Avoid null pointer exceptions from empty tables without snapshot

[raunaqmorarka]

Description

It is possible for optimize_manifests to fail

{"type":"java.lang.NullPointerException","message":"Cannot invoke \"org.apache.iceberg.Snapshot.allManifests(org.apache.iceberg.io.FileIO)\" because \"snapshot\" is null","suppressed":[],"stack":["io.trino.plugin.iceberg.IcebergMetadata.loadAllManifestsFromSnapshot(IcebergMetadata.java:4183)","io.trino.plugin.iceberg.IcebergMetadata.executeOptimizeManifests

Additional context and related issues

Release notes

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## Iceberg
* Fix failure in `optimize_manifests` for empty tables. ({issue}`26970`)

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Add a null guard for snapshot in loadAllManifestsFromSnapshot to return an empty manifest list when snapshot is null, preventing NullPointerException during optimize_manifests on empty tables.

Sequence diagram for manifest loading with null snapshot guard

sequenceDiagram
participant "IcebergMetadata"
participant "Table"
participant "Snapshot"
participant "ManifestFile List"

"IcebergMetadata"->>"Snapshot": Check if snapshot is null
alt snapshot is null
    "IcebergMetadata"->>"ManifestFile List": Return empty list
else snapshot is not null
    "IcebergMetadata"->>"Table": Get FileIO
    "IcebergMetadata"->>"Snapshot": allManifests(FileIO)
    "Snapshot"-->>"IcebergMetadata": List<ManifestFile>
end

Loading

Class diagram for updated loadAllManifestsFromSnapshot null guard

classDiagram
class IcebergMetadata {
    +List<ManifestFile> loadAllManifestsFromSnapshot(Table icebergTable, Snapshot snapshot)
}
class Snapshot {
    +List<ManifestFile> allManifests(FileIO io)
}
class Table {
    +FileIO io()
}
IcebergMetadata --> Snapshot : uses
IcebergMetadata --> Table : uses
Snapshot --> ManifestFile : returns list
Table --> FileIO : returns

%% Highlight: loadAllManifestsFromSnapshot now returns ImmutableList.of() if snapshot is null

Loading

File-Level Changes

Change	Details	Files
Null-check snapshot in loadAllManifestsFromSnapshot to prevent NPE	Insert an early return when snapshot is null Return an empty ImmutableList for null snapshots	plugin/trino-iceberg/src/main/java/io/trino/plugin/iceberg/IcebergMetadata.java

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[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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[grantatspothero]

Thank you for fixing this.

[wendigo]

Can we add a test?

[raunaqmorarka]

Can we add a test?

We already have a test for an empty table in io.trino.plugin.iceberg.procedure.TestIcebergOptimizeManifestsProcedure#testEmptyManifest. However, CREATE through Trino always creates a snapshot, I couldn't find a straightforward way of simulating this scenario.