Check if the universal library is updated before running lipo

src/lipo.rs

@@ -2,9 +2,28 @@ use crate::Result;
 use crate::cargo::Cargo;
 use crate::meta::Meta;
 use failure::ResultExt;
-use log::info;
+use log::{info, warn};
 use std::fs;
 use std::process::Command;
+use std::path::Path;
+
+fn should_update_output(
+    output: impl AsRef<Path>,
+    inputs: impl IntoIterator<Item = impl AsRef<Path>>,
+) -> std::io::Result<bool> {
+    let output_metadata = match fs::metadata(output) {
+        Ok(metadata) => metadata,
+        Err(_) => return Ok(true),
+    };
+    let output_mtime = output_metadata.modified()?;
+    for input in inputs {
+        let input_mtime = fs::metadata(input)?.modified()?;
+        if input_mtime > output_mtime {
+            return Ok(true);
+        }
+    }
+    Ok(false)
+}
 
 pub(crate) fn build(cargo: &Cargo, meta: &Meta, targets: &[impl AsRef<str>]) -> Result<()> {
     for package in meta.packages() {
@@ -37,6 +56,23 @@ pub(crate) fn build(cargo: &Cargo, meta: &Meta, targets: &[impl AsRef<str>]) ->
 
         output.push(&lib_name);
 
+        match should_update_output(&output, &inputs) {
+            Ok(true) => {}
+            Ok(false) => {
+                info!(
+                    "Universal library is up-to-date, skipping lipo invocation for {}",
+                    package.name()
+                );
+                continue;
+            }
+            Err(e) => {
+                warn!(
+                    "Failed to check if universal library for {:?} is up-to-date: {}",
+                    package.name(),
+                    e
+                )
+            }
+        }
         let mut cmd = Command::new("lipo");
         cmd.arg("-create").arg("-output").arg(output);
         cmd.args(inputs);