More Rust FFI adventures

We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.

nix-rust/src/error.rs

@@ -1,5 +1,30 @@
 #[derive(Debug)]
 pub enum Error {
+    IOError(std::io::Error),
     Misc(String),
-    Foreign(libc::c_void), // == std::exception_ptr
+    Foreign(CppException),
+}
+
+impl From<std::io::Error> for Error {
+    fn from(err: std::io::Error) -> Self {
+        Error::IOError(err)
+    }
+}
+
+impl From<Error> for CppException {
+    fn from(err: Error) -> Self {
+        match err {
+            Error::Foreign(ex) => ex,
+            Error::Misc(s) => unsafe { make_error(&s) },
+            Error::IOError(err) => unsafe { make_error(&err.to_string()) },
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug)]
+pub struct CppException(*const libc::c_void); // == std::exception_ptr*
+
+extern "C" {
+    fn make_error(s: &str) -> CppException;
 }

nix-rust/src/lib.rs

@@ -4,7 +4,30 @@ mod tarfile;
 
 pub use error::Error;
 
-#[no_mangle]
-pub extern "C" fn unpack_tarfile(source: foreign::Source, dest_dir: &str) {
-    tarfile::unpack_tarfile(source, dest_dir).unwrap();
+pub struct CBox<T> {
+    ptr: *mut libc::c_void,
+    phantom: std::marker::PhantomData<T>,
+}
+
+impl<T> CBox<T> {
+    fn new(t: T) -> Self {
+        unsafe {
+            let size = std::mem::size_of::<T>();
+            let ptr = libc::malloc(size);
+            eprintln!("PTR = {:?}, SIZE = {}", ptr, size);
+            *(ptr as *mut T) = t; // FIXME: probably UB
+            Self {
+                ptr,
+                phantom: std::marker::PhantomData,
+            }
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn unpack_tarfile(
+    source: foreign::Source,
+    dest_dir: &str,
+) -> CBox<Result<(), error::CppException>> {
+    CBox::new(tarfile::unpack_tarfile(source, dest_dir).map_err(|err| err.into()))
 }

nix-rust/src/tarfile.rs

@@ -10,19 +10,19 @@ pub fn unpack_tarfile(source: Source, dest_dir: &str) -> Result<(), Error> {
 
     let mut tar = Archive::new(source);
 
-    for file in tar.entries().unwrap() {
-        let mut file = file.unwrap();
+    for file in tar.entries()? {
+        let mut file = file?;
 
-        let dest_file = dest_dir.join(file.path().unwrap());
+        let dest_file = dest_dir.join(file.path()?);
 
-        fs::create_dir_all(dest_file.parent().unwrap()).unwrap();
+        fs::create_dir_all(dest_file.parent().unwrap())?;
 
         match file.header().entry_type() {
             tar::EntryType::Directory => {
-                fs::create_dir(dest_file).unwrap();
+                fs::create_dir(dest_file)?;
             }
             tar::EntryType::Regular => {
-                let mode = if file.header().mode().unwrap() & libc::S_IXUSR == 0 {
+                let mode = if file.header().mode()? & libc::S_IXUSR == 0 {
                     0o666
                 } else {
                     0o777
@@ -31,13 +31,11 @@ pub fn unpack_tarfile(source: Source, dest_dir: &str) -> Result<(), Error> {
                     .create(true)
                     .write(true)
                     .mode(mode)
-                    .open(dest_file)
-                    .unwrap();
-                io::copy(&mut file, &mut f).unwrap();
+                    .open(dest_file)?;
+                io::copy(&mut file, &mut f)?;
             }
             tar::EntryType::Symlink => {
-                std::os::unix::fs::symlink(file.header().link_name().unwrap().unwrap(), dest_file)
-                    .unwrap();
+                std::os::unix::fs::symlink(file.header().link_name()?.unwrap(), dest_file)?;
             }
             t => return Err(Error::Misc(format!("unsupported tar entry type '{:?}'", t))),
         }

src/libstore/builtins/unpack-channel.cc

@@ -27,7 +27,7 @@ void builtinUnpackChannel(const BasicDerivation & drv)
         decompressor->finish();
     });
 
-    unpack_tarfile(*source, out);
+    unpack_tarfile(*source, out).use()->unwrap();
 
     auto entries = readDirectory(out);
     if (entries.size() != 1)

src/libstore/rust.cc

@@ -0,0 +1,12 @@
+#include "logging.hh"
+#include "rust.hh"
+
+namespace nix {
+
+extern "C" std::exception_ptr * make_error(rust::StringSlice s)
+{
+    // FIXME: leak
+    return new std::exception_ptr(std::make_exception_ptr(Error(std::string(s.ptr, s.size))));
+}
+
+}

src/libstore/rust.hh

@@ -4,7 +4,8 @@ namespace rust {
 
 // Depending on the internal representation of Rust slices is slightly
 // evil...
-template<typename T> struct Slice
+template<typename T>
+struct Slice
 {
     T * ptr;
     size_t size;
@@ -37,8 +38,64 @@ struct Source
     }
 };
 
+/* C++ representation of Rust's Result<T, CppException>. */
+template<typename T>
+struct Result
+{
+    unsigned int tag;
+
+    union {
+        T data;
+        std::exception_ptr * exc;
+    };
+
+    /* Rethrow the wrapped exception or return the wrapped value. */
+    T unwrap()
+    {
+        if (tag == 0)
+            return data;
+        else if (tag == 1)
+            std::rethrow_exception(*exc);
+        else
+            abort();
+    }
+};
+
+template<typename T>
+struct CBox
+{
+    T * ptr;
+
+    T * operator ->()
+    {
+        return ptr;
+    }
+
+    CBox(T * ptr) : ptr(ptr) { }
+    CBox(const CBox &) = delete;
+    CBox(CBox &&) = delete;
+
+    ~CBox()
+    {
+        free(ptr);
+    }
+};
+
+// Grrr, this is only needed because 'extern "C"' functions don't
+// support non-POD return types (and CBox has a destructor so it's not
+// POD).
+template<typename T>
+struct CBox2
+{
+    T * ptr;
+    CBox<T> use()
+    {
+        return CBox(ptr);
+    }
+};
+
 }
 
 extern "C" {
-    void unpack_tarfile(rust::Source source, rust::StringSlice dest_dir);
+    rust::CBox2<rust::Result<std::tuple<>>> unpack_tarfile(rust::Source source, rust::StringSlice dest_dir);
 }