move encode_and_write_metadata to rustc_metadata::fs

compiler/rustc_metadata/Cargo.toml

@@ -12,6 +12,7 @@ odht = { version = "0.3.1", features = ["nightly"] }
 snap = "1"
 tracing = "0.1"
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+tempfile = "3.2"
 rustc_middle = { path = "../rustc_middle" }
 rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }

compiler/rustc_metadata/src/fs.rs

@@ -1,8 +1,15 @@
+use crate::{encode_metadata, EncodedMetadata};
+
 use rustc_data_structures::temp_dir::MaybeTempDir;
+use rustc_hir::def_id::LOCAL_CRATE;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::config::{CrateType, OutputFilenames, OutputType};
+use rustc_session::output::filename_for_metadata;
 use rustc_session::Session;
+use tempfile::Builder as TempFileBuilder;
 
 use std::fs;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 // FIXME(eddyb) maybe include the crate name in this?
 pub const METADATA_FILENAME: &str = "lib.rmeta";
@@ -22,3 +29,84 @@ pub fn emit_metadata(sess: &Session, metadata: &[u8], tmpdir: &MaybeTempDir) ->
 
     out_filename
 }
+
+pub fn encode_and_write_metadata(
+    tcx: TyCtxt<'_>,
+    outputs: &OutputFilenames,
+) -> (EncodedMetadata, bool) {
+    #[derive(PartialEq, Eq, PartialOrd, Ord)]
+    enum MetadataKind {
+        None,
+        Uncompressed,
+        Compressed,
+    }
+
+    let metadata_kind = tcx
+        .sess
+        .crate_types()
+        .iter()
+        .map(|ty| match *ty {
+            CrateType::Executable | CrateType::Staticlib | CrateType::Cdylib => MetadataKind::None,
+
+            CrateType::Rlib => MetadataKind::Uncompressed,
+
+            CrateType::Dylib | CrateType::ProcMacro => MetadataKind::Compressed,
+        })
+        .max()
+        .unwrap_or(MetadataKind::None);
+
+    let metadata = match metadata_kind {
+        MetadataKind::None => EncodedMetadata::new(),
+        MetadataKind::Uncompressed | MetadataKind::Compressed => encode_metadata(tcx),
+    };
+
+    let _prof_timer = tcx.sess.prof.generic_activity("write_crate_metadata");
+
+    let need_metadata_file = tcx.sess.opts.output_types.contains_key(&OutputType::Metadata);
+    if need_metadata_file {
+        let crate_name = tcx.crate_name(LOCAL_CRATE);
+        let out_filename = filename_for_metadata(tcx.sess, crate_name.as_str(), outputs);
+        // To avoid races with another rustc process scanning the output directory,
+        // we need to write the file somewhere else and atomically move it to its
+        // final destination, with an `fs::rename` call. In order for the rename to
+        // always succeed, the temporary file needs to be on the same filesystem,
+        // which is why we create it inside the output directory specifically.
+        let metadata_tmpdir = TempFileBuilder::new()
+            .prefix("rmeta")
+            .tempdir_in(out_filename.parent().unwrap())
+            .unwrap_or_else(|err| tcx.sess.fatal(&format!("couldn't create a temp dir: {}", err)));
+        let metadata_tmpdir = MaybeTempDir::new(metadata_tmpdir, tcx.sess.opts.cg.save_temps);
+        let metadata_filename = emit_metadata(tcx.sess, metadata.raw_data(), &metadata_tmpdir);
+        if let Err(e) = non_durable_rename(&metadata_filename, &out_filename) {
+            tcx.sess.fatal(&format!("failed to write {}: {}", out_filename.display(), e));
+        }
+        if tcx.sess.opts.json_artifact_notifications {
+            tcx.sess
+                .parse_sess
+                .span_diagnostic
+                .emit_artifact_notification(&out_filename, "metadata");
+        }
+    }
+
+    let need_metadata_module = metadata_kind == MetadataKind::Compressed;
+
+    (metadata, need_metadata_module)
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn non_durable_rename(src: &Path, dst: &Path) -> std::io::Result<()> {
+    std::fs::rename(src, dst)
+}
+
+/// This function attempts to bypass the auto_da_alloc heuristic implemented by some filesystems
+/// such as btrfs and ext4. When renaming over a file that already exists then they will "helpfully"
+/// write back the source file before committing the rename in case a developer forgot some of
+/// the fsyncs in the open/write/fsync(file)/rename/fsync(dir) dance for atomic file updates.
+///
+/// To avoid triggering this heuristic we delete the destination first, if it exists.
+/// The cost of an extra syscall is much lower than getting descheduled for the sync IO.
+#[cfg(target_os = "linux")]
+pub fn non_durable_rename(src: &Path, dst: &Path) -> std::io::Result<()> {
+    let _ = std::fs::remove_file(dst);
+    std::fs::rename(src, dst)
+}