EBML: Stop requiring explicit locking of reader

Signed-off-by: Serial <69764315+Serial-ATA@users.noreply.github.com>

Author: Serial-ATA

src/ebml/element_reader.rs

@@ -3,6 +3,7 @@ use crate::error::Result;
 use crate::macros::{decode_err, try_vec};
 
 use std::io::Read;
+use std::ops::{Deref, DerefMut};
 
 use byteorder::{BigEndian, ReadBytesExt};
 use lofty_attr::ebml_master_elements;
@@ -229,7 +230,11 @@ where
 		self.ctx.previous_master_length = self.ctx.master_length;
 	}
 
-	fn next_master(&mut self) -> Result<ElementReaderYield> {
+	fn goto_next_master(&mut self) -> Result<ElementReaderYield> {
+		if self.ctx.master_length != 0 {
+			self.skip(self.ctx.master_length)?;
+		}
+
 		let header = ElementHeader::read(
 			&mut self.reader,
 			self.ctx.max_id_length,
@@ -249,15 +254,6 @@ where
 		)))
 	}
 
-	/// Lock the reader to the current master element
-	pub(crate) fn lock(&mut self) {
-		self.ctx.locked = true;
-	}
-
-	pub(crate) fn unlock(&mut self) {
-		self.ctx.locked = false;
-	}
-
 	pub(crate) fn goto_previous_master(&mut self) -> Result<()> {
 		if let Some(previous_master) = self.ctx.previous_master {
 			self.ctx.current_master = Some(previous_master);
@@ -270,15 +266,15 @@ where
 
 	pub(crate) fn next(&mut self) -> Result<ElementReaderYield> {
 		let Some(current_master) = self.ctx.current_master else {
-			return self.next_master();
+			return self.goto_next_master();
 		};
 
 		if self.ctx.master_length == 0 {
 			if self.ctx.locked {
 				return Ok(ElementReaderYield::Eof);
 			}
 
-			return self.next_master();
+			return self.goto_next_master();
 		}
 
 		let header = ElementHeader::read(self, self.ctx.max_id_length, self.ctx.max_size_length)?;
@@ -310,11 +306,33 @@ where
 		Ok(ElementReaderYield::Child((*child, header.size.value())))
 	}
 
+	fn lock(&mut self) {
+		self.ctx.locked = true;
+	}
+
+	fn unlock(&mut self) {
+		self.ctx.locked = false;
+	}
+
+	pub(crate) fn children(&mut self) -> ElementChildIterator<'_, R> {
+		self.lock();
+		ElementChildIterator::new(self)
+	}
+
 	pub(crate) fn skip(&mut self, length: u64) -> Result<()> {
 		std::io::copy(&mut self.by_ref().take(length), &mut std::io::sink())?;
 		Ok(())
 	}
 
+	pub(crate) fn skip_element(&mut self, element_header: ElementHeader) -> Result<()> {
+		log::debug!(
+			"Encountered unknown EBML element in header: {:X}",
+			element_header.id.0
+		);
+		self.skip(element_header.size.value())?;
+		Ok(())
+	}
+
 	pub(crate) fn read_signed_int(&mut self, element_length: u64) -> Result<i64> {
 		// https://www.rfc-editor.org/rfc/rfc8794.html#section-7.1
 		// A Signed Integer Element MUST declare a length from zero to eight octets
@@ -390,3 +408,68 @@ where
 		todo!()
 	}
 }
+
+pub(crate) struct ElementChildIterator<'a, R>
+where
+	R: Read,
+{
+	reader: &'a mut ElementReader<R>,
+}
+
+impl<'a, R> ElementChildIterator<'a, R>
+where
+	R: Read,
+{
+	pub(crate) fn new(reader: &'a mut ElementReader<R>) -> Self {
+		Self { reader }
+	}
+
+	pub(crate) fn next(&mut self) -> Result<Option<ElementReaderYield>> {
+		match self.reader.next() {
+			Ok(ElementReaderYield::Unknown(header)) => {
+				self.reader.skip_element(header)?;
+				self.next()
+			},
+			Ok(ElementReaderYield::Eof) => Ok(None),
+			Err(e) => Err(e),
+			element => element.map(Some),
+		}
+	}
+
+	pub(crate) fn master_exhausted(&self) -> bool {
+		self.reader.ctx.master_length == 0
+	}
+
+	pub(crate) fn inner(&mut self) -> &mut ElementReader<R> {
+		self.reader
+	}
+}
+
+impl<'a, R> Deref for ElementChildIterator<'a, R>
+where
+	R: Read,
+{
+	type Target = ElementReader<R>;
+
+	fn deref(&self) -> &Self::Target {
+		self.reader
+	}
+}
+
+impl<'a, R> DerefMut for ElementChildIterator<'a, R>
+where
+	R: Read,
+{
+	fn deref_mut(&mut self) -> &mut Self::Target {
+		self.reader
+	}
+}
+
+impl<'a, R> Drop for ElementChildIterator<'a, R>
+where
+	R: Read,
+{
+	fn drop(&mut self) {
+		self.reader.unlock();
+	}
+}

