feat(capi): add mechanism to get panic message as const char *

Previously, when an error occurred in the rust side, the panic message
would get printed to stderr, then the c function would return 1 to
indicate error.

This commit adds the ability to disable the automatic prints of panic
messages and adds functions to get the panic message as a const char *
to allow user better control on how to display error messages.

tfhe/c_api_tests/test_high_level_error.c

@@ -0,0 +1,60 @@
+// If this test break the c_api doc needs to be updated
+
+#include "tfhe.h"
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+int main(void) {
+  tfhe_error_disable_automatic_prints();
+
+  int ok = 0;
+  // Prepare the config builder for the high level API and choose which types to enable
+  ConfigBuilder *builder;
+  Config *config;
+
+  config_builder_default(&builder);
+  config_builder_build(builder, &config);
+
+  ClientKey *client_key = NULL;
+  ServerKey *server_key = NULL;
+
+  generate_keys(config, &client_key, &server_key);
+
+  // Intentionally forget the set_server_key to test error
+  //  set_server_key(server_key);
+
+  FheUint128 *lhs = NULL;
+  FheUint128 *rhs = NULL;
+  FheUint128 *result = NULL;
+
+  U128 clear_lhs = {.w0 = 10, .w1 = 20};
+  U128 clear_rhs = {.w0 = 1, .w1 = 2};
+
+  ok = fhe_uint128_try_encrypt_with_client_key_u128(clear_lhs, client_key, &lhs);
+  assert(ok == 0);
+
+  ok = fhe_uint128_try_encrypt_with_client_key_u128(clear_rhs, client_key, &rhs);
+  assert(ok == 0);
+
+  const char *last_error = tfhe_error_get_last();
+  assert(last_error != NULL);
+  assert(strcmp(last_error, "no error") == 0);
+
+  // Compute the subtraction
+  ok = fhe_uint128_sub(lhs, rhs, &result);
+  assert(ok == 1);
+
+  last_error = tfhe_error_get_last();
+  assert(last_error != NULL);
+  printf("Error message Received from tfhe-rs: '%s'\n", last_error);
+
+  // Destroy the ciphertexts
+  fhe_uint128_destroy(lhs);
+  fhe_uint128_destroy(rhs);
+
+  // Destroy the keys
+  client_key_destroy(client_key);
+  server_key_destroy(server_key);
+
+  return EXIT_SUCCESS;
+}

