Address review comments

crates/proc_macro_api/src/lib.rs

@@ -15,7 +15,7 @@ use std::{
     ffi::OsStr,
     io,
     path::{Path, PathBuf},
-    sync::Arc,
+    sync::{Arc, Mutex},
 };
 
 use tt::{SmolStr, Subtree};
@@ -27,7 +27,7 @@ pub use version::{read_dylib_info, RustCInfo};
 
 #[derive(Debug, Clone)]
 struct ProcMacroProcessExpander {
-    process: Arc<ProcMacroProcessSrv>,
+    process: Arc<Mutex<ProcMacroProcessSrv>>,
     dylib_path: PathBuf,
     name: SmolStr,
 }
@@ -56,14 +56,24 @@ impl base_db::ProcMacroExpander for ProcMacroProcessExpander {
             env: env.iter().map(|(k, v)| (k.to_string(), v.to_string())).collect(),
         };
 
-        let result: ExpansionResult = self.process.send_task(msg::Request::ExpansionMacro(task))?;
+        let result: ExpansionResult = self
+            .process
+            .lock()
+            .unwrap_or_else(|e| e.into_inner())
+            .send_task(msg::Request::ExpansionMacro(task))?;
         Ok(result.expansion)
     }
 }
 
 #[derive(Debug)]
 pub struct ProcMacroClient {
-    process: Arc<ProcMacroProcessSrv>,
+    /// Currently, the proc macro process expands all procedural macros sequentially.
+    ///
+    /// That means that concurrent salsa requests may block each other when expanding proc macros,
+    /// which is unfortunate, but simple and good enough for the time being.
+    ///
+    /// Therefore, we just wrap the `ProcMacroProcessSrv` in a mutex here.
+    process: Arc<Mutex<ProcMacroProcessSrv>>,
 }
 
 impl ProcMacroClient {
@@ -73,7 +83,7 @@ impl ProcMacroClient {
         args: impl IntoIterator<Item = impl AsRef<OsStr>>,
     ) -> io::Result<ProcMacroClient> {
         let process = ProcMacroProcessSrv::run(process_path, args)?;
-        Ok(ProcMacroClient { process: Arc::new(process) })
+        Ok(ProcMacroClient { process: Arc::new(Mutex::new(process)) })
     }
 
     pub fn by_dylib_path(&self, dylib_path: &Path) -> Vec<ProcMacro> {
@@ -93,7 +103,12 @@ impl ProcMacroClient {
             }
         }
 
-        let macros = match self.process.find_proc_macros(dylib_path) {
+        let macros = match self
+            .process
+            .lock()
+            .unwrap_or_else(|e| e.into_inner())
+            .find_proc_macros(dylib_path)
+        {
             Err(err) => {
                 eprintln!("Failed to find proc macros. Error: {:#?}", err);
                 return vec![];