From 7c77c0c68c2a4def4e60ca07fc713d4110538e15 Mon Sep 17 00:00:00 2001
From: Sotaro Hikita <bering1814@gmail.com>
Date: Sun, 20 Sep 2026 01:33:41 +0900
Subject: [PATCH] GH-1300: Use Locale.ROOT when building C Data Interface
 format strings
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Format.asString built the FixedSizeList, FixedSizeBinary and Decimal
format strings with String.format and no explicit Locale, so JVMs whose
default locale uses digits outside ASCII exported strings such as "+w:٨"
that other Arrow implementations cannot parse.

Closes #1300.
---
 c/src/main/java/org/apache/arrow/c/Format.java  |  8 ++++----
 .../java/org/apache/arrow/c/FormatTest.java     | 17 +++++++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/c/src/main/java/org/apache/arrow/c/Format.java b/c/src/main/java/org/apache/arrow/c/Format.java
index 7ce99614d2..7f546fca2c 100644
--- a/c/src/main/java/org/apache/arrow/c/Format.java
+++ b/c/src/main/java/org/apache/arrow/c/Format.java
@@ -63,10 +63,10 @@ static String asString(ArrowType arrowType) {
         {
           ArrowType.Decimal type = (ArrowType.Decimal) arrowType;
           if (type.getBitWidth() == 128) {
-            return String.format("d:%d,%d", type.getPrecision(), type.getScale());
+            return String.format(Locale.ROOT, "d:%d,%d", type.getPrecision(), type.getScale());
           }
           return String.format(
-              "d:%d,%d,%d", type.getPrecision(), type.getScale(), type.getBitWidth());
+              Locale.ROOT, "d:%d,%d,%d", type.getPrecision(), type.getScale(), type.getBitWidth());
         }
       case Duration:
         {
@@ -88,12 +88,12 @@ static String asString(ArrowType arrowType) {
       case FixedSizeBinary:
         {
           ArrowType.FixedSizeBinary type = (ArrowType.FixedSizeBinary) arrowType;
-          return String.format("w:%d", type.getByteWidth());
+          return String.format(Locale.ROOT, "w:%d", type.getByteWidth());
         }
       case FixedSizeList:
         {
           ArrowType.FixedSizeList type = (ArrowType.FixedSizeList) arrowType;
-          return String.format("+w:%d", type.getListSize());
+          return String.format(Locale.ROOT, "+w:%d", type.getListSize());
         }
       case FloatingPoint:
         {
diff --git a/c/src/test/java/org/apache/arrow/c/FormatTest.java b/c/src/test/java/org/apache/arrow/c/FormatTest.java
index c773324330..3691ca095d 100644
--- a/c/src/test/java/org/apache/arrow/c/FormatTest.java
+++ b/c/src/test/java/org/apache/arrow/c/FormatTest.java
@@ -20,6 +20,7 @@
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.Locale;
 import org.apache.arrow.vector.types.DateUnit;
 import org.apache.arrow.vector.types.FloatingPointPrecision;
 import org.apache.arrow.vector.types.IntervalUnit;
@@ -156,4 +157,20 @@ public void testAsType()
     assertThrows(UnsupportedOperationException.class, () -> Format.asType(":", 0L));
     assertThrows(NumberFormatException.class, () -> Format.asType("w:1,2,3", 0L));
   }
+
+  @Test
+  public void testAsStringIgnoresDefaultLocale() {
+    // Locales that use digits other than 0-9 (Arabic-Indic, Bengali, Devanagari, ...)
+    // must not leak into the format string, which other implementations parse as ASCII.
+    Locale saved = Locale.getDefault();
+    try {
+      Locale.setDefault(Locale.forLanguageTag("ar-EG"));
+      assertEquals("d:10,2", Format.asString(new ArrowType.Decimal(10, 2, 128)));
+      assertEquals("d:10,2,256", Format.asString(new ArrowType.Decimal(10, 2, 256)));
+      assertEquals("w:16", Format.asString(new ArrowType.FixedSizeBinary(16)));
+      assertEquals("+w:8", Format.asString(new ArrowType.FixedSizeList(8)));
+    } finally {
+      Locale.setDefault(saved);
+    }
+  }
 }