toolkit: the tool program, read_file and write_file

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

crates/toolkit/src/input.rs

@@ -0,0 +1,48 @@
+//! Reading and parsing the arguments `brokerd` puts on `toolkit`'s standard input.
+
+use std::io::Read;
+
+pub const MAX_INPUT: usize = 2 * 1024 * 1024;
+
+#[derive(Debug)]
+pub enum InputError {
+    TooLarge,
+    NotUtf8,
+    Io(std::io::Error),
+}
+
+impl std::fmt::Display for InputError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            InputError::TooLarge => write!(f, "the arguments are larger than {MAX_INPUT} bytes"),
+            InputError::NotUtf8 => write!(f, "the arguments are not UTF-8"),
+            InputError::Io(e) => write!(f, "cannot read the arguments: {e}"),
+        }
+    }
+}
+
+impl std::error::Error for InputError {}
+
+impl From<std::io::Error> for InputError {
+    fn from(e: std::io::Error) -> Self {
+        InputError::Io(e)
+    }
+}
+
+/// All of `stdin`, at most MAX_INPUT bytes, as UTF-8. Read at most MAX_INPUT + 1 bytes
+/// (`Read::take`); more than MAX_INPUT is TooLarge.
+pub fn read_input(stdin: &mut dyn std::io::Read) -> Result<String, InputError> {
+    let mut buf = Vec::new();
+    let n = stdin.take(MAX_INPUT as u64 + 1).read_to_end(&mut buf)?;
+    if n > MAX_INPUT {
+        return Err(InputError::TooLarge);
+    }
+    String::from_utf8(buf).map_err(|_| InputError::NotUtf8)
+}
+
+/// Parse `text` as the arguments of tool `name`; the error is the whole line for stderr:
+/// "toolkit: {name}: the arguments do not parse: {serde's error}".
+pub fn parse<T: serde::de::DeserializeOwned>(name: &str, text: &str) -> Result<T, String> {
+    serde_json::from_str(text)
+        .map_err(|e| format!("toolkit: {name}: the arguments do not parse: {e}"))
+}