GH-50462: [C++][Gandiva] fix out-of-bounds read in translate_utf8_utf8_utf8

cpp/src/gandiva/gdv_function_stubs_test.cc

@@ -1202,6 +1202,23 @@ TEST(TestGdvFnStubs, TestTranslate) {
   EXPECT_STREQ(result, "");
   EXPECT_THAT(ctx.get_error(),
               ::testing::HasSubstr("Would overflow maximum output size"));
+
+  // A byte > 127 selects the multi-byte path. A truncated trailing glyph (0xE2
+  // claims a 3-byte character but only one byte is present) must not be read past
+  // the end of the input; it is passed through as a single byte. Exact-sized
+  // buffers let ASAN catch any over-read here.
+  const char truncated_in[] = {'a', static_cast<char>(0xE2)};
+  result = translate_utf8_utf8_utf8(ctx_ptr, truncated_in, 2, "x", 1, "y", 1, &out_len);
+  EXPECT_EQ(std::string(truncated_in, 2), std::string(result, out_len));
+
+  // A valid multi-byte input character absent from FROM previously over-read FROM
+  // by one byte at the end-of-list sentinel.
+  const char euro[] = {static_cast<char>(0xE2), static_cast<char>(0x82),
+                       static_cast<char>(0xAC)};
+  const char from_one[] = {'a'};
+  const char to_one[] = {'b'};
+  result = translate_utf8_utf8_utf8(ctx_ptr, euro, 3, from_one, 1, to_one, 1, &out_len);
+  EXPECT_EQ(std::string(euro, 3), std::string(result, out_len));
 }
 
 TEST(TestGdvFnStubs, TestToUtcTimezone) {

cpp/src/gandiva/gdv_string_function_stubs.cc

@@ -779,6 +779,11 @@ const char* translate_utf8_utf8_utf8(int64_t context, const char* in, int32_t in
     for (int32_t in_for = 0; in_for < in_len; in_for += len_char_in) {
       // Updating len to char in this position
       len_char_in = gdv_fn_utf8_char_length(in[in_for]);
+      // A truncated or invalid lead byte at the tail would make the copy below read
+      // past IN, and a zero width would spin the loop forever; consume one byte.
+      if (len_char_in == 0 || in_for + len_char_in > in_len) {
+        len_char_in = 1;
+      }
       // Making copy to std::string with length for this char position
       std::string insert_copy_key(in + in_for, len_char_in);
       if (subs_list.find(insert_copy_key) != subs_list.end()) {
@@ -790,10 +795,6 @@ const char* translate_utf8_utf8_utf8(int64_t context, const char* in, int32_t in
         }
       } else {
         for (int32_t from_for = 0; from_for <= from_len; from_for += len_char_from) {
-          // Updating len to char in this position
-          len_char_from = gdv_fn_utf8_char_length(from[from_for]);
-          // Making copy to std::string with length for this char position
-          std::string copy_from_compare(from + from_for, len_char_from);
           if (from_for == from_len) {
             // If it's not in the FROM list, just add it to the map and the result.
             std::string insert_copy_value(in + in_for, len_char_in);
@@ -806,6 +807,15 @@ const char* translate_utf8_utf8_utf8(int64_t context, const char* in, int32_t in
             break;
           }
 
+          // Updating len to char in this position
+          len_char_from = gdv_fn_utf8_char_length(from[from_for]);
+          // Clamp a truncated or invalid lead byte to the remaining bytes so the copy
+          // below never reads past FROM and the loop always advances.
+          if (len_char_from == 0 || from_for + len_char_from > from_len) {
+            len_char_from = 1;
+          }
+          // Making copy to std::string with length for this char position
+          std::string copy_from_compare(from + from_for, len_char_from);
           if (insert_copy_key != copy_from_compare) {
             // If this character does not exist in FROM list, don't need treatment
             continue;
@@ -818,6 +828,10 @@ const char* translate_utf8_utf8_utf8(int64_t context, const char* in, int32_t in
             // If exist and the start_compare is in range, add to map with the
             // corresponding TO in position start_compare
             len_char_to = gdv_fn_utf8_char_length(to[start_compare]);
+            // Clamp a truncated or invalid lead byte to the remaining bytes of TO.
+            if (len_char_to == 0 || start_compare + len_char_to > to_len) {
+              len_char_to = 1;
+            }
             std::string insert_copy_value(to + start_compare, len_char_to);
             // Insert in map to next loops
             subs_list.insert(