shred: remove usage of Vec::set_len() (#1738)

* shred: use a fixed-size array for BytesGenerator

src/uu/shred/src/shred.rs

@@ -119,6 +119,7 @@ struct BytesGenerator<'a> {
     exact: bool, // if false, every block's size is block_size
     gen_type: PassType<'a>,
     rng: Option<RefCell<ThreadRng>>,
+    bytes: [u8; BLOCK_SIZE],
 }
 
 impl<'a> BytesGenerator<'a> {
@@ -128,6 +129,8 @@ impl<'a> BytesGenerator<'a> {
             _ => None,
         };
 
+        let bytes = [0; BLOCK_SIZE];
+
         BytesGenerator {
             total_bytes,
             bytes_generated: Cell::new(0u64),
@@ -135,25 +138,35 @@ impl<'a> BytesGenerator<'a> {
             exact,
             gen_type,
             rng,
+            bytes,
         }
     }
-}
 
-impl<'a> Iterator for BytesGenerator<'a> {
-    type Item = Box<[u8]>;
+    pub fn reset(&mut self, total_bytes: u64, gen_type: PassType<'a>) {
+        if let PassType::Random = gen_type {
+            if self.rng.is_none() {
+                self.rng = Some(RefCell::new(rand::thread_rng()));
+            }
+        }
 
-    fn next(&mut self) -> Option<Box<[u8]>> {
+        self.total_bytes = total_bytes;
+        self.gen_type = gen_type;
+
+        self.bytes_generated.set(0);
+    }
+
+    pub fn next(&mut self) -> Option<&[u8]> {
         // We go over the total_bytes limit when !self.exact and total_bytes isn't a multiple
         // of self.block_size
         if self.bytes_generated.get() >= self.total_bytes {
             return None;
         }
 
-        let this_block_size: usize = {
+        let this_block_size = {
             if !self.exact {
                 self.block_size
             } else {
-                let bytes_left: u64 = self.total_bytes - self.bytes_generated.get();
+                let bytes_left = self.total_bytes - self.bytes_generated.get();
                 if bytes_left >= self.block_size as u64 {
                     self.block_size
                 } else {
@@ -162,17 +175,12 @@ impl<'a> Iterator for BytesGenerator<'a> {
             }
         };
 
-        let mut bytes: Vec<u8> = Vec::with_capacity(this_block_size);
+        let bytes = &mut self.bytes[..this_block_size];
 
         match self.gen_type {
             PassType::Random => {
-                // This is ok because the vector was
-                // allocated with the same capacity
-                unsafe {
-                    bytes.set_len(this_block_size);
-                }
                 let mut rng = self.rng.as_ref().unwrap().borrow_mut();
-                rng.fill(&mut bytes[..]);
+                rng.fill(bytes);
             }
             PassType::Pattern(pattern) => {
                 let skip = {
@@ -182,10 +190,17 @@ impl<'a> Iterator for BytesGenerator<'a> {
                         (pattern.len() as u64 % self.bytes_generated.get()) as usize
                     }
                 };
-                // Same range as 0..this_block_size but we start with the right index
-                for i in skip..this_block_size + skip {
-                    let index = i % pattern.len();
-                    bytes.push(pattern[index]);
+
+                // Copy the pattern in chunks rather than simply one byte at a time
+                let mut i = 0;
+                while i < this_block_size {
+                    let start = (i + skip) % pattern.len();
+                    let end = (this_block_size - i).min(pattern.len());
+                    let len = end - start;
+
+                    bytes[i..i + len].copy_from_slice(&pattern[start..end]);
+
+                    i += len;
                 }
             }
         };
@@ -193,7 +208,7 @@ impl<'a> Iterator for BytesGenerator<'a> {
         let new_bytes_generated = self.bytes_generated.get() + this_block_size as u64;
         self.bytes_generated.set(new_bytes_generated);
 
-        Some(bytes.into_boxed_slice())
+        Some(bytes)
     }
 }
 
@@ -443,6 +458,10 @@ fn wipe_file(
             .open(path)
             .expect("Failed to open file for writing");
 
+        // NOTE: it does not really matter what we set for total_bytes and gen_type here, so just
+        //       use bogus values
+        let mut generator = BytesGenerator::new(0, PassType::Pattern(&[]), exact);
+
         for (i, pass_type) in pass_sequence.iter().enumerate() {
             if verbose {
                 let pass_name: String = pass_name(*pass_type);
@@ -467,7 +486,8 @@ fn wipe_file(
                 }
             }
             // size is an optional argument for exactly how many bytes we want to shred
-            do_pass(&mut file, path, *pass_type, size, exact).expect("File write pass failed");
+            do_pass(&mut file, path, &mut generator, *pass_type, size)
+                .expect("File write pass failed");
             // Ignore failed writes; just keep trying
         }
     }
@@ -477,22 +497,22 @@ fn wipe_file(
     }
 }
 
-fn do_pass(
+fn do_pass<'a>(
     file: &mut File,
     path: &Path,
-    generator_type: PassType,
+    generator: &mut BytesGenerator<'a>,
+    generator_type: PassType<'a>,
     given_file_size: Option<u64>,
-    exact: bool,
 ) -> Result<(), io::Error> {
     file.seek(SeekFrom::Start(0))?;
 
     // Use the given size or the whole file if not specified
     let size: u64 = given_file_size.unwrap_or(get_file_size(path)?);
 
-    let generator = BytesGenerator::new(size, generator_type, exact);
+    generator.reset(size, generator_type);
 
-    for block in generator {
-        file.write_all(&*block)?;
+    while let Some(block) = generator.next() {
+        file.write_all(block)?;
     }
 
     file.sync_data()?;