From 9018f3d51aa615350f0c35595c8692393981ab7f Mon Sep 17 00:00:00 2001
From: Aaditya Srinivasan <aadityasri03@gmail.com>
Date: Fri, 26 Jun 2026 11:06:10 +0530
Subject: [PATCH] GH-50231: Handle unset Substrait extension mapping type

---
 cpp/src/arrow/engine/substrait/serde_test.cc  | 29 +++++++++++++++++++
 .../arrow/engine/substrait/util_internal.h    |  4 ++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/cpp/src/arrow/engine/substrait/serde_test.cc b/cpp/src/arrow/engine/substrait/serde_test.cc
index 039920066afa..4de5480ef3cd 100644
--- a/cpp/src/arrow/engine/substrait/serde_test.cc
+++ b/cpp/src/arrow/engine/substrait/serde_test.cc
@@ -1552,6 +1552,35 @@ TEST(Substrait, InvalidMinimumVersion) {
   ASSERT_RAISES(Invalid, DeserializePlans(*buf, [] { return kNullConsumer; }));
 }
 
+TEST(Substrait, InvalidEmptyExtensionDeclaration) {
+  ASSERT_OK_AND_ASSIGN(auto buf, internal::SubstraitFromJSON("Plan", R"({
+    "version": { "major_number": 9999, "minor_number": 9999, "patch_number": 9999 },
+    "relations": [{
+      "rel": {
+        "read": {
+          "base_schema": {
+            "names": ["A"],
+            "struct": {
+              "types": [{
+                "i32": {}
+              }]
+            }
+          },
+          "named_table": {
+            "names": ["x"]
+          }
+        }
+      }
+    }],
+    "extensionUris": [],
+    "extensions": [
+      {}
+    ]
+  })"));
+
+  ASSERT_RAISES(Invalid, DeserializePlans(*buf, [] { return kNullConsumer; }));
+}
+
 TEST(Substrait, JoinPlanBasic) {
   std::string substrait_json = R"({
   "version": { "major_number": 9999, "minor_number": 9999, "patch_number": 9999 },
diff --git a/cpp/src/arrow/engine/substrait/util_internal.h b/cpp/src/arrow/engine/substrait/util_internal.h
index d812bbf7b85f..2a0e5d55e3c9 100644
--- a/cpp/src/arrow/engine/substrait/util_internal.h
+++ b/cpp/src/arrow/engine/substrait/util_internal.h
@@ -79,7 +79,9 @@ Result<ExtensionSet> GetExtensionSetFromMessage(
       }
 
       default:
-        Unreachable();
+        return Status::Invalid(
+            "Invalid Substrait SimpleExtensionDeclaration: mapping_type is unset or "
+            "unknown");
     }
   }