fix(tvix/eval): only finalise formal arguments if defaulting r/6336

When dealing with a formal argument in a function argument pattern that has a default expression, there are two different things that can happen at runtime: Either we select its value from the passed attribute successfully or we need to use the default expression. Both of these may be thunks and both of these may need finalisers. However, in the former case this is taken care of elsewhere, the value will always be finalised already if necessary. In the latter case we may need to finalise the thunk resulting from the default expression. However, the thunk corresponding to the expression may never end up in the local's stack slot. Since finalisation goes by stack slot (and not constants), we need to prevent a case where we don't fall back to the default expression, but finalise anyways. Previously, we worked around this by making `OpFinalise` ignore non-thunks. Since finalisation of already evaluated thunks still crashed, the faulty compilation of function pattern arguments could still cause a crash. As a new approach, we reinstate the old behavior of `OpFinalise` to crash whenever encountering something that is either not a thunk or doesn't need finalisation. This can also help catching (similar) miscompilations in the future. To then prevent the crash, we need to track whether we have fallen back or not at runtime. This is done using an additional phantom on the stack that holds a new `FinaliseRequest` value. When it comes to finalisation we check this value and conditionally execute `OpFinalise` based on its value. Resolves b/261 and b/265 (partially). Change-Id: Ic04fb80ec671a2ba11fa645090769c335fb7f58b Reviewed-on: https://cl.tvl.fyi/c/depot/+/8705 Reviewed-by: tazjin <tazjin@tvl.su> Tested-by: BuildkiteCI Autosubmit: sterni <sternenseemann@systemli.org>
author: sterni <sternenseemann@systemli.org> 2023-06-03T00·10+0200
committer: sterni <sternenseemann@systemli.org> 2023-06-20T10·07+0000
commit: 4516cd09c51b7a19707de0a5ba171c9592241a18 (patch)
tree: 4ae73147e753aa5e510f43dd4b8029fe51642a14 /tvix/eval/src/compiler/mod.rs
parent: 6656b865b608f7417b5a4b5df1e20811f48c7a82 (diff)
1 files changed, 165 insertions, 34 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 436e504af2a6..b7b9192571f7 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -80,6 +80,47 @@ impl LambdaCtx {
     }
 }
 
+/// When compiling functions with an argument attribute set destructuring pattern,
+/// we need to do multiple passes over the declared formal arguments when setting
+/// up their local bindings (similarly to `let … in` expressions and recursive
+/// attribute sets. For this purpose, this struct is used to represent the two
+/// kinds of formal arguments:
+///
+/// - `TrackedFormal::NoDefault` is always required and causes an evaluation error
+///   if the corresponding attribute is missing in a function call.
+/// - `TrackedFormal::WithDefault` may be missing in the passed attribute set—
+///   in which case a `default_expr` will be evaluated and placed in the formal
+///   argument's local variable slot.
+enum TrackedFormal {
+    NoDefault {
+        local_idx: LocalIdx,
+        pattern_entry: ast::PatEntry,
+    },
+    WithDefault {
+        local_idx: LocalIdx,
+        /// Extra phantom local used for coordinating runtime dispatching not observable to
+        /// the language user. Detailed description in `compile_param_pattern()`.
+        finalise_request_idx: LocalIdx,
+        default_expr: ast::Expr,
+        pattern_entry: ast::PatEntry,
+    },
+}
+
+impl TrackedFormal {
+    fn pattern_entry(&self) -> &ast::PatEntry {
+        match self {
+            TrackedFormal::NoDefault { pattern_entry, .. } => pattern_entry,
+            TrackedFormal::WithDefault { pattern_entry, .. } => pattern_entry,
+        }
+    }
+    fn local_idx(&self) -> LocalIdx {
+        match self {
+            TrackedFormal::NoDefault { local_idx, .. } => *local_idx,
+            TrackedFormal::WithDefault { local_idx, .. } => *local_idx,
+        }
+    }
+}
+
 /// The map of globally available functions and other values that
 /// should implicitly be resolvable in the global scope.
 pub(crate) type GlobalsMap = HashMap<&'static str, Value>;
@@ -894,10 +935,19 @@ impl Compiler<'_> {
     ///    in <body>
     /// ```
     ///
-    /// The only tricky bit being that bindings have to fail if too
-    /// many arguments are provided. This is done by emitting a
-    /// special instruction that checks the set of keys from a
-    /// constant containing the expected keys.
+    /// However, there are two properties of pattern function arguments that can
+    /// not be compiled by desugaring in this way:
+    ///
+    /// 1. Bindings have to fail if too many arguments are provided. This is
+    ///    done by emitting a special instruction that checks the set of keys
+    ///    from a constant containing the expected keys.
+    /// 2. Formal arguments with a default expression are (as an optimization and
+    ///    because it is simpler) not wrapped in another thunk, instead compiled
+    ///    and accessed separately. This means that the default expression may
+    ///    never make it into the local's stack slot if the argument is provided
+    ///    by the caller. We need to take this into account and skip any
+    ///    operations specific to the expression like thunk finalisation in such
+    ///    cases.
     fn compile_param_pattern(&mut self, pattern: &ast::Pattern) -> Formals {
         let span = self.span_for(pattern);
         let set_idx = match pattern.pat_bind() {
@@ -905,10 +955,10 @@ impl Compiler<'_> {
             None => self.scope_mut().declare_phantom(span, true),
         };
 
