Add static mutex lock for mutable error handler fn handle

Add tests directory with a function to check the function
`register_error_handler`

Author: 9prady9

Cargo.toml

@@ -15,6 +15,7 @@ build         = "build.rs"
 libc = "0.1.10"
 num  = "0.1.27"
 time = "0.1.32"
+lazy_static = "0.1.*"
 
 [build-dependencies]
 rustc-serialize = "0.3.16"

src/error.rs

@@ -1,11 +1,19 @@
+
 use defines::AfError;
 use std::error::Error;
+use std::sync::Mutex;
 
 pub type ErrorCallback = Fn(AfError);
 
 pub static mut HANDLE_ERROR: &'static ErrorCallback = &handle_error_general; 
 
+lazy_static! {
+    static ref HANDLE_ERROR_LOCK: Mutex<i32> = Mutex::new(0);
+}
+
+#[allow(unused_must_use)]
 pub fn register_error_handler(callback: &'static ErrorCallback) {
+    HANDLE_ERROR_LOCK.lock().unwrap();
     unsafe {
         HANDLE_ERROR = callback;
     }

src/lib.rs

@@ -2,6 +2,9 @@
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://arrayfire.com/docs/rust")]
 
+#[macro_use]
+extern crate lazy_static;
+
 pub use array::Array;
 pub use array::{print};
 mod array;
@@ -50,7 +53,7 @@ mod defines;
 pub use dim4::Dim4;
 mod dim4;
 
-pub use error::{register_error_handler,handle_error_general};
+pub use error::{ErrorCallback, register_error_handler,handle_error_general};
 mod error;
 
 pub use index::{Indexer, index, row, rows, col, cols, slice, slices,

tests/lib.rs

@@ -0,0 +1,42 @@
+extern crate arrayfire as af;
+
+use std::error::Error;
+use std::thread;
+use std::time::Duration;
+use af::*;
+
+pub fn handler_sample1(error_code: AfError) {
+    println!("Error handler sample1");
+    match error_code {
+        AfError::SUCCESS => {}, /* No-op */
+        _ => panic!("Error message: {}", error_code.description()),
+    }
+}
+
+pub fn handler_sample2(error_code: AfError) {
+    println!("Error handler sample2");
+    match error_code {
+        AfError::SUCCESS => {}, /* No-op */
+        _ => panic!("Error message: {}", error_code.description()),
+    }
+}
+
+pub static HANDLE1: &'static ErrorCallback = &handler_sample1;
+pub static HANDLE2: &'static ErrorCallback = &handler_sample2;
+
+#[test]
+fn check_error_handler_mutation() {
+
+    for i in 0..4 {
+        thread::spawn(move || {
+            if i%2==0 {
+                register_error_handler(HANDLE2);
+            } else {
+                register_error_handler(HANDLE1);
+            }
+        });
+    }
+
+    thread::sleep(Duration::from_millis(50));
+
+}