Restrict piece length

- Must be greater than zero
- Must be a power of two (but can override with `--allow uneven-piece-length`
- Must be greater than 16KiB (but can override with `--allow small-piece-length`
- Must be less than u32 max

type: changed

src/bytes.rs

@@ -12,6 +12,12 @@ const YI: u128 = ZI << 10;
 #[derive(Debug, PartialEq, Copy, Clone)]
 pub(crate) struct Bytes(pub(crate) u128);
 
+impl Bytes {
+  pub(crate) fn is_power_of_two(self) -> bool {
+    self.0 == 0 || self.0 & (self.0 - 1) == 0
+  }
+}
+
 fn float_to_int(x: f64) -> u128 {
   #![allow(
     clippy::as_conversions,

src/common.rs

@@ -2,7 +2,7 @@
 pub(crate) use std::{
   borrow::Cow,
   cmp::{Ordering, Reverse},
-  collections::{BTreeMap, HashMap},
+  collections::{BTreeMap, BTreeSet, HashMap},
   convert::{Infallible, TryInto},
   env,
   ffi::{OsStr, OsString},
@@ -44,7 +44,7 @@ pub(crate) use crate::{
 // structs and enums
 pub(crate) use crate::{
   bytes::Bytes, env::Env, error::Error, file_info::FileInfo, hasher::Hasher, info::Info,
-  metainfo::Metainfo, mode::Mode, opt::Opt, platform::Platform, style::Style,
+  lint::Lint, metainfo::Metainfo, mode::Mode, opt::Opt, platform::Platform, style::Style,
   subcommand::Subcommand, torrent::Torrent, use_color::UseColor,
 };
 

src/env.rs

@@ -91,6 +91,17 @@ impl Env {
           self.err_style.message().suffix(),
         )
         .ok();
+
+        if let Some(lint) = error.lint() {
+          writeln!(
+            &mut self.err,
+            "{}: This check can be disabled with `--allow {}`.",
+            self.err_style.message().paint("note"),
+            lint.name()
+          )
+          .ok();
+        }
+
         Err(EXIT_FAILURE)
       }
     } else {

src/error.rs

@@ -37,7 +37,13 @@ pub(crate) enum Error {
     bytes,
     Bytes(u32::max_value().into())
   ))]
-  PieceLength { bytes: Bytes },
+  PieceLengthTooLarge { bytes: Bytes },
+  #[snafu(display("Piece length `{}` is not an even power of two", bytes))]
+  PieceLengthUneven { bytes: Bytes },
+  #[snafu(display("Piece length must be at least 16 KiB"))]
+  PieceLengthSmall,
+  #[snafu(display("Piece length cannot be zero"))]
+  PieceLengthZero,
   #[snafu(display("Serialization failed: {}", source))]
   Serialize { source: serde_bencode::Error },
   #[snafu(display("Failed to write to standard error: {}", source))]
@@ -51,6 +57,18 @@ pub(crate) enum Error {
     feature
   ))]
   Unstable { feature: &'static str },
+  #[snafu(display("Unknown lint: {}", text))]
+  LintUnknown { text: String },
+}
+
+impl Error {
+  pub(crate) fn lint(&self) -> Option<Lint> {
+    match self {
+      Self::PieceLengthUneven { .. } => Some(Lint::UnevenPieceLength),
+      Self::PieceLengthSmall { .. } => Some(Lint::SmallPieceLength),
+      _ => None,
+    }
+  }
 }
 