-        // At call time, the attribute set is already at the top of
-        // the stack.
+        // At call time, the attribute set is already at the top of the stack.
         self.scope_mut().mark_initialised(set_idx);
         self.emit_force(pattern);
+        // Evaluation fails on a type error, even if the argument(s) are unused.
         self.push_op(OpCode::OpAssertAttrs, pattern);
 
         let ellipsis = pattern.ellipsis_token().is_some();
@@ -919,51 +969,131 @@ impl Compiler<'_> {
         // Similar to `let ... in ...`, we now do multiple passes over
         // the bindings to first declare them, then populate them, and
         // then finalise any necessary recursion into the scope.
-        let mut entries: Vec<(LocalIdx, ast::PatEntry)> = vec![];
+        let mut entries: Vec<TrackedFormal> = vec![];
         let mut arguments = HashMap::default();
 
         for entry in pattern.pat_entries() {
             let ident = entry.ident().unwrap();
             let idx = self.declare_local(&ident, ident.to_string());
-            let has_default = entry.default().is_some();
-            entries.push((idx, entry));
-            arguments.insert(ident.into(), has_default);
+
+            arguments.insert(ident.into(), entry.default().is_some());
+
+            if let Some(default_expr) = entry.default() {
+                entries.push(TrackedFormal::WithDefault {
+                    local_idx: idx,
+                    // This phantom is used to track at runtime (!) whether we need to
+                    // finalise the local's stack slot or not. The relevant instructions are
+                    // emitted in the second pass where the mechanism is explained as well.
+                    finalise_request_idx: {
+                        let span = self.span_for(&default_expr);
+                        self.scope_mut().declare_phantom(span, false)
+                    },
+                    default_expr,
+                    pattern_entry: entry,
+                });
+            } else {
+                entries.push(TrackedFormal::NoDefault {
+                    local_idx: idx,
+                    pattern_entry: entry,
+                });
+            }
         }
 
         // For each of the bindings, push the set on the stack and
         // attempt to select from it.
         let stack_idx = self.scope().stack_index(set_idx);
-        for (idx, entry) in (&entries).into_iter() {
+        for tracked_formal in (&entries).into_iter() {
             self.push_op(OpCode::OpGetLocal(stack_idx), pattern);
-            self.emit_literal_ident(&entry.ident().unwrap());
+            self.emit_literal_ident(&tracked_formal.pattern_entry().ident().unwrap());
+
+            let idx = tracked_formal.local_idx();
 
