fix: prevent UnionExec panic with empty inputs

[EeshanBembi]

Summary

This PR fixes a panic in UnionExec when constructed with empty inputs, replacing the crash with proper error handling and descriptive error messages.

Fixes: #17052

Problem

When UnionExec::new(vec![]) was called with an empty input vector, it would panic with:

thread '...' panicked at datafusion/physical-plan/src/union.rs:542:24:
index out of bounds: the len is 0 but the index is 0

This occurred because union_schema() directly accessed inputs[0] without checking if the array was empty.

Solution

Core Changes

1. Made UnionExec::new() return Result<Self>:

   • Added validation: returns error if inputs.is_empty()
   • Provides clear error message: "UnionExec requires at least one input"

2. Made union_schema() return Result<SchemaRef>:

   • Added empty input validation before accessing inputs[0]
   • Returns descriptive error: "Cannot create union schema from empty inputs"

3. Updated all call sites (7 files):

   • physical_planner.rs - Core DataFusion integration
   • repartition/mod.rs - Internal dependencies
   • 4 test files - Updated to handle Result return type

Error Handling

• Before: Index out of bounds panic (unhelpful)
• After: Clear error messages that guide users

// Before: panic!
let union = UnionExec::new(vec![]); // PANIC!

// After: proper error handling
match UnionExec::new(vec![]) {
    Ok(_) => { /* use union */ }
    Err(e) => println!("Error: {}", e); // "UnionExec requires at least one input"
}

Testing

Added 4 comprehensive tests:

1. test_union_empty_inputs() - Verifies empty input validation
2. test_union_schema_empty_inputs() - Tests schema creation with empty inputs
3. test_union_single_input() - Ensures single input still works
4. test_union_multiple_inputs_still_works() - Verifies existing functionality unchanged

Test Results:

• ✅ All new tests pass
• ✅ All existing union tests pass (8/8)
• ✅ All physical planner integration tests pass

Backward Compatibility

• Existing functionality unchanged for valid inputs (≥1 input)
• Only adds error handling for previously crashing invalid inputs
• API change: UnionExec::new() now returns Result<Self> instead of Self

This is a breaking change but justified because:

1. The previous behavior (panic) was incorrect
2. Empty inputs are invalid by design (no logical meaning)
3. Consistent with logical Union which requires ≥2 inputs
4. Better error handling improves user experience

Files Changed

• datafusion/physical-plan/src/union.rs - Core fix + tests (main changes)
• datafusion/core/src/physical_planner.rs - Handle Result return
• datafusion/physical-plan/src/repartition/mod.rs - Update internal calls
• 4 test files - Update test utilities and test cases

The fix provides robust error handling while maintaining all existing functionality for valid use cases.

[alamb]

Thanks @EeshanBembi

[alamb]

I think the assertion check for is_err is redundant as unwrap_err will panic if result is not an err

[alamb]

this is technically an API change -- maybe to make it easier on others, we can make a new function called try_new that has the error checking, and deprecate the existing new function per https://datafusion.apache.org/contributor-guide/api-health.html#deprecation-guidelines