fmonjalet opened a new pull request, #16031:
URL: https://github.com/apache/datafusion/pull/16031
## Which issue does this PR close?
- Closes #16030
## Rationale for this change
Some substrait plans can cause DataFusion to trigger stack overflow,
crashing the hosting process and preventing to run them. This stack overflow is
caused by very large recursion levels. See issue for more details.
This kind of issue may also occur in the SQL --> Logical plan path, but I
did not investigate it yet. The "ideal" fix would be not to rely on recursion
for most operations on plans, but this sounds like an unreasonably large and
complex code change for now.
## What changes are included in this PR?
When transforming a substrait function call to DataFusion logical plan,
if the substrait function maps to a DataFusion binary operator, but has
more than 2 arguments, it is mapped to a tree of BinaryExpr. This
BinaryExpr tree is not balanced, and its depth is the number of
arguments:
```
Op
/ \
arg1 Op
/ \
arg2 ...
/ \
argN Op
```
Since many functions manipulating the logical plan are recursive, it
means that N arguments result in an `O(N)` recursion, leading to stack
overflows for large N (1000 for example).
Transforming these function calls into a balanced tree mitigates the
issue:
```
.__ Op __.
/ \
Op Op
/ \ / \
... ... ... ...
/ \ / \ / \ / \
arg1 ... argN
```
The recursion depth is now `O(log2(N))`, meaning that 1000 arguments
results in a depth of ~10, and it would take 2^1000 arguments to reach a
depth of 1000, which is a vastly unreasonable amount of data.
Therefore, it's not possible to use this flaw anymore to trigger stack
overflows in processes running DataFusion.
The new balanced tree preserves the order of execution (`arg1` evaluated
before `arg2`) and has the same order of magnitude of operator nodes.
The main change is that there is now at least `O(log2(N))` operators
evaluated,
whereas before the minimum was 1, if the first operator exited after
evaluating
only its left side.
## Are these changes tested?
Yes
## Are there any user-facing changes?
No, the change to the logical plan is semantically equivalent and should not
be exposed to users.
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