initial support for kernel data download (#2765)

* initial support for kernel data download

* fix vec backend for tests

* cleanup after rebase

Cargo.lock

@@ -786,6 +786,7 @@ dependencies = [
  "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 1.0.90 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 1.0.90 (registry+https://github.com/rust-lang/crates.io-index)",
+ "tempfile 3.0.7 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -869,6 +870,7 @@ dependencies = [
  "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 1.0.90 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 1.0.90 (registry+https://github.com/rust-lang/crates.io-index)",
+ "tempfile 3.0.7 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -1784,6 +1786,14 @@ dependencies = [
  "ucd-util 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "remove_dir_all"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "winapi 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "ring"
 version = "0.13.5"
@@ -2025,6 +2035,19 @@ dependencies = [
  "unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "tempfile"
+version = "3.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "cfg-if 0.1.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "libc 0.2.51 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "redox_syscall 0.1.54 (registry+https://github.com/rust-lang/crates.io-index)",
+ "remove_dir_all 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "winapi 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "term"
 version = "0.5.2"
@@ -2749,6 +2772,7 @@ dependencies = [
 "checksum redox_users 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "3fe5204c3a17e97dde73f285d49be585df59ed84b50a872baf416e73b62c3828"
 "checksum regex 1.1.5 (registry+https://github.com/rust-lang/crates.io-index)" = "559008764a17de49a3146b234641644ed37d118d1ef641a0bb573d146edc6ce0"
 "checksum regex-syntax 0.6.6 (registry+https://github.com/rust-lang/crates.io-index)" = "dcfd8681eebe297b81d98498869d4aae052137651ad7b96822f09ceb690d0a96"
+"checksum remove_dir_all 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "3488ba1b9a2084d38645c4c08276a1752dcbf2c7130d74f1569681ad5d2799c5"
 "checksum ring 0.13.5 (registry+https://github.com/rust-lang/crates.io-index)" = "2c4db68a2e35f3497146b7e4563df7d4773a2433230c5e4b448328e31740458a"
 "checksum ripemd160 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "482aa56cc68aaeccdaaff1cc5a72c247da8bbad3beb174ca5741f274c22883fb"
 "checksum rustc-demangle 0.1.14 (registry+https://github.com/rust-lang/crates.io-index)" = "ccc78bfd5acd7bf3e89cffcf899e5cb1a52d6fafa8dec2739ad70c9577a57288"
@@ -2781,6 +2805,7 @@ dependencies = [
 "checksum syn 0.14.9 (registry+https://github.com/rust-lang/crates.io-index)" = "261ae9ecaa397c42b960649561949d69311f08eeaea86a65696e6e46517cf741"
 "checksum syn 0.15.31 (registry+https://github.com/rust-lang/crates.io-index)" = "d2b4cfac95805274c6afdb12d8f770fa2d27c045953e7b630a81801953699a9a"
 "checksum synstructure 0.10.1 (registry+https://github.com/rust-lang/crates.io-index)" = "73687139bf99285483c96ac0add482c3776528beac1d97d444f6e91f203a2015"
+"checksum tempfile 3.0.7 (registry+https://github.com/rust-lang/crates.io-index)" = "b86c784c88d98c801132806dadd3819ed29d8600836c4088e855cdf3e178ed8a"
 "checksum term 0.5.2 (registry+https://github.com/rust-lang/crates.io-index)" = "edd106a334b7657c10b7c540a0106114feadeb4dc314513e97df481d5d966f42"
 "checksum term_size 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)" = "9e5b9a66db815dcfd2da92db471106457082577c3c278d4138ab3e3b4e189327"
 "checksum termcolor 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)" = "4096add70612622289f2fdcdbd5086dc81c1e2675e6ae58d6c4f62a16c6d7f2f"

