port in args and env. slightly different style

building up a bunch of deferred errors in the CtxBuilder sucks. so does
reporting errors right away. Idk what to do here?

crates/test-programs/tests/wasm_tests/runtime.rs

@@ -25,7 +25,7 @@ pub fn instantiate(
     // Additionally register any preopened directories if we have them.
     let mut builder = wasi_c2::WasiCtx::builder();
 
-    builder.arg(bin_name).arg(".").inherit_stdio();
+    builder.arg(bin_name)?.arg(".")?.inherit_stdio();
 
     /*
         if let Some(workspace) = workspace {

crates/wasi-c2/src/ctx.rs

@@ -1,5 +1,6 @@
 use crate::dir::{DirCaps, DirEntry, WasiDir};
 use crate::file::{FileCaps, FileEntry, WasiFile};
+use crate::string_array::{StringArray, StringArrayError};
 use crate::table::Table;
 use crate::Error;
 use std::cell::{RefCell, RefMut};
@@ -7,6 +8,8 @@ use std::path::PathBuf;
 use std::rc::Rc;
 
 pub struct WasiCtx {
+    pub(crate) args: StringArray,
+    pub(crate) env: StringArray,
     table: Rc<RefCell<Table>>,
 }
 
@@ -17,6 +20,8 @@ impl WasiCtx {
 
     pub fn new() -> Self {
         WasiCtx {
+            args: StringArray::new(),
+            env: StringArray::new(),
             table: Rc::new(RefCell::new(Table::new())),
         }
     }
@@ -64,9 +69,9 @@ impl WasiCtxBuilder {
     pub fn build(self) -> Result<WasiCtx, Error> {
         Ok(self.0)
     }
-    pub fn arg(&mut self, _arg: &str) -> &mut Self {
-        // Intentionally left blank. We do not handle arguments yet.
-        self
+    pub fn arg(&mut self, arg: &str) -> Result<&mut Self, StringArrayError> {
+        self.0.args.push(arg.to_owned())?;
+        Ok(self)
     }
     pub fn inherit_stdio(&mut self) -> &mut Self {
         self.0.insert_file(

crates/wasi-c2/src/lib.rs

@@ -6,9 +6,11 @@ mod error;
 mod file;
 pub mod snapshots;
 pub mod stdio;
+mod string_array;
 pub mod table;
 
 pub use ctx::WasiCtx;
 pub use dir::{DirCaps, WasiDir};
 pub use error::Error;
 pub use file::{FileCaps, WasiFile};
+pub use string_array::StringArrayError;

crates/wasi-c2/src/snapshots/preview_1.rs

@@ -106,11 +106,11 @@ impl<'a> wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
         argv: &GuestPtr<'b, GuestPtr<'b, u8>>,
         argv_buf: &GuestPtr<'b, u8>,
     ) -> Result<(), Error> {
-        unimplemented!()
+        self.args.write_to_guest(argv_buf, argv)
     }
 
     fn args_sizes_get(&self) -> Result<(types::Size, types::Size), Error> {
-        unimplemented!()
+        Ok((self.args.number_elements(), self.args.cumulative_size()))
     }
 
     fn environ_get<'b>(
@@ -118,11 +118,11 @@ impl<'a> wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
         environ: &GuestPtr<'b, GuestPtr<'b, u8>>,
         environ_buf: &GuestPtr<'b, u8>,
     ) -> Result<(), Error> {
-        unimplemented!()
+        self.env.write_to_guest(environ_buf, environ)
     }
 
     fn environ_sizes_get(&self) -> Result<(types::Size, types::Size), Error> {
-        unimplemented!()
+        Ok((self.env.number_elements(), self.env.cumulative_size()))
     }
 
     fn clock_res_get(&self, id: types::Clockid) -> Result<types::Timestamp, Error> {

crates/wasi-c2/src/string_array.rs

@@ -0,0 +1,71 @@
+use crate::Error;
+use wiggle::GuestPtr;
+
+pub struct StringArray {
+    elems: Vec<String>,
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum StringArrayError {
+    #[error("Number of elements exceeds 2^32")]
+    NumberElements,
+    #[error("Element size exceeds 2^32")]
+    ElementSize,
+    #[error("Cumulative size exceeds 2^32")]
+    CumulativeSize,
+}
+
+impl StringArray {
+    pub fn new() -> Self {
+        StringArray { elems: Vec::new() }
+    }
+
+    pub fn push(&mut self, elem: String) -> Result<(), StringArrayError> {
+        if self.elems.len() + 1 > std::u32::MAX as usize {
+            return Err(StringArrayError::NumberElements);
+        }
+        if elem.as_bytes().len() + 1 > std::u32::MAX as usize {
+            return Err(StringArrayError::ElementSize);
+        }
+        if self.cumulative_size() as usize + elem.as_bytes().len() + 1 > std::u32::MAX as usize {
+            return Err(StringArrayError::CumulativeSize);
+        }
+        self.elems.push(elem);
+        Ok(())
+    }
+
+    pub fn number_elements(&self) -> u32 {
+        self.elems.len() as u32
+    }
+
+    pub fn cumulative_size(&self) -> u32 {
+        self.elems
+            .iter()
+            .map(|e| e.as_bytes().len() + 1)
+            .sum::<usize>() as u32
+    }
+
+    pub fn write_to_guest<'a>(
+        &self,
+        buffer: &GuestPtr<'a, u8>,
+        element_heads: &GuestPtr<'a, GuestPtr<'a, u8>>,
+    ) -> Result<(), Error> {
+        let element_heads = element_heads.as_array(self.number_elements());
+        let buffer = buffer.as_array(self.cumulative_size());
+        let mut cursor = 0;
+        for (elem, head) in self.elems.iter().zip(element_heads.iter()) {
+            let bytes = elem.as_bytes();
+            let len = bytes.len() as u32;
+            {
+                let elem_buffer = buffer
+                    .get_range(cursor..(cursor + len))
+                    .ok_or(Error::Inval)?; // Elements don't fit in buffer provided
+                elem_buffer.copy_from_slice(bytes)?;
+            }
+            buffer.get(cursor + len).ok_or(Error::Inval)?.write(0)?; // 0 terminate
+            head?.write(buffer.get(cursor).expect("already validated"))?;
+            cursor += len + 1;
+        }
+        Ok(())
+    }
+}