From 4896095e37c0368681f376b3fa5b3cab1a5155c7 Mon Sep 17 00:00:00 2001
From: Julian Frimmel <julian@fri-me.de>
Date: Wed, 18 Sep 2024 21:18:29 +0200
Subject: [PATCH] Cleanup host-lookup in driver code

The old code was very messy for multiple reasons: the code worked almost
entirely with `Vec<u8>` (a.k.a. byte strings) instead of actual strings,
and thus needed to perform all string operations by hand. The search was
also linear (although the performance penalty is negligible in this
case). Another bad thing is, that the code always executed the fallback
path of querying `rustc -vV` even if the normal path would have worked
(which are the majority of cases), therefore the code needed a redesign.

This PR pulls the search over a specific command output out into its own
function and runs the primary path first and only runs the fallback path
if necessary. This makes the code much easier to read and under- stand.
The new function also works on strings, making its implementation much
simpler.
---
 src/driver.rs | 42 ++++++++++++++++++++----------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/src/driver.rs b/src/driver.rs
index e42e74e..0604baf 100644
--- a/src/driver.rs
+++ b/src/driver.rs
@@ -7,7 +7,18 @@ use std::path::Path;
 use std::process::Command;
 
 /// The prefix line for the target host output.
-const HOST_PREFIX: &[u8] = b"host: ";
+const HOST_PREFIX: &str = "host: ";
+
+/// Search for [`HOST_PREFIX`] inside the command output and extract its value.
+fn search_for_host(command: &mut Command) -> Option<String> {
+    let output = command.output().ok()?.stdout;
+    let output = String::from_utf8(output).ok()?;
+
+    output
+        .lines()
+        .find(|line| line.starts_with(HOST_PREFIX))
+        .map(|host_line| host_line.trim_start_matches(HOST_PREFIX).to_string())
+}
 
 /// Act as a driver for `cargo run`/`cargo test`, but with special runner.
 ///
@@ -21,27 +32,14 @@ pub fn driver() -> io::Result<bool> {
     let cargo = env::var_os("CARGO").expect("CARGO environment variable is not set");
     let rustc = Path::new(&cargo).with_file_name("rustc");
 
-    /* get the output of `cargo version -v` */
-    let cargo_info = Command::new(&cargo)
-        .args(&["version", "-v"])
-        .output()?
-        .stdout;
-    /* old cargo versions don't include the host-field, so fall back to rustc */
-    let rustc_info = Command::new(rustc).arg("-vV").output()?.stdout;
-    let info = [cargo_info, rustc_info].concat();
-
-    /* get the host information (all after the "host: ..." line) */
-    let host = info
-        .windows(HOST_PREFIX.len())
-        .position(|window| window == HOST_PREFIX)
-        .expect("Host information not present in `cargo version -v` or `rustc -vV");
-    let host: String = info
-        .into_iter()
-        .skip(host)
-        .skip(HOST_PREFIX.len())
-        .take_while(|&x| x != b'\n')
-        .map(char::from)
-        .collect();
+    // Search for the host currently running to be able to override the runner.
+    // The host field is extracted from `cargo version -v` if possible, since
+    // this relies entirely on the used `cargo` binary. Older versions of cargo
+    // don't provide the host in that output, though, so there is a fallback to
+    // `rustc -vV` in that case.
+    let host = search_for_host(Command::new(&cargo).arg("version").arg("-v"))
+        .or_else(|| search_for_host(Command::new(rustc).arg("rustc").arg("-vV")))
+        .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "could not determine host"))?;
 
     /* convert to runner env variable */
     let host = host.replace('-', "_").replace('.', "_").to_uppercase();