src/ebml/read.rs

@@ -69,69 +69,60 @@ where
 		Err(e) => return Err(e),
 	}
 
-	element_reader.lock();
-
-	loop {
+	let mut child_reader = element_reader.children();
+	while let Some(child) = child_reader.next()? {
 		let ident;
 		let data_ty;
 		let size;
 
-		let res = element_reader.next()?;
-		match res {
+		match child {
 			// The only expected master element in the header is `DocTypeExtension`
 			ElementReaderYield::Master((ElementIdent::DocTypeExtension, _)) => continue,
 			ElementReaderYield::Child((child, size_)) => {
 				ident = child.ident;
 				data_ty = child.data_type;
 				size = size_;
 			},
-			ElementReaderYield::Unknown(element) => {
-				log::debug!(
-					"Encountered unknown EBML element in header: {:X}",
-					element.id.0
-				);
-				element_reader.skip(element.size.value())?;
-				continue;
-			},
 			_ => break,
 		}
 
 		if ident == ElementIdent::EBMLMaxIDLength {
-			properties.header.max_id_length = element_reader.read_unsigned_int(size)? as u8;
-			element_reader.set_max_id_length(properties.header.max_id_length);
+			properties.header.max_id_length = child_reader.read_unsigned_int(size)? as u8;
+			child_reader.set_max_id_length(properties.header.max_id_length);
 			continue;
 		}
 
 		if ident == ElementIdent::EBMLMaxSizeLength {
-			properties.header.max_size_length = element_reader.read_unsigned_int(size)? as u8;
-			element_reader.set_max_size_length(properties.header.max_size_length);
+			properties.header.max_size_length = child_reader.read_unsigned_int(size)? as u8;
+			child_reader.set_max_size_length(properties.header.max_size_length);
 			continue;
 		}
 
 		// Anything else in the header is unnecessary, and only read for the properties
 		// struct
 		if !parse_options.read_properties {
-			element_reader.skip(size)?;
+			child_reader.skip(size)?;
 			continue;
 		}
 
 		match ident {
 			ElementIdent::EBMLVersion => {
-				properties.header.version = element_reader.read_unsigned_int(size)?
+				properties.header.version = child_reader.read_unsigned_int(size)?
 			},
 			ElementIdent::EBMLReadVersion => {
-				properties.header.read_version = element_reader.read_unsigned_int(size)?
-			},
-			ElementIdent::DocType => {
-				properties.header.doc_type = element_reader.read_string(size)?
+				properties.header.read_version = child_reader.read_unsigned_int(size)?
 			},
+			ElementIdent::DocType => properties.header.doc_type = child_reader.read_string(size)?,
 			ElementIdent::DocTypeVersion => {
-				properties.header.doc_type_version = element_reader.read_unsigned_int(size)?
+				properties.header.doc_type_version = child_reader.read_unsigned_int(size)?
 			},
-			_ => element_reader.skip(size)?,
+			_ => child_reader.skip(size)?,
 		}
 	}
 