api/src/handlers.rs

@@ -33,6 +33,7 @@ use self::pool_api::PoolInfoHandler;
 use self::pool_api::PoolPushHandler;
 use self::server_api::IndexHandler;
 use self::server_api::StatusHandler;
+use self::server_api::KernelDownloadHandler;
 use self::transactions_api::TxHashSetHandler;
 use crate::auth::{BasicAuthMiddleware, GRIN_BASIC_REALM};
 use crate::chain;
@@ -135,6 +136,9 @@ pub fn build_router(
 		chain: Arc::downgrade(&chain),
 		peers: Arc::downgrade(&peers),
 	};
+	let kernel_download_handler = KernelDownloadHandler {
+		peers: Arc::downgrade(&peers),
+	};
 	let txhashset_handler = TxHashSetHandler {
 		chain: Arc::downgrade(&chain),
 	};
@@ -165,6 +169,7 @@ pub fn build_router(
 	router.add_route("/v1/chain/validate", Arc::new(chain_validation_handler))?;
 	router.add_route("/v1/txhashset/*", Arc::new(txhashset_handler))?;
 	router.add_route("/v1/status", Arc::new(status_handler))?;
+	router.add_route("/v1/kerneldownload", Arc::new(kernel_download_handler))?;
 	router.add_route("/v1/pool", Arc::new(pool_info_handler))?;
 	router.add_route("/v1/pool/push", Arc::new(pool_push_handler))?;
 	router.add_route("/v1/peers/all", Arc::new(peers_all_handler))?;

api/src/handlers/server_api.rs

@@ -19,7 +19,7 @@ use crate::rest::*;
 use crate::router::{Handler, ResponseFuture};
 use crate::types::*;
 use crate::web::*;
-use hyper::{Body, Request};
+use hyper::{Body, Request, StatusCode};
 use std::sync::Weak;
 
 // RESTful index of available api endpoints
@@ -36,6 +36,29 @@ impl Handler for IndexHandler {
 	}
 }
 
+pub struct KernelDownloadHandler {
+	pub peers: Weak<p2p::Peers>,
+}
+
+impl Handler for KernelDownloadHandler {
+	fn post(&self, _req: Request<Body>) -> ResponseFuture {
+		if let Some(peer) = w_fut!(&self.peers).most_work_peer() {
+			match peer.send_kernel_data_request() {
+				Ok(_) => response(StatusCode::OK, "{}"),
+				Err(e) => response(
+					StatusCode::INTERNAL_SERVER_ERROR,
+					format!("requesting kernel data from peer failed: {:?}", e),
+				),
+			}
+		} else {
+			response(
+				StatusCode::INTERNAL_SERVER_ERROR,
+				format!("requesting kernel data from peer failed (no peers)"),
+			)
+		}
+	}
+}
+
 /// Status handler. Post a summary of the server status
 /// GET /v1/status
 pub struct StatusHandler {

chain/src/chain.rs

@@ -23,6 +23,7 @@ use crate::core::core::{
 };
 use crate::core::global;
 use crate::core::pow;
+use crate::core::ser::{Readable, StreamingReader};
 use crate::error::{Error, ErrorKind};
 use crate::pipe;
 use crate::store;
@@ -36,6 +37,7 @@ use crate::util::{Mutex, RwLock, StopState};
 use grin_store::Error::NotFoundErr;
 use std::collections::HashMap;
 use std::fs::{self, File};
+use std::io::Read;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;
@@ -659,6 +661,28 @@ impl Chain {
 		self.txhashset.read().roots()
 	}
 
+	/// Provides a reading view into the current kernel state.
+	pub fn kernel_data_read(&self) -> Result<File, Error> {
+		let txhashset = self.txhashset.read();
+		txhashset::rewindable_kernel_view(&txhashset, |view| view.kernel_data_read())
+	}
+
+	/// Writes kernels provided to us (via a kernel data download).
+	/// Currently does not write these to disk and simply deserializes
+	/// the provided data.
+	/// TODO - Write this data to disk and validate the rebuilt kernel MMR.
+	pub fn kernel_data_write(&self, reader: &mut Read) -> Result<(), Error> {
+		let mut count = 0;
+		let mut stream = StreamingReader::new(reader, Duration::from_secs(1));
+		while let Ok(_kernel) = TxKernelEntry::read(&mut stream) {
+			count += 1;
+		}
+
+		debug!("kernel_data_write: read {} kernels", count);
+
+		Ok(())
+	}
+
 	/// Provides a reading view into the current txhashset state as well as
 	/// the required indexes for a consumer to rewind to a consistent state
 	/// at the provided block hash.

chain/src/txhashset/rewindable_kernel_view.rs

@@ -14,6 +14,8 @@
 
 //! Lightweight readonly view into kernel MMR for convenience.
 
