Entry::new returns Err when entry point isn't found (#390)

* Check for entry point in EntryCustom::new and return error

* Split off into load_checked

* Remove string from MissingEntryPoint

* Change MissingEntryPoint error message

ash/src/entry.rs

@@ -3,6 +3,7 @@ use crate::prelude::*;
 use crate::vk;
 use crate::RawPtr;
 use std::error::Error;
+use std::ffi::CStr;
 use std::fmt;
 use std::mem;
 use std::os::raw::c_char;
@@ -163,11 +164,15 @@ impl<L> EntryV1_2 for EntryCustom<L> {
 }
 
 impl<L> EntryCustom<L> {
-    pub fn new_custom<Load>(mut lib: L, mut load: Load) -> Self
+    pub fn new_custom<Load>(
+        mut lib: L,
+        mut load: Load,
+    ) -> std::result::Result<Self, MissingEntryPoint>
     where
         Load: FnMut(&mut L, &::std::ffi::CStr) -> *const c_void,
     {
-        let static_fn = vk::StaticFn::load(|name| load(&mut lib, name));
+        // Bypass the normal StaticFn::load so we can return an error
+        let static_fn = vk::StaticFn::load_checked(|name| load(&mut lib, name))?;
 
         let entry_fn_1_0 = vk::EntryFnV1_0::load(|name| unsafe {
             mem::transmute(static_fn.get_instance_proc_addr(vk::Instance::null(), name.as_ptr()))
@@ -181,13 +186,13 @@ impl<L> EntryCustom<L> {
             mem::transmute(static_fn.get_instance_proc_addr(vk::Instance::null(), name.as_ptr()))
         });
 
-        EntryCustom {
+        Ok(EntryCustom {
             static_fn,
             entry_fn_1_0,
             entry_fn_1_1,
             entry_fn_1_2,
             lib,
-        }
+        })
     }
 
     #[doc = "<https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/vkEnumerateInstanceVersion.html>"]
@@ -226,3 +231,34 @@ impl<L> EntryCustom<L> {
         }
     }
 }
+
+impl vk::StaticFn {
+    pub fn load_checked<F>(mut _f: F) -> Result<Self, MissingEntryPoint>
+    where
+        F: FnMut(&::std::ffi::CStr) -> *const c_void,
+    {
+        // TODO: Make this a &'static CStr once CStr::from_bytes_with_nul_unchecked is const
+        static ENTRY_POINT: &[u8] = b"vkGetInstanceProcAddr\0";
+
+        Ok(vk::StaticFn {
+            get_instance_proc_addr: unsafe {
+                let cname = CStr::from_bytes_with_nul_unchecked(ENTRY_POINT);
+                let val = _f(cname);
+                if val.is_null() {
+                    return Err(MissingEntryPoint);
+                } else {
+                    ::std::mem::transmute(val)
+                }
+            },
+        })
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct MissingEntryPoint;
+impl std::fmt::Display for MissingEntryPoint {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error> {
+        write!(f, "Cannot load `vkGetInstanceProcAddr` symbol from library")
+    }
+}
+impl std::error::Error for MissingEntryPoint {}

ash/src/entry_libloading.rs

@@ -1,4 +1,5 @@
 use crate::entry::EntryCustom;
+use crate::entry::MissingEntryPoint;
 use libloading::Library;
 use std::error::Error;
 use std::ffi::OsStr;
@@ -25,17 +26,32 @@ const LIB_PATH: &str = "libvulkan.dylib";
 pub type Entry = EntryCustom<Arc<Library>>;
 
 #[derive(Debug)]
-pub struct LoadingError(libloading::Error);
+pub enum LoadingError {
+    LibraryLoadFailure(libloading::Error),
+    MissingEntryPoint(MissingEntryPoint),
+}
 
 impl fmt::Display for LoadingError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Display::fmt(&self.0, f)
+        match self {
+            LoadingError::LibraryLoadFailure(err) => fmt::Display::fmt(err, f),
+            LoadingError::MissingEntryPoint(err) => fmt::Display::fmt(err, f),
+        }
     }
 }
 
 impl Error for LoadingError {
     fn source(&self) -> Option<&(dyn Error + 'static)> {
-        Error::source(&self.0)
+        Some(match self {
+            LoadingError::LibraryLoadFailure(err) => err,
+            LoadingError::MissingEntryPoint(err) => err,
+        })
+    }
+}
+
+impl From<MissingEntryPoint> for LoadingError {
+    fn from(err: MissingEntryPoint) -> Self {
+        LoadingError::MissingEntryPoint(err)
     }
 }
 
@@ -71,13 +87,15 @@ impl EntryCustom<Arc<Library>> {
     /// `dlopen`ing native libraries is inherently unsafe. The safety guidelines
     /// for [`Library::new`] and [`Library::get`] apply here.
     pub unsafe fn with_library(path: impl AsRef<OsStr>) -> Result<Entry, LoadingError> {
-        let lib = Library::new(path).map_err(LoadingError).map(Arc::new)?;
+        let lib = Library::new(path)
+            .map_err(LoadingError::LibraryLoadFailure)
+            .map(Arc::new)?;
 
         Ok(Self::new_custom(lib, |vk_lib, name| {
             vk_lib
                 .get(name.to_bytes_with_nul())
                 .map(|symbol| *symbol)
                 .unwrap_or(ptr::null_mut())
-        }))
+        })?)
     }
 }