Fix NPE race in NettyResponseFuture.cancel (#2042) #2088
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hyperxpro
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May 9, 2025
client/src/main/java/org/asynchttpclient/netty/NettyResponseFuture.java
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Pull Request Overview
This PR fixes a race condition that could lead to a NullPointerException when cancelling a response by copying the channel reference to a local variable.
- Introduces a local variable "ch" to capture the channel in one atomic operation.
- Prevents a TOCTOU race condition by ensuring the local reference remains valid during subsequent operations.
hyperxpro
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May 9, 2025
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Fixes #2042
This is a typical TOCTOU (time-of-check/time-of-use) race https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use.
The NPE was occurring because the channel field could be set to null by another thread between the check and its use:
if (channel != null) { // time-of-check
Channels.setDiscard(channel); // time-of-use
Channels.silentlyCloseChannel(channel);
}
By copying channel into a local variable in one atomic read, we ensure that—even if another thread changes the field—the local reference remains valid.
P.S. It is hard to write a deterministic test that fails consistently, so this PR only includes the code fix.