+use std::fs::File;
+
 use crate::core::core::pmmr::RewindablePMMR;
 use crate::core::core::{BlockHeader, TxKernel};
 use crate::error::{Error, ErrorKind};
@@ -78,4 +80,14 @@ impl<'a> RewindableKernelView<'a> {
 		}
 		Ok(())
 	}
+
+	/// Read the "raw" kernel backend data file (via temp file for consistent view on data).
+	pub fn kernel_data_read(&self) -> Result<File, Error> {
+		let file = self
+			.pmmr
+			.backend()
+			.data_as_temp_file()
+			.map_err(|_| ErrorKind::FileReadErr("Data file woes".into()))?;
+		Ok(file)
+	}
 }

core/src/core/pmmr/backend.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::fs::File;
+
 use croaring::Bitmap;
 
 use crate::core::hash::Hash;
@@ -59,6 +61,11 @@ pub trait Backend<T: PMMRable> {
 	/// triggered removal).
 	fn remove(&mut self, position: u64) -> Result<(), String>;
 
+	/// Creates a temp file containing the contents of the underlying data file
+	/// from the backend storage. This allows a caller to see a consistent view
+	/// of the data without needing to lock the backend storage.
+	fn data_as_temp_file(&self) -> Result<File, String>;
+
 	/// Release underlying datafiles and locks
 	fn release_files(&mut self);
 

core/src/core/pmmr/rewindable_pmmr.rs

@@ -49,6 +49,11 @@ where
 		}
 	}
 
+	/// Reference to the underlying storage backend.
+	pub fn backend(&'a self) -> &Backend<T> {
+		self.backend
+	}
+
 	/// Build a new readonly PMMR pre-initialized to
 	/// last_pos with the provided backend.
 	pub fn at(backend: &'a B, last_pos: u64) -> RewindablePMMR<'_, T, B> {

core/tests/vec_backend.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::fs::File;
+
 use self::core::core::hash::{DefaultHashable, Hash};
 use self::core::core::pmmr::{self, Backend};
 use self::core::core::BlockHeader;
@@ -103,6 +105,10 @@ impl<T: PMMRable> Backend<T> for VecBackend<T> {
 		Some(data.as_elmt())
 	}
 
+	fn data_as_temp_file(&self) -> Result<File, String> {
+		unimplemented!()
+	}
+
 	fn leaf_pos_iter(&self) -> Box<Iterator<Item = u64> + '_> {
 		unimplemented!()
 	}

p2p/Cargo.toml

@@ -18,6 +18,7 @@ num = "0.1"
 rand = "0.6"
 serde = "1"
 serde_derive = "1"
+tempfile = "3.0.5"
 log = "0.4"
 chrono = { version = "0.4.4", features = ["serde"] }
 

p2p/src/msg.rs

@@ -74,6 +74,8 @@ enum_from_primitive! {
 		BanReason = 18,
 		GetTransaction = 19,
 		TransactionKernel = 20,
+		KernelDataRequest = 21,
+		KernelDataResponse = 22,
 	}
 }
 
@@ -111,6 +113,8 @@ fn max_msg_size(msg_type: Type) -> u64 {
 		Type::BanReason => 64,
 		Type::GetTransaction => 32,
 		Type::TransactionKernel => 32,
+		Type::KernelDataRequest => 0,
+		Type::KernelDataResponse => 8,
 	}
 }
 
@@ -714,3 +718,30 @@ impl Readable for TxHashSetArchive {
 		})
 	}
 }
+
+pub struct KernelDataRequest {}
+
+impl Writeable for KernelDataRequest {
+	fn write<W: Writer>(&self, _writer: &mut W) -> Result<(), ser::Error> {
+		Ok(())
+	}
+}
+
+pub struct KernelDataResponse {
+	/// Size in bytes of the attached kernel data file.
+	pub bytes: u64,
+}
+
+impl Writeable for KernelDataResponse {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		writer.write_u64(self.bytes)?;
+		Ok(())
+	}
+}
+
+impl Readable for KernelDataResponse {
+	fn read(reader: &mut dyn Reader) -> Result<KernelDataResponse, ser::Error> {
+		let bytes = reader.read_u64()?;
+		Ok(KernelDataResponse { bytes })
+	}
+}

