toolkit: the tool program, read_file and write_file

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/toolkit/src/files.rs

@@ -0,0 +1,68 @@
+//! The first two tools, `read_file` and `write_file`. `shell` and `http_fetch` come later; until
+//! then `run` never reaches them, so they stay unknown tools.
+
+use std::fs;
+use std::io::Read;
+use std::path::Path;
+
+use proto::tools::{ReadFileArgs, WriteFileArgs};
+
+use crate::Outcome;
+
+pub const MAX_READ: usize = 1024 * 1024;
+
+fn tool_err(tool: &str, path: &str, why: &str) -> Outcome {
+    Outcome::tool_error(format!("{tool}: {path}: {why}"))
+}
+
+pub fn read_file(args: &ReadFileArgs) -> Outcome {
+    let path = args.path.as_str();
+
+    let is_dir = match fs::metadata(path) {
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+            return tool_err("read_file", path, "no such file");
+        }
+        Err(e) => return tool_err("read_file", path, &e.to_string()),
+        Ok(meta) => meta.is_dir(),
+    };
+    if is_dir {
+        return tool_err("read_file", path, "is a directory");
+    }
+
+    let file = match fs::File::open(path) {
+        Err(e) => return tool_err("read_file", path, &e.to_string()),
+        Ok(f) => f,
+    };
+
+    let mut buf = Vec::new();
+    let n = match file.take(MAX_READ as u64 + 1).read_to_end(&mut buf) {
+        Err(e) => return tool_err("read_file", path, &e.to_string()),
+        Ok(n) => n,
+    };
+    if n > MAX_READ {
+        return tool_err("read_file", path, &format!("larger than {MAX_READ} bytes"));
+    }
+
+    match String::from_utf8(buf) {
+        Ok(text) => Outcome::done(text),
+        Err(_) => tool_err("read_file", path, "not UTF-8 text"),
+    }
+}
+
+pub fn write_file(args: &WriteFileArgs) -> Outcome {
+    let path = args.path.as_str();
+    let content = args.content.as_str();
+
+    match Path::new(path).parent() {
+        Some(p) if p.is_dir() => {}
+        _ => return tool_err("write_file", path, "the directory does not exist"),
+    }
+    if Path::new(path).is_dir() {
+        return tool_err("write_file", path, "is a directory");
+    }
+    if let Err(e) = fs::write(path, content) {
+        return tool_err("write_file", path, &e.to_string());
+    }
+
+    Outcome::done(format!("wrote {} bytes to {path}", content.len()))
+}