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Fix race condition in cycle detector block sent handling #3666

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merged 1 commit into from
Sep 26, 2020
Merged

Fix race condition in cycle detector block sent handling #3666

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3 changes: 3 additions & 0 deletions .release-notes/cd-double-free.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
## Fix race condition that can result in a segfault

We recently introduced some improvements to the handling of garbage collection for short-lived actors when the cycle detector is running. In the process, we introduced a race condition in the runtime that could result in an actor being garbage collected twice which if it were to occur, would result in a crashing program.
11 changes: 9 additions & 2 deletions src/libponyrt/actor/actor.c
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Expand Up @@ -192,9 +192,16 @@ static bool handle_message(pony_ctx_t* ctx, pony_actor_t* actor,
#endif

pony_assert(!ponyint_is_cycle(actor));
if(has_flag(actor, FLAG_BLOCKED) && !has_flag(actor, FLAG_BLOCKED_SENT))
if(has_flag(actor, FLAG_BLOCKED)
&& !has_flag(actor, FLAG_BLOCKED_SENT)
&& (actor->gc.rc > 0))
{
// We're blocked, send block message.
// We're blocked, send block message if:
// - the actor hasn't already sent one
// - the actor aren't a zombie aka rc == 0
//
// Sending multiple "i'm blocked" messages to the cycle detector
// will result in actor potentially being freed more than once.
set_flag(actor, FLAG_BLOCKED_SENT);
pony_assert(ctx->current == actor);
ponyint_cycle_block(actor, &actor->gc);
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