From 66ab70f99178f816262584755cd3a0eadbcc7457 Mon Sep 17 00:00:00 2001
From: Elichai Turkel <elichai.turkel@gmail.com>
Date: Fri, 16 Aug 2019 14:48:03 -0400
Subject: [PATCH] Added a C Ptr trait that returns a null pointer for ZSTs

---
 src/ffi.rs          | 34 +++++++++++++++++++++++++++++++++-
 src/key.rs          | 14 +++++++++++++-
 src/lib.rs          | 12 ++++++++++++
 src/macros.rs       | 18 ++++++++++++++++++
 src/recovery/mod.rs | 19 +++++++++++++++++++
 5 files changed, 95 insertions(+), 2 deletions(-)

diff --git a/src/ffi.rs b/src/ffi.rs
index 3f28490..fab655d 100644
--- a/src/ffi.rs
+++ b/src/ffi.rs
@@ -16,7 +16,7 @@
 //! # FFI bindings
 //! Direct bindings to the underlying C library functions. These should
 //! not be needed for most users.
-use core::{mem, hash, slice};
+use core::{mem, hash, slice, ptr};
 use types::*;
 
 /// Flag for context to enable no precomputation
@@ -366,6 +366,38 @@ unsafe fn strlen(mut str_ptr: *const c_char) -> usize {
 }
 
 
+/// A trait for producing pointers that will always be valid in C. (assuming NULL pointer is a valid no-op)
+/// Rust doesn't promise what pointers does it give to ZST (https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts)
+/// In case the type is empty this trait will give a NULL pointer, which should be handled in C.
+/// 
+pub(crate) trait CPtr {
+    type Target;
+    fn as_c_ptr(&self) -> *const Self::Target;
+    fn as_mut_c_ptr(&mut self) -> *mut Self::Target;
+}
+
+impl<T> CPtr for [T] {
+    type Target = T;
+    fn as_c_ptr(&self) -> *const Self::Target {
+        if self.is_empty() {
+            ptr::null()
+        } else {
+            self.as_ptr()
+        }
+    }
+
+    fn as_mut_c_ptr(&mut self) -> *mut Self::Target {
+        if self.is_empty() {
+            ptr::null::<Self::Target>() as *mut _
+        } else {
+            self.as_mut_ptr()
+        }
+    }
+}
+
+
+
+
 #[cfg(feature = "fuzztarget")]
 mod fuzz_dummy {
     extern crate std;
diff --git a/src/key.rs b/src/key.rs
index 85eb92e..e4464ff 100644
--- a/src/key.rs
+++ b/src/key.rs
@@ -24,7 +24,7 @@ use super::Error::{self, InvalidPublicKey, InvalidSecretKey};
 use Signing;
 use Verification;
 use constants;
-use ffi;
+use ffi::{self, CPtr};
 
 /// Secret 256-bit key used as `x` in an ECDSA signature
 pub struct SecretKey([u8; constants::SECRET_KEY_SIZE]);
@@ -355,6 +355,18 @@ impl PublicKey {
     }
 }
 
+impl CPtr for PublicKey {
+    type Target = ffi::PublicKey;
+    fn as_c_ptr(&self) -> *const Self::Target {
+        self.as_ptr()
+    }
+
+    fn as_mut_c_ptr(&mut self) -> *mut Self::Target {
+        self.as_mut_ptr()
+    }
+}
+
+
 /// Creates a new public key from a FFI public key
 impl From<ffi::PublicKey> for PublicKey {
     #[inline]
diff --git a/src/lib.rs b/src/lib.rs
index fd24ab6..450e193 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -161,6 +161,7 @@ pub use key::PublicKey;
 pub use context::*;
 use core::marker::PhantomData;
 use core::ops::Deref;
+use ffi::CPtr;
 
 /// An ECDSA signature
 #[derive(Copy, Clone, PartialEq, Eq)]
@@ -380,6 +381,17 @@ impl Signature {
     }
 }
 
+impl CPtr for Signature {
+    type Target = ffi::Signature;
+    fn as_c_ptr(&self) -> *const Self::Target {
+        self.as_ptr()
+    }
+
+    fn as_mut_c_ptr(&mut self) -> *mut Self::Target {
+        self.as_mut_ptr()
+    }
+}
+
 /// Creates a new signature from a FFI signature
 impl From<ffi::Signature> for Signature {
     #[inline]
diff --git a/src/macros.rs b/src/macros.rs
index 3b41467..6a8354f 100644
--- a/src/macros.rs
+++ b/src/macros.rs
@@ -122,6 +122,24 @@ macro_rules! impl_array_newtype {
                 &dat[..]
             }
         }
+        impl ::ffi::CPtr for $thing {
+            type Target = $ty;
+            fn as_c_ptr(&self) -> *const Self::Target {
+                if self.is_empty() {
+                    ::core::ptr::null()
+                } else {
+                    self.as_ptr()
+                }
+            }
+
+            fn as_mut_c_ptr(&mut self) -> *mut Self::Target {
+                if self.is_empty() {
+                    ::core::ptr::null::<Self::Target>() as *mut _
+                } else {
+                    self.as_mut_ptr()
+                }
+            }
+        }
     }
 }
 
diff --git a/src/recovery/mod.rs b/src/recovery/mod.rs
index aecefb1..bf0c9f6 100644
--- a/src/recovery/mod.rs
+++ b/src/recovery/mod.rs
@@ -23,6 +23,7 @@ use super::{Secp256k1, Message, Error, Signature, Verification, Signing};
 use super::ffi as super_ffi;
 pub use key::SecretKey;
 pub use key::PublicKey;
+use self::super_ffi::CPtr;
 
 mod ffi;
 
@@ -82,6 +83,12 @@ impl RecoverableSignature {
         &self.0 as *const _
     }
 
+    /// Obtains a raw mutable pointer suitable for use with FFI functions
+    #[inline]
+    pub fn as_mut_ptr(&mut self) -> *mut ffi::RecoverableSignature {
+        &mut self.0 as *mut _
+    }
+
     #[inline]
     /// Serializes the recoverable signature in compact format
     pub fn serialize_compact(&self) -> (RecoveryId, [u8; 64]) {
@@ -116,6 +123,18 @@ impl RecoverableSignature {
     }
 }
 
+
+impl CPtr for RecoverableSignature {
+    type Target = ffi::RecoverableSignature;
+    fn as_c_ptr(&self) -> *const Self::Target {
+        self.as_ptr()
+    }
+
+    fn as_mut_c_ptr(&mut self) -> *mut Self::Target {
+        self.as_mut_ptr()
+    }
+}
+
 /// Creates a new recoverable signature from a FFI one
 impl From<ffi::RecoverableSignature> for RecoverableSignature {
     #[inline]