Slightly optimize cargo vendor

I've noticed recently that `cargo vendor` feels really sluggish and
slow, and apparently this is primarily because we delete the registry
caches and re-extract all the tarballs. This commit implements one
possible optimization without changing this, however, which is that
currently we both copy a file and checksum it, but that ends up reading
all the contents twice. Those two functions are now folded into one,
shaving about 3s locally from Cargo's own vendor times.

src/cargo/ops/vendor.rs

@@ -9,6 +9,8 @@ use serde::Serialize;
 use std::collections::HashSet;
 use std::collections::{BTreeMap, BTreeSet, HashMap};
 use std::fs;
+use std::fs::File;
+use std::io::{Read, Write};
 use std::path::{Path, PathBuf};
 
 pub struct VendorOptions<'a> {
@@ -182,6 +184,7 @@ fn sync(
     }
 
     let mut sources = BTreeSet::new();
+    let mut tmp_buf = [0; 64 * 1024];
     for (id, pkg) in ids.iter() {
         // Next up, copy it to the vendor directory
         let src = pkg
@@ -216,7 +219,7 @@ fn sync(
         let pathsource = PathSource::new(src, id.source_id(), config);
         let paths = pathsource.list_files(pkg)?;
         let mut map = BTreeMap::new();
-        cp_sources(src, &paths, &dst, &mut map)
+        cp_sources(src, &paths, &dst, &mut map, &mut tmp_buf)
             .chain_err(|| format!("failed to copy over vendored sources for: {}", id))?;
 
         // Finally, emit the metadata about this package
@@ -299,6 +302,7 @@ fn cp_sources(
     paths: &[PathBuf],
     dst: &Path,
     cksums: &mut BTreeMap<String, String>,
+    tmp_buf: &mut [u8],
 ) -> CargoResult<()> {
     for p in paths {
         let relative = p.strip_prefix(&src).unwrap();
@@ -334,9 +338,27 @@ fn cp_sources(
 
         paths::create_dir_all(dst.parent().unwrap())?;
 
-        paths::copy(&p, &dst)?;
-        let cksum = Sha256::new().update_path(dst)?.finish_hex();
+        let cksum = copy_and_checksum(&p, &dst, tmp_buf)?;
         cksums.insert(relative.to_str().unwrap().replace("\\", "/"), cksum);
     }
     Ok(())
 }
+
+fn copy_and_checksum(src_path: &Path, dst_path: &Path, buf: &mut [u8]) -> CargoResult<String> {
+    let mut src = File::open(src_path).chain_err(|| format!("failed to open {:?}", src_path))?;
+    let mut dst =
+        File::create(dst_path).chain_err(|| format!("failed to create {:?}", dst_path))?;
+    let mut cksum = Sha256::new();
+    loop {
+        let n = src
+            .read(buf)
+            .chain_err(|| format!("failed to read from {:?}", src_path))?;
+        if n == 0 {
+            break Ok(cksum.finish_hex());
+        }
+        let data = &buf[..n];
+        cksum.update(data);
+        dst.write_all(data)
+            .chain_err(|| format!("failed to write to {:?}", dst_path))?;
+    }
+}