Retry opening proc-macro DLLs a few times on Windows.

compiler/rustc_metadata/src/creader.rs

@@ -33,6 +33,7 @@ use rustc_target::spec::{PanicStrategy, TargetTriple};
 use proc_macro::bridge::client::ProcMacro;
 use std::ops::Fn;
 use std::path::Path;
+use std::time::Duration;
 use std::{cmp, env};
 
 #[derive(Clone)]
@@ -689,8 +690,7 @@ impl<'a> CrateLoader<'a> {
     ) -> Result<&'static [ProcMacro], CrateError> {
         // Make sure the path contains a / or the linker will search for it.
         let path = env::current_dir().unwrap().join(path);
-        let lib = unsafe { libloading::Library::new(path) }
-            .map_err(|err| CrateError::DlOpen(err.to_string()))?;
+        let lib = load_dylib(&path, 5).map_err(|err| CrateError::DlOpen(err))?;
 
         let sym_name = self.sess.generate_proc_macro_decls_symbol(stable_crate_id);
         let sym = unsafe { lib.get::<*const &[ProcMacro]>(sym_name.as_bytes()) }
@@ -1093,3 +1093,41 @@ fn alloc_error_handler_spans(sess: &Session, krate: &ast::Crate) -> Vec<Span> {
     visit::walk_crate(&mut f, krate);
     f.spans
 }
+
+// On Windows the compiler would sometimes intermittently fail to open the
+// proc-macro DLL with `Error::LoadLibraryExW`. It is suspected that something in the
+// system still holds a lock on the file, so we retry a few times before calling it
+// an error.
+fn load_dylib(path: &Path, max_attempts: usize) -> Result<libloading::Library, String> {
+    assert!(max_attempts > 0);
+
+    let mut last_error = None;
+
+    for attempt in 0..max_attempts {
+        match unsafe { libloading::Library::new(&path) } {
+            Ok(lib) => {
+                if attempt > 0 {
+                    debug!(
+                        "Loaded proc-macro `{}` after {} attempts.",
+                        path.display(),
+                        attempt + 1
+                    );
+                }
+                return Ok(lib);
+            }
+            Err(err) => {
+                // Only try to recover from this specific error.
+                if !matches!(err, libloading::Error::LoadLibraryExW { .. }) {
+                    return Err(err.to_string());
+                }
+
+                last_error = Some(err);
+                std::thread::sleep(Duration::from_millis(100));
+                debug!("Failed to load proc-macro `{}`. Retrying.", path.display());
+            }
+        }
+    }
+
+    debug!("Failed to load proc-macro `{}` even after {} attempts.", path.display(), max_attempts);
+    Err(format!("{} (retried {} times)", last_error.unwrap(), max_attempts))
+}