MERGE should raise when multiple source rows match target row

[mrjsj]

Environment

Delta-rs version: 0.16.4

Binding: Python 0.16.4

Environment:

• Cloud provider: None
• OS: MacOS Sonoma 14.4
• Other:

---

Bug

What happened:
TableMerger.when_matched_update_all() inserts records when there is a match on multiple source columns
TableMerger.when_matched_update() inserts records when there is a match on multiple source columns

What you expected to happen:
TableMerger.when_matched_update_all() should throw an error if a target record matches multiple source records
TableMerger.when_matched_update() should throw an error if a target record matches multiple source records

They should never insert new records.

How to reproduce it:
Using polars 0.20.7

import polars as pl
from deltalake import DeltaTable

base_df = pl.DataFrame(
    {
        "id": [1, 2],
        "attr": ["x", "y"]
    }
)

base_df.write_delta("./test_cdc", mode="overwrite")

dt = DeltaTable("./test_cdc")

print(pl.DataFrame(dt.to_pyarrow_table()))

cdc_df = pl.DataFrame(
    {
        "id": [1,1,1,2,2],
        "attr": ["a","b","c","d","e"],
        "op": ["U", "U", "U", "U", "U"]
    }
)


(
    dt.merge(
        cdc_df.to_arrow(),
        "s.id = t.id",
        source_alias="s",
        target_alias="t",
    )
    .when_matched_update(
        updates={"t.attr": "s.attr"},
        predicate="s.op = 'U'")
    .execute()
)

print(pl.DataFrame(dt.to_pyarrow_table()))

Gives the following output

base table

shape: (2, 2)
┌─────┬──────┐
│ id  ┆ attr │
│ --- ┆ ---  │
│ i64 ┆ str  │
╞═════╪══════╡
│ 1   ┆ x    │
│ 2   ┆ y    │
└─────┴──────┘

After TableMerger is executed

shape: (5, 2)
┌─────┬──────┐
│ id  ┆ attr │
│ --- ┆ ---  │
│ i64 ┆ str  │
╞═════╪══════╡
│ 2   ┆ d    │
│ 2   ┆ e    │
│ 1   ┆ a    │
│ 1   ┆ b    │
│ 1   ┆ c    │
└─────┴──────┘

More details:
Info on the specific case in slack: https://delta-users.slack.com/archives/C013LCAEB98/p1712673309723829