
diff --git a/minijinja/tests/inputs/loop_break_one_shot_iter_peek.txt b/minijinja/tests/inputs/loop_break_one_shot_iter_peek.txt
new file mode 100644
index 0000000..5b6d5c1
--- /dev/null
+++ b/minijinja/tests/inputs/loop_break_one_shot_iter_peek.txt
@@ -0,0 +1,5 @@
+{}
+---
+peeking loses items:
+{% for item in one_shot_iterator %}{{ item }}(next: {{ loop.nextitem }}){% if item == 0 %}{% break %}{% endif %}{% endfor %}
+{%- for item in one_shot_iterator %}{{ item }}(next: {{ loop.nextitem }}){% endfor %}
\ No newline at end of file
diff --git a/minijinja/tests/inputs/loop_controls.txt b/minijinja/tests/inputs/loop_controls.txt
index e301352..61a6ec5 100644
--- a/minijinja/tests/inputs/loop_controls.txt
+++ b/minijinja/tests/inputs/loop_controls.txt
@@ -4,4 +4,4 @@
   {%- if item is odd %}{% continue %}{% endif %}
   {%- if item > 5 %}{% break %}{% endif %}
   {{- item }}
-{%- endfor %}
+{%- endfor %}
\ No newline at end of file
diff --git a/minijinja/tests/snapshots/test_templates__vm@loop_break_one_shot_iter.txt.snap b/minijinja/tests/snapshots/test_templates__vm@loop_break_one_shot_iter.txt.snap
new file mode 100644
index 0000000..7e14d15
--- /dev/null
+++ b/minijinja/tests/snapshots/test_templates__vm@loop_break_one_shot_iter.txt.snap
@@ -0,0 +1,8 @@
+---
+source: minijinja/tests/test_templates.rs
+description: "normal iteration does not lose items:\n{% for item in one_shot_iterator %}{{ item }}{% if item == 1 %}{% break %}{% endif %}{% endfor %}\n{%- for item in one_shot_iterator %}{{ item }}{% endfor %}"
+info: {}
+input_file: minijinja/tests/inputs/loop_break_one_shot_iter.txt
+---
+normal iteration does not lose items:
+012
diff --git a/minijinja/tests/snapshots/test_templates__vm@loop_break_one_shot_iter_peek.txt.snap b/minijinja/tests/snapshots/test_templates__vm@loop_break_one_shot_iter_peek.txt.snap
new file mode 100644
index 0000000..024f156
--- /dev/null
+++ b/minijinja/tests/snapshots/test_templates__vm@loop_break_one_shot_iter_peek.txt.snap
@@ -0,0 +1,8 @@
+---
+source: minijinja/tests/test_templates.rs
+description: "peeking loses items:\n{% for item in one_shot_iterator %}{{ item }}(next: {{ loop.nextitem }}){% if item == 0 %}{% break %}{% endif %}{% endfor %}\n{%- for item in one_shot_iterator %}{{ item }}(next: {{ loop.nextitem }}){% endfor %}"
+info: {}
+input_file: minijinja/tests/inputs/loop_break_one_shot_iter_peek.txt
+---
+peeking loses items:
+0(next: 1)2(next: )