chore(nint): move from //users/sterni to //nix

Since //web/bubblegum depends on nint, we need to move it to a non user
directory to conform with the policy established via cl/3434.
Note that this likely doesn't mean greater stability (which isn't
really implied in depot anyways), since I still would like to use a more
elaborate calling convention to allow for additional useful features.

Change-Id: I616f905d8df13e3363674aab69a797b0d39fdd79
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3506
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

nix/nint/nint.rs

@@ -0,0 +1,110 @@
+extern crate serde_json;
+
+use serde_json::Value;
+use std::ffi::OsString;
+use std::os::unix::ffi::{OsStringExt, OsStrExt};
+use std::io::{Error, ErrorKind, Write};
+use std::process::Command;
+
+fn render_nix_string(s: &OsString) -> OsString {
+    let mut rendered = Vec::new();
+
+    rendered.extend(b"\"");
+
+    for b in s.as_os_str().as_bytes() {
+        match char::from(*b) {
+            '\"' => rendered.extend(b"\\\""),
+            '\\' => rendered.extend(b"\\\\"),
+            '$'  => rendered.extend(b"\\$"),
+            _    => rendered.push(*b),
+        }
+    }
+
+    rendered.extend(b"\"");
+
+    OsString::from_vec(rendered)
+}
+
+fn render_nix_list(arr: &[OsString]) -> OsString {
+    let mut rendered = Vec::new();
+
+    rendered.extend(b"[ ");
+
+    for el in arr {
+        rendered.extend(render_nix_string(el).as_os_str().as_bytes());
+        rendered.extend(b" ");
+    }
+
+    rendered.extend(b"]");
+
+    OsString::from_vec(rendered)
+}
+
+fn main() -> std::io::Result<()> {
+    let mut nix_args = Vec::new();
+
+    let mut args = std::env::args_os().into_iter();
+    let mut in_args = true;
+
+    let mut argv: Vec<OsString> = Vec::new();
+
+    // skip argv[0]
+    args.next();
+
+    loop {
+        let arg = match args.next() {
+            Some(a) => a,
+            None => break,
+        };
+
+        if !arg.to_str().map(|s| s.starts_with("-")).unwrap_or(false) {
+            in_args = false;
+        }
+
+        if in_args {
+            match(arg.to_str()) {
+                Some("--arg") | Some("--argstr") => {
+                    nix_args.push(arg);
+                    nix_args.push(args.next().unwrap());
+                    nix_args.push(args.next().unwrap());
+                    Ok(())
+                }
+                _ => Err(Error::new(ErrorKind::Other, "unknown argument")),
+            }?
+        } else {
+            argv.push(arg);
+        }
+    }
+
+    if argv.len() < 1 {
+        Err(Error::new(ErrorKind::Other, "missing argv"))
+    } else {
+        let cd = std::env::current_dir()?.into_os_string();
+
+        nix_args.push(OsString::from("--arg"));
+        nix_args.push(OsString::from("currentDir"));
+        nix_args.push(cd);
+
+        nix_args.push(OsString::from("--arg"));
+        nix_args.push(OsString::from("argv"));
+        nix_args.push(render_nix_list(&argv[..]));
+
+        nix_args.push(OsString::from("--eval"));
+        nix_args.push(OsString::from("--json"));
+
+        nix_args.push(argv[0].clone());
+
+        let run = Command::new("nix-instantiate")
+                          .args(nix_args)
+                          .output()?;
+
+        match serde_json::from_slice(&run.stdout[..]) {
+            Ok(Value::String(s)) => Ok(print!("{}", s)),
+            Ok(_) => Err(Error::new(ErrorKind::Other, "output must be a string")),
+            _ => {
+                std::io::stderr().write_all(&run.stderr[..]);
+                Err(Error::new(ErrorKind::Other, "internal nix error"))
+            },
+        }
+    }
+}