From 377bda4da3591051a3f357e246da28704ae965aa Mon Sep 17 00:00:00 2001
From: Anas Khan <83116240+anxkhn@users.noreply.github.com>
Date: Sun, 5 Jul 2026 05:08:54 +0530
Subject: [PATCH] Core: Fix AvroEncoderUtil header error printing literal %02X

AvroEncoderUtil.decode validated the magic bytes with
Preconditions.checkState(cond, "Unrecognized header bytes: 0x%02X 0x%02X",
header0, header1). Guava's Preconditions formats messages with
Strings.lenientFormat, which only understands the %s placeholder, so the
%02X specifiers were emitted verbatim and the two byte arguments were
appended as decimal in a trailing bracket. A mismatched header therefore
reported "Unrecognized header bytes: 0x%02X 0x%02X [16, 32]" instead of the
intended hex, obscuring the actual bytes.

Build the message with String.format(Locale.ROOT, ...) and pass it as the
single %s argument, matching the existing handling in IcebergDecoder.decode.
Add a regression test asserting the uppercase hex is rendered.

Generated-by: opencode
Signed-off-by: Anas Khan <83116240+anxkhn@users.noreply.github.com>
---
 .../java/org/apache/iceberg/avro/AvroEncoderUtil.java    | 6 +++---
 .../org/apache/iceberg/avro/TestAvroEncoderUtil.java     | 9 +++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/core/src/main/java/org/apache/iceberg/avro/AvroEncoderUtil.java b/core/src/main/java/org/apache/iceberg/avro/AvroEncoderUtil.java
index 0db8d7dd5f9f..97892bf8fcb2 100644
--- a/core/src/main/java/org/apache/iceberg/avro/AvroEncoderUtil.java
+++ b/core/src/main/java/org/apache/iceberg/avro/AvroEncoderUtil.java
@@ -23,6 +23,7 @@
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
+import java.util.Locale;
 import org.apache.avro.LogicalTypes;
 import org.apache.avro.Schema;
 import org.apache.avro.io.BinaryDecoder;
@@ -73,9 +74,8 @@ public static <T> T decode(byte[] data) throws IOException {
       byte header1 = dataInput.readByte();
       Preconditions.checkState(
           header0 == MAGIC_BYTES[0] && header1 == MAGIC_BYTES[1],
-          "Unrecognized header bytes: 0x%02X 0x%02X",
-          header0,
-          header1);
+          "%s",
+          String.format(Locale.ROOT, "Unrecognized header bytes: 0x%02X 0x%02X", header0, header1));
 
       // Read avro schema
       Schema avroSchema = new Schema.Parser().parse(dataInput.readUTF());
diff --git a/core/src/test/java/org/apache/iceberg/avro/TestAvroEncoderUtil.java b/core/src/test/java/org/apache/iceberg/avro/TestAvroEncoderUtil.java
index 456093501b72..0b853971706c 100644
--- a/core/src/test/java/org/apache/iceberg/avro/TestAvroEncoderUtil.java
+++ b/core/src/test/java/org/apache/iceberg/avro/TestAvroEncoderUtil.java
@@ -19,6 +19,7 @@
 package org.apache.iceberg.avro;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 import java.io.IOException;
 import java.util.List;
@@ -27,6 +28,7 @@
 import org.apache.avro.generic.GenericData;
 import org.apache.iceberg.data.DataTestBase;
 import org.apache.iceberg.types.Type;
+import org.junit.jupiter.api.Test;
 
 public class TestAvroEncoderUtil extends DataTestBase {
   @Override
@@ -68,4 +70,11 @@ protected void writeAndValidate(org.apache.iceberg.Schema schema) throws IOExcep
       assertThat(record.toString()).isEqualTo(expectedRecord.toString());
     }
   }
+
+  @Test
+  public void decodeRejectsUnrecognizedHeaderBytes() {
+    assertThatThrownBy(() -> AvroEncoderUtil.decode(new byte[] {(byte) 0x10, (byte) 0x20}))
+        .isInstanceOf(IllegalStateException.class)
+        .hasMessage("Unrecognized header bytes: 0x10 0x20");
+  }
 }