From 183982640716658b77c35545de88912cb3453d5f Mon Sep 17 00:00:00 2001
From: Pedro Piccino <pedropiccino@Pedros-Mac-mini.local>
Date: Sat, 11 Apr 2026 22:36:51 -0300
Subject: [PATCH] fix: handle empty String fields in to_byte_array (issue
 #1272)

Empty String fields previously produced [0u8; 32], which caused the
Poseidon hasher to fail with custom program error 1 on-chain.
Now empty strings use a non-zero sentinel byte to avoid this error.

Fixes #1272
---
 program-libs/hasher/src/to_byte_array.rs   | 9 +++++++++
 program-libs/hasher/tests/to_byte_array.rs | 8 ++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/program-libs/hasher/src/to_byte_array.rs b/program-libs/hasher/src/to_byte_array.rs
index a8f1f4edb3..5cd2810768 100644
--- a/program-libs/hasher/src/to_byte_array.rs
+++ b/program-libs/hasher/src/to_byte_array.rs
@@ -180,6 +180,10 @@ impl ToByteArray for String {
 
     /// Max allowed String length is 31 bytes.
     /// For longer strings hash to field size or provide a custom implementation.
+    ///
+    /// Empty strings are represented with a leading 1 byte at index 0 to avoid
+    /// an all-zero `[0u8; 32]` representation, which can cause errors in the
+    /// Poseidon hasher on Solana (see issue #1272).
     fn to_byte_array(&self) -> Result<[u8; 32], HasherError> {
         let bytes = self.as_bytes();
         let mut result = [0u8; 32];
@@ -187,6 +191,11 @@ impl ToByteArray for String {
         if byte_len > 31 {
             return Err(HasherError::InvalidInputLength(31, bytes.len()));
         }
+        if byte_len == 0 {
+            // Use a non-zero sentinel to avoid all-zero representation.
+            // This prevents Poseidon syscall errors on Solana (issue #1272).
+            result[0] = 1;
+        }
         result[32 - byte_len..].copy_from_slice(bytes);
         Ok(result)
     }
diff --git a/program-libs/hasher/tests/to_byte_array.rs b/program-libs/hasher/tests/to_byte_array.rs
index 71b2ad69e8..fd34be1039 100644
--- a/program-libs/hasher/tests/to_byte_array.rs
+++ b/program-libs/hasher/tests/to_byte_array.rs
@@ -216,11 +216,15 @@ fn test_to_byte_array_u8_arrays() {
 
 #[test]
 fn test_to_byte_array_string() {
-    // Test with empty string
+    // Test with empty string - should produce a non-zero representation
+    // to avoid Poseidon errors (issue #1272)
     let empty_string = "".to_string();
     let result = empty_string.to_byte_array().unwrap();
-    let expected = [0u8; 32];
+    let mut expected = [0u8; 32];
+    expected[0] = 1; // non-zero sentinel for empty strings
     assert_eq!(result, expected);
+    // Ensure it's not all zeros
+    assert_ne!(result, [0u8; 32]);
 
     // Test with short string
     let short_string = "foobar".to_string();