p2p/src/peer.rs

@@ -15,6 +15,7 @@
 use crate::util::{Mutex, RwLock};
 use std::fmt;
 use std::fs::File;
+use std::io::Read;
 use std::net::{Shutdown, TcpStream};
 use std::path::PathBuf;
 use std::sync::Arc;
@@ -26,7 +27,9 @@ use crate::core::pow::Difficulty;
 use crate::core::ser::Writeable;
 use crate::core::{core, global};
 use crate::handshake::Handshake;
-use crate::msg::{self, BanReason, GetPeerAddrs, Locator, Ping, TxHashSetRequest, Type};
+use crate::msg::{
+	self, BanReason, GetPeerAddrs, KernelDataRequest, Locator, Ping, TxHashSetRequest, Type,
+};
 use crate::protocol::Protocol;
 use crate::types::{
 	Capabilities, ChainAdapter, Error, NetAdapter, P2PConfig, PeerAddr, PeerInfo, ReasonForBan,
@@ -379,6 +382,13 @@ impl Peer {
 		)
 	}
 
+	pub fn send_kernel_data_request(&self) -> Result<(), Error> {
+		debug!("Asking {} for kernel data.", self.info.addr);
+		self.connection
+			.lock()
+			.send(&KernelDataRequest {}, msg::Type::KernelDataRequest)
+	}
+
 	/// Stops the peer, closing its connection
 	pub fn stop(&self) {
 		self.connection.lock().close();
@@ -521,6 +531,14 @@ impl ChainAdapter for TrackingAdapter {
 		self.adapter.get_block(h)
 	}
 
+	fn kernel_data_read(&self) -> Result<File, chain::Error> {
+		self.adapter.kernel_data_read()
+	}
+
+	fn kernel_data_write(&self, reader: &mut Read) -> Result<bool, chain::Error> {
+		self.adapter.kernel_data_write(reader)
+	}
+
 	fn txhashset_read(&self, h: Hash) -> Option<TxHashSetRead> {
 		self.adapter.txhashset_read(h)
 	}

p2p/src/peers.rs

@@ -15,6 +15,7 @@
 use crate::util::RwLock;
 use std::collections::HashMap;
 use std::fs::File;
+use std::io::Read;
 use std::path::PathBuf;
 use std::sync::Arc;
 
@@ -594,6 +595,14 @@ impl ChainAdapter for Peers {
 		self.adapter.get_block(h)
 	}
 
+	fn kernel_data_read(&self) -> Result<File, chain::Error> {
+		self.adapter.kernel_data_read()
+	}
+
+	fn kernel_data_write(&self, reader: &mut Read) -> Result<bool, chain::Error> {
+		self.adapter.kernel_data_write(reader)
+	}
+
 	fn txhashset_read(&self, h: Hash) -> Option<TxHashSetRead> {
 		self.adapter.txhashset_read(h)
 	}

p2p/src/protocol.rs

@@ -15,17 +15,19 @@
 use rand::{thread_rng, Rng};
 use std::cmp;
 use std::fs::{self, File, OpenOptions};
-use std::io::{BufWriter, Write};
+use std::io::{BufWriter, Seek, SeekFrom, Write};
 use std::sync::Arc;
 
+use chrono::prelude::Utc;
+use tempfile::tempfile;
+
 use crate::conn::{Message, MessageHandler, Response};
 use crate::core::core::{self, hash::Hash, CompactBlock};
 use crate::util::{RateCounter, RwLock};
-use chrono::prelude::Utc;
 
 use crate::msg::{
-	BanReason, GetPeerAddrs, Headers, Locator, PeerAddrs, Ping, Pong, TxHashSetArchive,
-	TxHashSetRequest, Type,
+	BanReason, GetPeerAddrs, Headers, KernelDataResponse, Locator, PeerAddrs, Ping, Pong,
+	TxHashSetArchive, TxHashSetRequest, Type,
 };
 use crate::types::{Error, NetAdapter, PeerInfo};
 