tfhe/src/c_api/error.rs

@@ -0,0 +1,116 @@
+use std::any::Any;
+use std::cell::RefCell;
+use std::ffi::{c_char, CString};
+use std::panic::AssertUnwindSafe;
+
+enum LastError {
+    None,
+    Message(CString),
+    // Only used when there was a memory panic happens
+    // when trying to store the last panic message into the CString above
+    NoMemory,
+}
+
+impl LastError {
+    const NO_MEMORY_MSG: [c_char; 10] = [
+        b'n' as c_char,
+        b'o' as c_char,
+        b' ' as c_char,
+        b'm' as c_char,
+        b'e' as c_char,
+        b'm' as c_char,
+        b'o' as c_char,
+        b'r' as c_char,
+        b'y' as c_char,
+        b'\0' as c_char,
+    ];
+
+    const NO_ERROR_MSG: [c_char; 9] = [
+        b'n' as c_char,
+        b'o' as c_char,
+        b' ' as c_char,
+        b'e' as c_char,
+        b'r' as c_char,
+        b'r' as c_char,
+        b'o' as c_char,
+        b'r' as c_char,
+        b'\0' as c_char,
+    ];
+
+    fn as_ptr(&self) -> *const c_char {
+        match self {
+            Self::None => Self::NO_ERROR_MSG.as_ptr(),
+            Self::Message(cstring) => cstring.as_ptr(),
+            Self::NoMemory => Self::NO_MEMORY_MSG.as_ptr(),
+        }
+    }
+
+    /// Does not include the nul-byte
+    fn len(&self) -> usize {
+        match self {
+            Self::None => Self::NO_ERROR_MSG.len() - 1,
+            Self::Message(cstring) => cstring.as_bytes().len(),
+            Self::NoMemory => Self::NO_MEMORY_MSG.len() - 1,
+        }
+    }
+}
+
+std::thread_local! {
+   pub(in crate::c_api) static LAST_LOCAL_ERROR: RefCell<LastError> = const { RefCell::new(LastError::None) };
+}
+
+pub(in crate::c_api) fn replace_last_error_with_panic_payload(payload: &Box<dyn Any + Send>) {
+    LAST_LOCAL_ERROR.with(|local_error| {
+        let _previous_error = local_error.replace(panic_payload_to_error(payload));
+    });
+}
+
+fn panic_payload_to_error(payload: &Box<dyn Any + Send>) -> LastError {
+    // Add a catch panic as technically the to_vec could fail
+    let result = std::panic::catch_unwind(AssertUnwindSafe(|| {
+        // Rust doc says:
+        // An invocation of the panic!() macro in Rust 2021 or later
+        // will always result in a panic payload of type &'static str or String.
+        payload
+            .downcast_ref::<&str>()
+            .map_or_else(|| b"panic occurred".to_vec(), |s| s.as_bytes().to_vec())
+    }));
+
+    result.map_or_else(
+        |_| LastError::NoMemory,
+        |bytes| LastError::Message(CString::new(bytes).unwrap()),
+    )
+}
+
+/// Returns a pointer to a nul-terminated string describing the last error in the thread.
+///
+/// * This pointer is only valid for as long as no other tfhe-rs function are called within this
+///   thread.
+///
+/// This should be used to directly print/display or copy the error.
+#[no_mangle]
+pub unsafe extern "C" fn tfhe_error_get_last() -> *const c_char {
+    LAST_LOCAL_ERROR.with_borrow(|last_error| last_error.as_ptr())
+}
+
+/// Returns the length of the current error message stored
+///
+/// The length **DOES NOT INCLUDE** the nul byte
+#[no_mangle]
+pub unsafe extern "C" fn tfhe_error_get_size() -> usize {
+    LAST_LOCAL_ERROR.with_borrow(|last_error| last_error.len())
+}
+
+/// Disables panic prints to stderr when a thread panics
+#[no_mangle]
+pub unsafe extern "C" fn tfhe_error_disable_automatic_prints() {
+    std::panic::set_hook(Box::new(|_panic_info| {}));
+}
+
+/// Enables panic prints to stderr when a thread panics
+#[no_mangle]
+pub unsafe extern "C" fn tfhe_error_enable_automatic_prints() {
+    // if the current hook is the default one, 'taking' it
+    // will still make is registered
+    let _ = std::panic::take_hook();
+}

tfhe/src/c_api/mod.rs

@@ -3,6 +3,7 @@
 pub mod boolean;
 pub mod buffer;
 pub mod core_crypto;
+pub mod error;
 #[cfg(feature = "high-level-c-api")]
 pub mod high_level_api;
 #[cfg(feature = "shortint-c-api")]

tfhe/src/c_api/utils.rs

@@ -6,7 +6,10 @@ where
 {
     match std::panic::catch_unwind(std::panic::AssertUnwindSafe(closure)) {
         Ok(_) => 0,
-        _ => 1,
+        Err(err) => {
+            super::error::replace_last_error_with_panic_payload(&err);
+            1
+        }
     }
 }
 

tfhe/src/high_level_api/errors.rs

@@ -32,7 +32,7 @@ impl Display for UninitializedServerKey {
         write!(
             f,
             "The server key was not properly initialized.\n\
-             Did you forget to call `set_server_key` in the current thread ?
+             Did you forget to call `set_server_key` in the current thread ?\
             ",
         )
     }