From 1596511175ba9234d6f883813affa8db30f0d3bb Mon Sep 17 00:00:00 2001
From: Calvin Rose <calsrose@gmail.com>
Date: Wed, 10 Apr 2019 23:29:40 -0400
Subject: [PATCH] Fix undefined behavior bug with errors.

janet_vm_return_reg should only be set when janet_continue
is called. Otherwise, a panic may dump it's error message in
the wrong place, resulting in undefined behavior (often showing
the last return value or worse, segfaulting).
---
 src/core/vm.c     | 11 +++--------
 test/suite3.janet |  6 ++++++
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/src/core/vm.c b/src/core/vm.c
index 7a6a17d3..6513a587 100644
--- a/src/core/vm.c
+++ b/src/core/vm.c
@@ -578,7 +578,6 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in, JanetFiberStatus status)
             janet_fiber_cframe(fiber, janet_unwrap_cfunction(callee));
             Janet ret = janet_unwrap_cfunction(callee)(argc, fiber->data + fiber->frame);
             janet_fiber_popframe(fiber);
-            /*if (fiber->frame == 0) vm_return(JANET_SIGNAL_OK, ret);*/
             stack = fiber->data + fiber->frame;
             stack[A] = ret;
             vm_checkgc_pcnext();
@@ -638,6 +637,7 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in, JanetFiberStatus status)
         if (sig != JANET_SIGNAL_OK && !(child->flags & (1 << sig)))
             vm_return(sig, retreg);
         fiber->child = NULL;
+        stack = fiber->data + fiber->frame;
         stack[A] = retreg;
         vm_checkgc_pcnext();
     }
@@ -749,9 +749,6 @@ static JanetSignal run_vm(JanetFiber *fiber, Janet in, JanetFiberStatus status)
 }
 
 Janet janet_call(JanetFunction *fun, int32_t argc, const Janet *argv) {
-    Janet ret;
-    Janet *old_return_reg = janet_vm_return_reg;
-
     /* Check entry conditions */
     if (!janet_vm_fiber)
         janet_panic("janet_call failed because there is no current fiber");
@@ -768,7 +765,6 @@ Janet janet_call(JanetFunction *fun, int32_t argc, const Janet *argv) {
     /* Set up */
     int32_t oldn = janet_vm_stackn++;
     int handle = janet_gclock();
-    janet_vm_return_reg = &ret;
 
     /* Run vm */
     JanetSignal signal = run_vm(janet_vm_fiber,
@@ -776,13 +772,12 @@ Janet janet_call(JanetFunction *fun, int32_t argc, const Janet *argv) {
                                 JANET_STATUS_ALIVE);
 
     /* Teardown */
-    janet_vm_return_reg = old_return_reg;
     janet_vm_stackn = oldn;
     janet_gcunlock(handle);
 
-    if (signal != JANET_SIGNAL_OK) janet_panicv(ret);
+    if (signal != JANET_SIGNAL_OK) janet_panicv(*janet_vm_return_reg);
 
-    return ret;
+    return *janet_vm_return_reg;
 }
 
 /* Enter the main vm loop */
diff --git a/test/suite3.janet b/test/suite3.janet
index 097eb26b..00fce95d 100644
--- a/test/suite3.janet
+++ b/test/suite3.janet
@@ -351,6 +351,12 @@
 (def t (put @{} :hi 1))
 (assert (deep= t @{:hi 1}) "regression #24")
 
+# Peg swallowing errors
+(assert (try (peg/match ~(/ '1 ,(fn [x] (nil x))) "x") ([err] err))
+        "errors should not be swallowed")
+(assert (try ((fn [x] (nil x))) ([err] err))
+        "errors should not be swallowed 2")
+
 # Tuple types
 
 (assert (= (tuple/type '(1 2 3)) :parens) "normal tuple")