 impl From<clap::Error> for Error {

src/lint.rs

@@ -0,0 +1,70 @@
+use crate::common::*;
+
+#[derive(Eq, PartialEq, Debug, Copy, Clone, Ord, PartialOrd)]
+pub(crate) enum Lint {
+  UnevenPieceLength,
+  SmallPieceLength,
+}
+
+const UNEVEN_PIECE_LENGTH: &str = "uneven-piece-length";
+const SMALL_PIECE_LENGTH: &str = "small-piece-length";
+
+impl Lint {
+  pub(crate) fn name(self) -> &'static str {
+    match self {
+      Self::UnevenPieceLength => UNEVEN_PIECE_LENGTH,
+      Self::SmallPieceLength => SMALL_PIECE_LENGTH,
+    }
+  }
+}
+
+impl FromStr for Lint {
+  type Err = Error;
+
+  fn from_str(text: &str) -> Result<Self, Self::Err> {
+    match text.replace('_', "-").to_lowercase().as_str() {
+      UNEVEN_PIECE_LENGTH => Ok(Self::UnevenPieceLength),
+      SMALL_PIECE_LENGTH => Ok(Self::SmallPieceLength),
+      _ => Err(Error::LintUnknown {
+        text: text.to_string(),
+      }),
+    }
+  }
+}
+
+impl Display for Lint {
+  fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+    write!(f, "{}", self.name())
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn from_str_ok() {
+    assert_eq!(
+      Lint::UnevenPieceLength,
+      "uneven_piece_length".parse().unwrap()
+    );
+
+    assert_eq!(
+      Lint::UnevenPieceLength,
+      "uneven-piece-length".parse().unwrap()
+    );
+
+    assert_eq!(
+      Lint::UnevenPieceLength,
+      "UNEVEN-piece-length".parse().unwrap()
+    );
+  }
+
+  #[test]
+  fn from_str_err() {
+    assert_matches!(
+      "foo".parse::<Lint>(),
+      Err(Error::LintUnknown { text }) if text == "foo"
+    );
+  }
+}

src/main.rs

@@ -52,6 +52,7 @@ mod hasher;
 mod info;
 mod into_u64;
 mod into_usize;
+mod lint;
 mod metainfo;
 mod mode;
 mod opt;

src/torrent/create.rs

@@ -15,6 +15,13 @@ pub(crate) struct Create {
     long_help = "Use `ANNOUNCE` as the primary tracker announce URL. To supply multiple announce URLs, also use `--announce-tier`."
   )]
   announce: Url,
+  #[structopt(
+    name = "ALLOW",
+    long = "allow",
+    help = "Use `ANNOUNCE` as the primary tracker announce URL.",
+    long_help = "Use `ANNOUNCE` as the primary tracker announce URL. To supply multiple announce URLs, also use `--announce-tier`."
+  )]
+  allowed_lints: Vec<Lint>,
   #[structopt(
     long = "announce-tier",
     name = "ANNOUNCE-TIER",
@@ -98,15 +105,57 @@ Note: Many BitTorrent clients do not implement the behavior described in BEP 12.
   private: bool,
 }
 
+struct Linter {
+  allowed: BTreeSet<Lint>,
+}
+
+impl Linter {
+  fn new() -> Linter {
+    Linter {
+      allowed: BTreeSet::new(),
+    }
+  }
+
+  fn allow(&mut self, allowed: impl IntoIterator<Item = Lint>) {
+    self.allowed.extend(allowed)
+  }
+
+  fn is_allowed(&self, lint: Lint) -> bool {
+    self.allowed.contains(&lint)
+  }
+
+  fn is_denied(&self, lint: Lint) -> bool {
+    !self.is_allowed(lint)
+  }
+}
+
 impl Create {
   pub(crate) fn run(self, env: &Env) -> Result<(), Error> {
-    let piece_length: u32 = self
-      .piece_length
-      .0
-      .try_into()
-      .map_err(|_| Error::PieceLength {
+    let mut linter = Linter::new();
+    linter.allow(self.allowed_lints.iter().cloned());
+
+    if linter.is_denied(Lint::UnevenPieceLength) && !self.piece_length.is_power_of_two() {
+      return Err(Error::PieceLengthUneven {
         bytes: self.piece_length,
-      })?;
+      });
+    }
+
+    let piece_length: u32 =
+      self
+        .piece_length
+        .0
+        .try_into()
+        .map_err(|_| Error::PieceLengthTooLarge {
+          bytes: self.piece_length,
+        })?;
+
+    if piece_length == 0 {
+      return Err(Error::PieceLengthZero);
+    }
+
+    if linter.is_denied(Lint::SmallPieceLength) && piece_length < 16 * 1024 {
+      return Err(Error::PieceLengthSmall);
+    }
 
     let input = env.resolve(&self.input);
 
