EBML: Support locking the ElementReader to the current master

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

src/ebml/element_reader.rs

@@ -148,6 +148,8 @@ ebml_master_elements! {
 }
 
 struct ElementReaderContext {
+	/// Previous master element
+	previous_master: Option<MasterElement>,
 	/// Current master element
 	current_master: Option<MasterElement>,
 	/// Remaining length of the master element
@@ -156,17 +158,24 @@ struct ElementReaderContext {
 	max_id_length: u8,
 	/// Maximum size in octets of all element data sizes
 	max_size_length: u8,
+	/// Whether the reader is locked to the current master element
+	///
+	/// This is set with [`ElementReader::lock`], and is used to prevent
+	/// the reader from reading past the end of the current master element.
+	locked: bool,
 }
 
 impl Default for ElementReaderContext {
 	fn default() -> Self {
 		Self {
+			previous_master: None,
 			current_master: None,
 			master_length: 0,
 			// https://www.rfc-editor.org/rfc/rfc8794.html#name-ebmlmaxidlength-element
 			max_id_length: 4,
 			// https://www.rfc-editor.org/rfc/rfc8794.html#name-ebmlmaxsizelength-element
 			max_size_length: 8,
+			locked: false,
 		}
 	}
 }
@@ -213,6 +222,7 @@ where
 			return Ok(ElementReaderYield::Unknown(header));
 		};
 
+		self.ctx.previous_master = self.ctx.current_master;
 		self.ctx.current_master = Some(*master);
 		self.ctx.master_length = header.size.value();
 		Ok(ElementReaderYield::Master((
@@ -221,12 +231,34 @@ 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);
+			Ok(())
+		} else {
+			decode_err!(@BAIL Ebml, "Expected a parent element to be available")
+		}
+	}
+
 	pub(crate) fn next(&mut self) -> Result<ElementReaderYield> {
 		let Some(current_master) = self.ctx.current_master else {
 			return self.next_master();
 		};
 
 		if self.ctx.master_length == 0 {
+			if self.ctx.locked {
+				return Ok(ElementReaderYield::Eof);
+			}
+
 			return self.next_master();
 		}
 
@@ -244,6 +276,21 @@ where
 			return Ok(ElementReaderYield::Unknown(header));
 		};
 
+		if child.data_type == ElementDataType::Master {
+			self.ctx.current_master = Some(
+				*MASTER_ELEMENTS
+					.get(&header.id)
+					.expect("Nested master elements should be defined at this level."),
+			);
+			self.ctx.master_length = header.size.value();
+
+			// We encountered a nested master element
+			return Ok(ElementReaderYield::Master((
+				child.ident,
+				header.size.value(),
+			)));
+		}
+
 		Ok(ElementReaderYield::Child((*child, header.size.value())))
 	}