Merge 'Small VDBE insn tweaks' from Jussi Saurio

1. allow calling op_null with Insn::BeginSubrtn
    - BeginSubrtn is identical to Null, but named differently so that
its use in context is clearer
2. Insn::Return: add possibility to fallthrough on non-integer values as
per sqlite spec

Closes #1588

core/translate/group_by.rs

@@ -760,6 +760,7 @@ pub fn group_by_emit_row_phase<'a>(
     });
     program.emit_insn(Insn::Return {
         return_reg: registers.reg_subrtn_acc_output_return_offset,
+        can_fallthrough: false,
     });
 
     program.resolve_label(labels.label_subrtn_acc_output, program.offset());
@@ -784,6 +785,7 @@ pub fn group_by_emit_row_phase<'a>(
     }
     program.emit_insn(Insn::Return {
         return_reg: registers.reg_subrtn_acc_output_return_offset,
+        can_fallthrough: false,
     });
 
     // Finalize aggregate values for output
@@ -916,6 +918,7 @@ pub fn group_by_emit_row_phase<'a>(
 
     program.emit_insn(Insn::Return {
         return_reg: registers.reg_subrtn_acc_output_return_offset,
+        can_fallthrough: false,
     });
 
     // Subroutine to clear accumulators for a new group
@@ -955,6 +958,7 @@ pub fn group_by_emit_row_phase<'a>(
     });
     program.emit_insn(Insn::Return {
         return_reg: registers.reg_subrtn_acc_clear_return_offset,
+        can_fallthrough: false,
     });
     program.preassign_label_to_next_insn(labels.label_group_by_end);
     Ok(())

core/vdbe/execute.rs

@@ -322,15 +322,17 @@ pub fn op_null(
     pager: &Rc<Pager>,
     mv_store: Option<&Rc<MvStore>>,
 ) -> Result<InsnFunctionStepResult> {
-    let Insn::Null { dest, dest_end } = insn else {
-        unreachable!("unexpected Insn {:?}", insn)
-    };
-    if let Some(dest_end) = dest_end {
-        for i in *dest..=*dest_end {
-            state.registers[i] = Register::Value(Value::Null);
+    match insn {
+        Insn::Null { dest, dest_end } | Insn::BeginSubrtn { dest, dest_end } => {
+            if let Some(dest_end) = dest_end {
+                for i in *dest..=*dest_end {
+                    state.registers[i] = Register::Value(Value::Null);
+                }
+            } else {
+                state.registers[*dest] = Register::Value(Value::Null);
+            }
         }
-    } else {
-        state.registers[*dest] = Register::Value(Value::Null);
+        _ => unreachable!("unexpected Insn {:?}", insn),
     }
     state.pc += 1;
     Ok(InsnFunctionStepResult::Step)
@@ -1809,7 +1811,11 @@ pub fn op_return(
     pager: &Rc<Pager>,
     mv_store: Option<&Rc<MvStore>>,
 ) -> Result<InsnFunctionStepResult> {
-    let Insn::Return { return_reg } = insn else {
+    let Insn::Return {
+        return_reg,
+        can_fallthrough,
+    } = insn
+    else {
         unreachable!("unexpected Insn {:?}", insn)
     };
     if let Value::Integer(pc) = state.registers[*return_reg].get_owned_value() {
@@ -1818,9 +1824,12 @@ pub fn op_return(
             .unwrap_or_else(|_| panic!("Return register is negative: {}", pc));
         state.pc = pc;
     } else {
-        return Err(LimboError::InternalError(
-            "Return register is not an integer".to_string(),
-        ));
+        if !*can_fallthrough {
+            return Err(LimboError::InternalError(
+                "Return register is not an integer".to_string(),
+            ));
+        }
+        state.pc += 1;
     }
     Ok(InsnFunctionStepResult::Step)
 }

core/vdbe/explain.rs

@@ -625,11 +625,14 @@ pub fn insn_to_str(
                 0,
                 "".to_string(),
             ),
-            Insn::Return { return_reg } => (
+            Insn::Return {
+                return_reg,
+                can_fallthrough,
+            } => (
                 "Return",
                 *return_reg as i32,
                 0,
-                0,
+                *can_fallthrough as i32,
                 Value::build_text(""),
                 0,
                 "".to_string(),

core/vdbe/insn.rs

@@ -429,8 +429,11 @@ pub enum Insn {
     },
 
     /// Returns to the program counter stored in register 'return_reg'.
+    /// If can_fallthrough is true, fall through to the next instruction
+    /// if return_reg does not contain an integer value. Otherwise raise an error.
     Return {
         return_reg: usize,
+        can_fallthrough: bool,
     },
 
     /// Write an integer value into a register.