Add basic GuestString support to wiggle (#13)

* Add basic GuestString support to wiggle

This commit adds basic `GuestString` support to `wiggle`. `GuestString`
is a wrapper around `GuestArray<'_, u8>` array type which itself can
be made into either an owned (cloned) Rust `String` or borrowed as
a reference `&str`. In both cases, `GuestString` ensures that the
underlying bytes are valid Unicode code units, throwing a `InvalidUtf8`
error if not.

This commit adds support *only* for passing in strings as arguments
in WASI. Marshalling of the return arg has not yet been implemented.
I'm not even sure it's possible without multi-value return args
feature of Wasm. It's not a major setback especially since the WASI
spec (and this includes even the `ephemeral` snapshot) doesn't
return strings anywhere. They are only ever passed in as arguments
to interface functions.

It should be noted that error returned in case of invalid UTF-8
requires a lot more love as it doesn't include anything besides
flagging an event that the string contained an invalid Unicode code unit.

* Borrow all of string's memory including nul-byte

Borrow all of string's underlying memory including the nul-byte.
This perhaps might not have a tremendous impact on anything, but
since the nul-byte is technically part of the WASI string, we should
include it in the borrow as well.

* Fill in wiggle-generate blanks for strings

* Print to screen passed string in proptest

* Strings are PointerLengthPairs!

* Fix generation of strings in compound types

* Update test with simple string strategy

* Generate better test strings

* Finalise proptest for strings

* Fix formatting

* Update crates/runtime/src/memory/string.rs

Removes unnecessary comment in code

* Apply Pat's suggestion to wrap Utf8Error as error

tests/main.rs

@@ -2,7 +2,7 @@ use proptest::prelude::*;
 use std::convert::TryFrom;
 use wiggle_runtime::{
     GuestArray, GuestError, GuestErrorType, GuestMemory, GuestPtr, GuestPtrMut, GuestRef,
-    GuestRefMut,
+    GuestRefMut, GuestString,
 };
 
 wiggle_generate::from_witx!({
@@ -136,6 +136,13 @@ impl foo::Foo for WasiCtx {
         })?;
         Ok(old_config ^ other_config)
     }
+
+    fn hello_string(&mut self, a_string: &GuestString<'_>) -> Result<u32, types::Errno> {
+        let as_ref = a_string.as_ref().expect("deref ptr should succeed");
+        let as_str = as_ref.as_str().expect("valid UTF-8 string");
+        println!("a_string='{}'", as_str);
+        Ok(as_str.len() as u32)
+    }
 }
 // Errno is used as a first return value in the functions above, therefore
 // it must implement GuestErrorType with type Context = WasiCtx.
@@ -867,3 +874,87 @@ proptest! {
         e.test()
     }
 }
+
+fn test_string_strategy() -> impl Strategy<Value = String> {
+    "\\p{Greek}{1,256}"
+}
+
+#[derive(Debug)]
+struct HelloStringExercise {
+    test_word: String,
+    string_ptr_loc: MemArea,
+    string_len_loc: MemArea,
+    return_ptr_loc: MemArea,
+}
+
+impl HelloStringExercise {
+    pub fn strat() -> BoxedStrategy<Self> {
+        (test_string_strategy(),)
+            .prop_flat_map(|(test_word,)| {
+                (
+                    Just(test_word.clone()),
+                    HostMemory::mem_area_strat(test_word.len() as u32),
+                    HostMemory::mem_area_strat(4),
+                    HostMemory::mem_area_strat(4),
+                )
+            })
+            .prop_map(
+                |(test_word, string_ptr_loc, string_len_loc, return_ptr_loc)| Self {
+                    test_word,
+                    string_ptr_loc,
+                    string_len_loc,
+                    return_ptr_loc,
+                },
+            )
+            .prop_filter("non-overlapping pointers", |e| {
+                non_overlapping_set(&[&e.string_ptr_loc, &e.string_len_loc, &e.return_ptr_loc])
+            })
+            .boxed()
+    }
+
+    pub fn test(&self) {
+        let mut ctx = WasiCtx::new();
+        let mut host_memory = HostMemory::new();
+        let mut guest_memory = GuestMemory::new(host_memory.as_mut_ptr(), host_memory.len() as u32);
+
+        // Populate string length
+        *guest_memory
+            .ptr_mut(self.string_len_loc.ptr)
+            .expect("ptr mut to string len")
+            .as_ref_mut()
+            .expect("deref ptr mut to string len") = self.test_word.len() as u32;
+
+        // Populate string in guest's memory
+        {
+            let mut next: GuestPtrMut<'_, u8> = guest_memory
+                .ptr_mut(self.string_ptr_loc.ptr)
+                .expect("ptr mut to the first byte of string");
+            for byte in self.test_word.as_bytes() {
+                *next.as_ref_mut().expect("deref mut") = *byte;
+                next = next.elem(1).expect("increment ptr by 1");
+            }
+        }
+
+        let res = foo::hello_string(
+            &mut ctx,
+            &mut guest_memory,
+            self.string_ptr_loc.ptr as i32,
+            self.string_len_loc.ptr as i32,
+            self.return_ptr_loc.ptr as i32,
+        );
+        assert_eq!(res, types::Errno::Ok.into(), "hello string errno");
+
+        let given = *guest_memory
+            .ptr::<u32>(self.return_ptr_loc.ptr)
+            .expect("ptr to return value")
+            .as_ref()
+            .expect("deref ptr to return value");
+        assert_eq!(self.test_word.len() as u32, given);
+    }
+}
+proptest! {
+    #[test]
+    fn hello_string(e in HelloStringExercise::strat()) {
+        e.test()
+    }
+}

tests/test.witx

@@ -72,4 +72,9 @@
     (result $error $errno)
     (result $new_config $car_config)
   )
+  (@interface func (export "hello_string")
+    (param $a_string string)
+    (result $error $errno)
+    (result $total_bytes u32)
+  )
 )