Auto merge of #7946 - hbina:add_stream_hashu64, r=alexcrichton

Fixes issue #7543

Added a helper function for `util::hex::hash_64` that uses streams
the content instead of reading through the entire content in one
go.

Edit: Should I add test cases for this?

Edit2: Per #7543

src/cargo/ops/cargo_package.rs

@@ -746,8 +746,8 @@ fn hash_all(path: &Path) -> CargoResult<HashMap<PathBuf, u64>> {
             let entry = entry?;
             let file_type = entry.file_type();
             if file_type.is_file() {
-                let contents = fs::read(entry.path())?;
-                let hash = util::hex::hash_u64(&contents);
+                let file = File::open(entry.path())?;
+                let hash = util::hex::hash_u64_file(&file)?;
                 result.insert(entry.path().to_path_buf(), hash);
             } else if file_type.is_symlink() {
                 let hash = util::hex::hash_u64(&fs::read_link(entry.path())?);

src/cargo/util/hex.rs

@@ -1,6 +1,8 @@
 #![allow(deprecated)]
 
+use std::fs::File;
 use std::hash::{Hash, Hasher, SipHasher};
+use std::io::Read;
 
 pub fn to_hex(num: u64) -> String {
     hex::encode(&[
@@ -21,6 +23,19 @@ pub fn hash_u64<H: Hash>(hashable: H) -> u64 {
     hasher.finish()
 }
 
+pub fn hash_u64_file(mut file: &File) -> std::io::Result<u64> {
+    let mut hasher = SipHasher::new_with_keys(0, 0);
+    let mut buf = [0; 64 * 1024];
+    loop {
+        let n = file.read(&mut buf)?;
+        if n == 0 {
+            break;
+        }
+        hasher.write(&buf[..n]);
+    }
+    Ok(hasher.finish())
+}
+
 pub fn short_hash<H: Hash>(hashable: &H) -> String {
     to_hex(hash_u64(hashable))
 }