GH-1246: validate decimal byte length before writing in setBigEndian

vector/src/main/java/org/apache/arrow/vector/Decimal256Vector.java

@@ -208,6 +208,14 @@ public void setBigEndian(int index, byte[] value) {
     BitVectorHelper.setBit(validityBuffer, index);
     final int length = value.length;
 
+    // Reject an oversized value before any bytes are written. The little-endian path below
+    // copies all `length` bytes into the fixed TYPE_WIDTH slot with unchecked native writes,
+    // so validating after the copy would leave adjacent memory already corrupted.
+    if (length > TYPE_WIDTH) {
+      throw new IllegalArgumentException(
+          "Invalid decimal value length. Valid length in [1 - 32], got " + length);
+    }
+
     // do the bound check.
     valueBuffer.checkBytes((long) index * TYPE_WIDTH, (long) (index + 1) * TYPE_WIDTH);
 
@@ -243,8 +251,6 @@ public void setBigEndian(int index, byte[] value) {
         return;
       }
     }
-    throw new IllegalArgumentException(
-        "Invalid decimal value length. Valid length in [1 - 32], got " + length);
   }
 
   /**
@@ -308,6 +314,11 @@ public void setBigEndianSafe(int index, long start, ArrowBuf buffer, int length)
     handleSafe(index);
     BitVectorHelper.setBit(validityBuffer, index);
 
+    if (length > TYPE_WIDTH) {
+      throw new IllegalArgumentException(
+          "Invalid decimal value length. Valid length in [1 - 32], got " + length);
+    }
+
     // do the bound checks.
     buffer.checkBytes(start, start + length);
     valueBuffer.checkBytes((long) index * TYPE_WIDTH, (long) (index + 1) * TYPE_WIDTH);

vector/src/main/java/org/apache/arrow/vector/DecimalVector.java

@@ -207,6 +207,14 @@ public void setBigEndian(int index, byte[] value) {
     BitVectorHelper.setBit(validityBuffer, index);
     final int length = value.length;
 
+    // Reject an oversized value before any bytes are written. The little-endian path below
+    // copies all `length` bytes into the fixed TYPE_WIDTH slot with unchecked native writes,
+    // so validating after the copy would leave adjacent memory already corrupted.
+    if (length > TYPE_WIDTH) {
+      throw new IllegalArgumentException(
+          "Invalid decimal value length. Valid length in [1 - 16], got " + length);
+    }
+
     // do the bound check.
     valueBuffer.checkBytes((long) index * TYPE_WIDTH, (long) (index + 1) * TYPE_WIDTH);
 
@@ -241,8 +249,6 @@ public void setBigEndian(int index, byte[] value) {
         return;
       }
     }
-    throw new IllegalArgumentException(
-        "Invalid decimal value length. Valid length in [1 - 16], got " + length);
   }
 
   /**
@@ -306,6 +312,11 @@ public void setBigEndianSafe(int index, long start, ArrowBuf buffer, int length)
     handleSafe(index);
     BitVectorHelper.setBit(validityBuffer, index);
 
+    if (length > TYPE_WIDTH) {
+      throw new IllegalArgumentException(
+          "Invalid decimal value length. Valid length in [1 - 16], got " + length);
+    }
+
     // do the bound checks.
     buffer.checkBytes(start, start + length);
     valueBuffer.checkBytes((long) index * TYPE_WIDTH, (long) (index + 1) * TYPE_WIDTH);

vector/src/test/java/org/apache/arrow/vector/TestDecimal256Vector.java

@@ -283,6 +283,25 @@ public void decimalBE2NE() {
     }
   }
 
+  @Test
+  public void setBigEndianOversizedDoesNotCorruptNeighbor() {
+    try (Decimal256Vector decimalVector =
+        TestUtils.newVector(
+            Decimal256Vector.class, "decimal", new ArrowType.Decimal(60, 0, 256), allocator)) {
+      decimalVector.allocateNew(2);
+
+      // A value whose low bytes are all non-zero, stored in the slot right after the target.
+      final BigInteger neighbor = new BigInteger("305419896"); // 0x12345678
+      decimalVector.setBigEndian(1, neighbor.toByteArray());
+
+      // A value longer than the 32-byte decimal must be rejected before anything is written.
+      assertThrows(
+          IllegalArgumentException.class, () -> decimalVector.setBigEndian(0, new byte[40]));
+
+      assertEquals(neighbor, decimalVector.getObject(1).unscaledValue());
+    }
+  }
+
   @Test
   public void setUsingArrowBufOfLEInts() {
     try (Decimal256Vector decimalVector =

vector/src/test/java/org/apache/arrow/vector/TestDecimalVector.java

@@ -18,6 +18,7 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
 
@@ -279,6 +280,25 @@ public void decimalBE2NE() {
     }
   }
 
+  @Test
+  public void setBigEndianOversizedDoesNotCorruptNeighbor() {
+    try (DecimalVector decimalVector =
+        TestUtils.newVector(
+            DecimalVector.class, "decimal", new ArrowType.Decimal(38, 0, 128), allocator)) {
+      decimalVector.allocateNew(2);
+
+      // A value whose low bytes are all non-zero, stored in the slot right after the target.
+      final BigInteger neighbor = new BigInteger("305419896"); // 0x12345678
+      decimalVector.setBigEndian(1, neighbor.toByteArray());
+
+      // A value longer than the 16-byte decimal must be rejected before anything is written.
+      assertThrows(
+          IllegalArgumentException.class, () -> decimalVector.setBigEndian(0, new byte[24]));
+
+      assertEquals(neighbor, decimalVector.getObject(1).unscaledValue());
+    }
+  }
+
   @Test
   public void setUsingArrowBufOfInts() {
     try (DecimalVector decimalVector =