Add method to create Asn1Time from time_t value

This is mostly just a rework of the earlier work done by @illegalprime
in his PR #673 and credit should go to him.

openssl-sys/src/asn1.rs

@@ -43,6 +43,7 @@ extern "C" {
     pub fn ASN1_TIME_diff(pday: *mut c_int, psec: *mut c_int, from: *const ASN1_TIME, to: *const ASN1_TIME) -> c_int;
     pub fn ASN1_TIME_free(tm: *mut ASN1_TIME);
     pub fn ASN1_TIME_print(b: *mut BIO, tm: *const ASN1_TIME) -> c_int;
+    pub fn ASN1_TIME_set(from: *mut ASN1_TIME, to: time_t) -> *mut ASN1_TIME;
 
     pub fn ASN1_INTEGER_free(x: *mut ASN1_INTEGER);
     pub fn ASN1_INTEGER_get(dest: *const ASN1_INTEGER) -> c_long;

openssl/src/asn1.rs

@@ -26,7 +26,7 @@
 //! ```
 use ffi;
 use foreign_types::{ForeignType, ForeignTypeRef};
-use libc::{c_char, c_int, c_long};
+use libc::{c_char, c_int, c_long, time_t};
 #[cfg(ossl102)]
 use std::cmp::Ordering;
 use std::ffi::CString;
@@ -237,6 +237,16 @@ impl Asn1Time {
         Asn1Time::from_period(days as c_long * 60 * 60 * 24)
     }
 
+    /// Creates a new time from the specified `time_t` value
+    pub fn from_unix(time: time_t) -> Result<Asn1Time, ErrorStack> {
+        ffi::init();
+
+        unsafe {
+            let handle = cvt_p(ffi::ASN1_TIME_set(ptr::null_mut(), time))?;
+            Ok(Asn1Time::from_ptr(handle))
+        }
+    }
+
     /// Creates a new time corresponding to the specified ASN1 time string.
     ///
     /// This corresponds to [`ASN1_TIME_set_string`].
@@ -542,6 +552,12 @@ mod tests {
         Asn1Time::from_str_x509("99991231235959Z").unwrap();
     }
 
+    #[test]
+    fn time_from_unix() {
+        let t = Asn1Time::from_unix(0).unwrap();
+        assert_eq!("Jan  1 00:00:00 1970 GMT", t.to_string());
+    }
+
     #[test]
     #[cfg(ossl102)]
     fn time_eq() {