fix(spans): Avoid unpack() crashes when merging sets#113442
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unpack() can crash if it gets more than 200 arguments. ref INC-2130
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lvthanh03
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Apr 20, 2026
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| -- at dest_key can be massive. if it is, SUNIONSTORE will copy the | ||
| -- entire target set again, which appears to be _worse_ than | ||
| -- copying the source set into lua memory and out again. | ||
| redis.call("sadd", dest_key, unpack(members)) | ||
| end | ||
| until cursor == "0" | ||
| end | ||
|
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| local num_spans = ARGV[1] | ||
| local parent_span_id = ARGV[2] | ||
| local has_root_span = ARGV[3] == "true" |
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Bug: The unpack(hset_args) call in the Lua script may exceed the argument limit if num_spans is large, as there is no explicit chunking for the hset operation.
Severity: HIGH
Suggested Fix
Apply a similar batching mechanism to the hset call as was done for the sadd calls in the same script. This involves iterating over the arguments in chunks to avoid exceeding the Lua stack limit when calling unpack.
Prompt for AI Agent
Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.
Location: src/sentry/scripts/spans/add-buffer.lua#L43-L70
Potential issue: The Lua script at `src/sentry/scripts/spans/add-buffer.lua` uses
`redis.call("hset", ..., unpack(hset_args))`. The `unpack` function has a stack limit of
around 8000 arguments. Since `hset_args` contains two entries per span, if the number of
spans exceeds approximately 3999, the call will fail with a stack overflow, causing a
runtime crash. While other calls like `sadd` were updated to handle large numbers of
arguments by batching, this `hset` call was not, despite the `max_spans_per_evalsha`
setting defaulting to an unlimited value (0). This appears to be an oversight.
Also affects:
src/sentry/scripts/spans/add-buffer.lua:124
Did we get this right? 👍 / 👎 to inform future reviews.
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we limit EVALSHA on the caller side
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unpack() can crash if it gets more than 200 arguments.
ref INC-2130