Detect truncated incr comp files

compiler/rustc_incremental/src/persist/file_format.rs

@@ -17,7 +17,8 @@ use rustc_session::Session;
 use std::borrow::Cow;
 use std::env;
 use std::fs;
-use std::io::{self, Read};
+use std::io::{self, Read, Write};
+use std::io::{Seek, SeekFrom};
 use std::path::{Path, PathBuf};
 
 /// The first few bytes of files generated by incremental compilation.
@@ -35,6 +36,8 @@ pub(crate) fn write_file_header(stream: &mut FileEncoder, sess: &Session) {
     assert_eq!(rustc_version.len(), (rustc_version.len() as u8) as usize);
     stream.emit_raw_bytes(&[rustc_version.len() as u8]);
     stream.emit_raw_bytes(rustc_version.as_bytes());
+    // Reserve space for the u64 where we will later write in the expected total size of this file.
+    stream.emit_raw_bytes(&0u64.to_le_bytes());
 }
 
 pub(crate) fn save_in<F>(sess: &Session, path_buf: PathBuf, name: &str, encode: F)
@@ -65,6 +68,8 @@ where
 
     write_file_header(&mut encoder, sess);
 
+    let len_offset = encoder.position() - 8;
+
     match encode(encoder) {
         Ok(position) => {
             sess.prof.artifact_size(
@@ -76,6 +81,17 @@ where
         }
         Err((path, err)) => sess.dcx().emit_fatal(errors::WriteNew { name, path, err }),
     }
+
+    if let Err(err) = write_file_len(&path_buf, len_offset as u64) {
+        sess.dcx().emit_fatal(errors::WriteNew { name, path: path_buf, err });
+    }
+}
+
+fn write_file_len(path: &Path, offset: u64) -> Result<(), std::io::Error> {
+    let mut file = fs::File::options().write(true).create(false).open(path)?;
+    let len = file.metadata()?.len();
+    file.seek(SeekFrom::Start(offset))?;
+    file.write_all(&len.to_le_bytes())
 }
 
 /// Reads the contents of a file with a file header as defined in this module.
@@ -147,6 +163,14 @@ pub fn read_file(
         }
     }
 
+    // Check FILE_LEN
+    {
+        let mut len = [0u8; 8];
+        file.read_exact(&mut len)?;
+        let len_in_header = u64::from_le_bytes(len);
+        assert_eq!(len_in_header, fs::metadata(path)?.len());
+    }
+
     let post_header_start_pos = file.position() as usize;
     Ok(Some((mmap, post_header_start_pos)))
 }