@@ -405,14 +454,14 @@ mod tests {
       "--announce",
       "http://bar",
       "--piece-length",
-      "1",
+      "64KiB",
     ]);
     fs::write(env.resolve("foo"), "").unwrap();
     env.run().unwrap();
     let torrent = env.resolve("foo.torrent");
     let bytes = fs::read(torrent).unwrap();
     let metainfo = serde_bencode::de::from_bytes::<Metainfo>(&bytes).unwrap();
-    assert_eq!(metainfo.info.piece_length, 1);
+    assert_eq!(metainfo.info.piece_length, 64 * 1024);
   }
 
   #[test]
@@ -441,7 +490,7 @@ mod tests {
       "--announce",
       "http://bar",
       "--piece-length",
-      "1",
+      "16KiB",
     ]);
     fs::write(env.resolve("foo"), "").unwrap();
     env.run().unwrap();
@@ -459,7 +508,7 @@ mod tests {
       "--announce",
       "http://bar",
       "--piece-length",
-      "1",
+      "32KiB",
     ]);
     let dir = env.resolve("foo");
     fs::create_dir(&dir).unwrap();
@@ -588,6 +637,8 @@ mod tests {
       "http://bar",
       "--piece-length",
       "1",
+      "--allow",
+      "small-piece-length",
     ]);
     let contents = "bar";
     fs::write(env.resolve("foo"), contents).unwrap();
@@ -732,4 +783,80 @@ mod tests {
 
     panic!("Failed to read `opened.txt`.");
   }
+
+  #[test]
+  fn uneven_piece_length() {
+    let mut env = environment(&[
+      "--input",
+      "foo",
+      "--announce",
+      "http://bar",
+      "--piece-length",
+      "17KiB",
+    ]);
+    assert_matches!(
+      env.run(),
+      Err(Error::PieceLengthUneven { bytes }) if bytes.0 == 17 * 1024
+    );
+  }
+
+  #[test]
+  fn uneven_piece_length_allow() {
+    let mut env = environment(&[
+      "--input",
+      "foo",
+      "--announce",
+      "http://bar",
+      "--piece-length",
+      "17KiB",
+      "--allow",
+      "uneven-piece-length",
+    ]);
+    let dir = env.resolve("foo");
+    fs::create_dir(&dir).unwrap();
+    env.run().unwrap();
+  }
+
+  #[test]
+  fn zero_piece_length() {
+    let mut env = environment(&[
+      "--input",
+      "foo",
+      "--announce",
+      "http://bar",
+      "--piece-length",
+      "0",
+    ]);
+    assert_matches!(env.run(), Err(Error::PieceLengthZero));
+  }
+
+  #[test]
+  fn small_piece_length() {
+    let mut env = environment(&[
+      "--input",
+      "foo",
+      "--announce",
+      "http://bar",
+      "--piece-length",
+      "8KiB",
+    ]);
+    assert_matches!(env.run(), Err(Error::PieceLengthSmall));
+  }
+
+  #[test]
+  fn small_piece_length_allow() {
+    let mut env = environment(&[
+      "--input",
+      "foo",
+      "--announce",
+      "http://bar",
+      "--piece-length",
+      "8KiB",
+      "--allow",
+      "small-piece-length",
+    ]);
+    let dir = env.resolve("foo");
+    fs::create_dir(&dir).unwrap();
+    env.run().unwrap();
+  }
 }