rel make: Avoid EMFILE when loading many modules (#122)

Author: encounter

src/cmd/rel.rs

@@ -300,7 +300,15 @@ fn make(args: MakeArgs) -> Result<()> {
     }
 
     // Load all modules
-    let mut files = paths.iter().map(|p| open_file(p, true)).collect::<Result<Vec<_>>>()?;
+    let mut files = paths
+        .iter()
+        .map(|p| {
+            let mut file = open_file(p, true)?;
+            // Immediately map to avoid keeping file handles open
+            file.map()?;
+            Ok(file)
+        })
+        .collect::<Result<Vec<_>>>()?;
     let modules = files
         .par_iter_mut()
         .enumerate()

src/vfs/std_fs.rs

@@ -86,14 +86,17 @@ impl Seek for StdFile {
 
 impl VfsFile for StdFile {
     fn map(&mut self) -> io::Result<&[u8]> {
-        let file = match self.file {
-            Some(ref mut file) => file,
-            None => self.file.insert(BufReader::new(fs::File::open(&self.path)?)),
-        };
         let mmap = match self.mmap {
             Some(ref mmap) => mmap,
-            None => self.mmap.insert(unsafe { memmap2::Mmap::map(file.get_ref())? }),
+            None => {
+                let file = match self.file {
+                    Some(ref mut file) => file,
+                    None => self.file.insert(BufReader::new(fs::File::open(&self.path)?)),
+                };
+                self.mmap.insert(unsafe { memmap2::Mmap::map(file.get_ref())? })
+            }
         };
+        self.file = None; // Drop the BufReader to avoid holding the file handle
         Ok(mmap)
     }