-	element_reader.unlock();
+	debug_assert!(
+		child_reader.master_exhausted(),
+		"There should be no remaining elements in the header"
+	);
 	Ok(())
 }

src/ebml/read/segment.rs

@@ -16,16 +16,14 @@ pub(super) fn read_from<R>(
 where
 	R: Read + Seek,
 {
-	element_reader.lock();
-
 	let mut tags = None;
 
-	loop {
-		let res = element_reader.next()?;
-		match res {
+	let mut children_reader = element_reader.children();
+	while let Some(child) = children_reader.next()? {
+		match child {
 			ElementReaderYield::Master((id, size)) => match id {
 				ElementIdent::Info => {
-					segment_info::read_from(element_reader, parse_options, properties)?
+					segment_info::read_from(children_reader.inner(), parse_options, properties)?
 				},
 				ElementIdent::Cluster => todo!("Support segment.Cluster"),
 				ElementIdent::Tracks => todo!("Support segment.Tracks"),
@@ -37,23 +35,15 @@ where
 					// elements, so we can just skip any useless ones.
 
 					log::debug!("Skipping EBML master element: {:?}", id);
-					element_reader.skip(size)?;
-					element_reader.goto_previous_master()?;
+					children_reader.skip(size)?;
+					children_reader.goto_previous_master()?;
 					continue;
 				},
 			},
-			ElementReaderYield::Unknown(element) => {
-				log::debug!("Skipping unknown EBML element: {:X}", element.id.0);
-				element_reader.skip(element.size.value())?;
-				continue;
-			},
-			ElementReaderYield::Eof => {
-				element_reader.unlock();
-				break;
-			},
-			_ => {
+			ElementReaderYield::Child(_) => {
 				decode_err!(@BAIL Ebml, "Segment element should only contain master elements")
 			},
+			_ => break,
 		}
 	}
 

src/ebml/read/segment_info.rs

@@ -14,25 +14,24 @@ pub(super) fn read_from<R>(
 where
 	R: Read + Seek,
 {
-	element_reader.lock();
+	let mut children_reader = element_reader.children();
 
-	loop {
-		let res = element_reader.next()?;
-		match res {
+	while let Some(child) = children_reader.next()? {
+		match child {
 			ElementReaderYield::Master((id, size)) => {
 				// We do not end up using information from any of the nested master
 				// elements, so we can just skip them.
 
 				log::debug!("Skipping EBML master element: {:?}", id);
-				element_reader.skip(size)?;
-				element_reader.goto_previous_master()?;
+				children_reader.skip(size)?;
+				children_reader.goto_previous_master()?;
 				continue;
 			},
 			ElementReaderYield::Child((child, size)) => {
 				match child.ident {
 					ElementIdent::TimecodeScale => {
 						properties.segment_info.timestamp_scale =
-							element_reader.read_unsigned_int(size)?;
+							children_reader.read_unsigned_int(size)?;
 
 						if properties.segment_info.timestamp_scale == 0 {
 							log::warn!("Segment.Info.TimecodeScale is 0, which is invalid");
@@ -42,33 +41,26 @@ where
 						}
 					},
 					ElementIdent::MuxingApp => {
-						properties.segment_info.muxing_app = element_reader.read_utf8(size)?
+						properties.segment_info.muxing_app = children_reader.read_utf8(size)?
 					},
 					ElementIdent::WritingApp => {
-						properties.segment_info.writing_app = element_reader.read_utf8(size)?
+						properties.segment_info.writing_app = children_reader.read_utf8(size)?
 					},
 					_ => {
 						// We do not end up using information from all of the segment
 						// elements, so we can just skip any useless ones.
 
 						log::debug!("Skipping EBML child element: {:?}", child.ident);
-						element_reader.skip(size)?;
+						children_reader.skip(size)?;
 						continue;
 					},
 				}
 			},
-			ElementReaderYield::Unknown(element) => {
-				log::debug!("Skipping unknown EBML element: {:X}", element.id.0);
-				element_reader.skip(element.size.value())?;
-				continue;
-			},
-			ElementReaderYield::Eof => {
-				element_reader.unlock();
-				break;
-			},
+			_ => break,
 		}
 	}
 
+	drop(children_reader);
 	element_reader.goto_previous_master()?;
 	Ok(())
 }