             // Use the same mechanism as `compile_select_or` if a
             // default value was provided, or simply select otherwise.
-            if let Some(default_expr) = entry.default() {
-                self.push_op(OpCode::OpAttrsTrySelect, &entry.ident().unwrap());
-
-                let jump_to_default =
-                    self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &default_expr);
-
-                let jump_over_default = self.push_op(OpCode::OpJump(JumpOffset(0)), &default_expr);
-
-                self.patch_jump(jump_to_default);
-
-                // Does not need to thunked since compile() already does so when necessary
-                self.compile(*idx, default_expr);
-
-                self.patch_jump(jump_over_default);
-            } else {
-                self.push_op(OpCode::OpAttrsSelect, &entry.ident().unwrap());
+            match tracked_formal {
+                TrackedFormal::WithDefault {
+                    default_expr,
+                    pattern_entry,
+                    ..
+                } => {
+                    // The tricky bit about compiling a formal argument with a default value
+                    // is that the default may be a thunk that may depend on the value of
+                    // other formal arguments, i.e. may need to be finalised. This
+                    // finalisation can only happen if we are actually using the default
+                    // value—otherwise OpFinalise will crash on an already finalised (or
+                    // non-thunk) value.
+                    //
+                    // Thus we use an additional local to track whether we wound up
+                    // defaulting or not. `FinaliseRequest(false)` indicates that we should
+                    // not finalise, as we did not default.
+                    //
+                    // We are being wasteful with VM stack space in case of default
+                    // expressions that don't end up needing to be finalised. Unfortunately
+                    // we only know better after compiling the default expression, so
+                    // avoiding unnecessary locals would mean we'd need to modify the chunk
+                    // after the fact.
+                    self.push_op(OpCode::OpAttrsTrySelect, &pattern_entry.ident().unwrap());
+                    let jump_to_default =
+                        self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), default_expr);
+
+                    self.emit_constant(Value::FinaliseRequest(false), default_expr);
+
+                    let jump_over_default =
+                        self.push_op(OpCode::OpJump(JumpOffset(0)), default_expr);
+
+                    self.patch_jump(jump_to_default);
+
+                    // Does not need to thunked since compile() already does so when necessary
+                    self.compile(idx, default_expr.clone());
+
+                    self.emit_constant(Value::FinaliseRequest(true), default_expr);
+
+                    self.patch_jump(jump_over_default);
+                }
+                TrackedFormal::NoDefault { pattern_entry, .. } => {
+                    self.push_op(OpCode::OpAttrsSelect, &pattern_entry.ident().unwrap());
+                }
             }
 
-            self.scope_mut().mark_initialised(*idx);
+            self.scope_mut().mark_initialised(idx);
+            if let TrackedFormal::WithDefault {
+                finalise_request_idx,
+                ..
+            } = tracked_formal
+            {
+                self.scope_mut().mark_initialised(*finalise_request_idx);
+            }
         }
 
-        for (idx, _) in (&entries).into_iter() {
-            if self.scope()[*idx].needs_finaliser {
-                let stack_idx = self.scope().stack_index(*idx);
-                self.push_op(OpCode::OpFinalise(stack_idx), pattern);
+        for tracked_formal in (&entries).into_iter() {
+            if self.scope()[tracked_formal.local_idx()].needs_finaliser {
+                let stack_idx = self.scope().stack_index(tracked_formal.local_idx());
+                match tracked_formal {
+                    TrackedFormal::NoDefault { .. } =>
+                        panic!("Tvix bug: local for pattern formal needs finaliser, but has no default expr"),
+                    TrackedFormal::WithDefault { finalise_request_idx, .. } => {
+                        let finalise_request_stack_idx = self.scope().stack_index(*finalise_request_idx);
+
+                        // TODO(sterni): better spans
+                        self.push_op(
+                            OpCode::OpGetLocal(finalise_request_stack_idx),
+                            pattern
+                        );
+                        let jump_over_finalise =
+                            self.push_op(
+                                OpCode::OpJumpIfNoFinaliseRequest(
+                                    JumpOffset(0)),
+                                pattern
+                            );
+                        self.push_op(
+                            OpCode::OpFinalise(stack_idx),
+                            pattern,
+                        );
+                        self.patch_jump(jump_over_finalise);
+                        // Get rid of finaliser request value on the stack
+                        self.push_op(OpCode::OpPop, pattern);
+                    }
+                }
             }
         }
 
@@ -1196,7 +1326,8 @@ impl Compiler<'_> {
             OpCode::OpJump(n)
             | OpCode::OpJumpIfFalse(n)
             | OpCode::OpJumpIfTrue(n)
-            | OpCode::OpJumpIfNotFound(n) => {
+            | OpCode::OpJumpIfNotFound(n)
+            | OpCode::OpJumpIfNoFinaliseRequest(n) => {
                 *n = offset;
             }
author	sterni <sternenseemann@systemli.org>	2023-06-03T00·10+0200
committer	sterni <sternenseemann@systemli.org>	2023-06-20T10·07+0000
commit	4516cd09c51b7a19707de0a5ba171c9592241a18 (patch)
tree	4ae73147e753aa5e510f43dd4b8029fe51642a14 /tvix/eval/src/compiler/mod.rs
parent	6656b865b608f7417b5a4b5df1e20811f48c7a82 (diff)