@@ -244,6 +246,54 @@ impl MessageHandler for Protocol {
 				Ok(None)
 			}
 
+			Type::KernelDataRequest => {
+				debug!("handle_payload: kernel_data_request");
+				let kernel_data = self.adapter.kernel_data_read()?;
+				let bytes = kernel_data.metadata()?.len();
+				let kernel_data_response = KernelDataResponse { bytes };
+				let mut response =
+					Response::new(Type::KernelDataResponse, &kernel_data_response, writer)?;
+				response.add_attachment(kernel_data);
+				Ok(Some(response))
+			}
+
+			Type::KernelDataResponse => {
+				let response: KernelDataResponse = msg.body()?;
+				debug!(
+					"handle_payload: kernel_data_response: bytes: {}",
+					response.bytes
+				);
+
+				let mut writer = BufWriter::new(tempfile()?);
+
+				let total_size = response.bytes as usize;
+				let mut remaining_size = total_size;
+
+				while remaining_size > 0 {
+					let size = msg.copy_attachment(remaining_size, &mut writer)?;
+					remaining_size = remaining_size.saturating_sub(size);
+
+					// Increase received bytes quietly (without affecting the counters).
+					// Otherwise we risk banning a peer as "abusive".
+					received_bytes.write().inc_quiet(size as u64);
+				}
+
+				// Remember to seek back to start of the file as the caller is likely
+				// to read this file directly without reopening it.
+				writer.seek(SeekFrom::Start(0))?;
+
+				let mut file = writer.into_inner().map_err(|_| Error::Internal)?;
+
+				debug!(
+					"handle_payload: kernel_data_response: file size: {}",
+					file.metadata().unwrap().len()
+				);
+
+				self.adapter.kernel_data_write(&mut file)?;
+
+				Ok(None)
+			}
+
 			Type::TxHashSetRequest => {
 				let sm_req: TxHashSetRequest = msg.body()?;
 				debug!(

p2p/src/serv.rs

@@ -13,11 +13,12 @@
 // limitations under the License.
 
 use std::fs::File;
+use std::io::{self, Read};
 use std::net::{Shutdown, SocketAddr, TcpListener, TcpStream};
 use std::path::PathBuf;
 use std::sync::Arc;
+use std::thread;
 use std::time::Duration;
-use std::{io, thread};
 
 use crate::chain;
 use crate::core::core;
@@ -280,6 +281,12 @@ impl ChainAdapter for DummyAdapter {
 	fn get_block(&self, _: Hash) -> Option<core::Block> {
 		None
 	}
+	fn kernel_data_read(&self) -> Result<File, chain::Error> {
+		unimplemented!()
+	}
+	fn kernel_data_write(&self, _reader: &mut Read) -> Result<bool, chain::Error> {
+		unimplemented!()
+	}
 	fn txhashset_read(&self, _h: Hash) -> Option<TxHashSetRead> {
 		unimplemented!()
 	}

p2p/src/types.rs

@@ -15,7 +15,7 @@
 use crate::util::RwLock;
 use std::convert::From;
 use std::fs::File;
-use std::io;
+use std::io::{self, Read};
 use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6};
 use std::path::PathBuf;
 
@@ -527,6 +527,10 @@ pub trait ChainAdapter: Sync + Send {
 	/// Gets a full block by its hash.
 	fn get_block(&self, h: Hash) -> Option<core::Block>;
 
+	fn kernel_data_read(&self) -> Result<File, chain::Error>;
+
+	fn kernel_data_write(&self, reader: &mut Read) -> Result<bool, chain::Error>;
+
 	/// Provides a reading view into the current txhashset state as well as
 	/// the required indexes for a consumer to rewind to a consistant state
 	/// at the provided block hash.

servers/src/common/adapters.rs

@@ -17,6 +17,7 @@
 
 use crate::util::RwLock;
 use std::fs::File;
+use std::io::Read;
 use std::path::PathBuf;
 use std::sync::{Arc, Weak};
 use std::thread;
@@ -344,6 +345,16 @@ impl p2p::ChainAdapter for NetToChainAdapter {
 		}
 	}
 
+	fn kernel_data_read(&self) -> Result<File, chain::Error> {
+		self.chain().kernel_data_read()
+	}
+
+	fn kernel_data_write(&self, reader: &mut Read) -> Result<bool, chain::Error> {
+		let res = self.chain().kernel_data_write(reader)?;
+		error!("***** kernel_data_write: {:?}", res);
+		Ok(true)
+	}
+
 	/// Provides a reading view into the current txhashset state as well as
 	/// the required indexes for a consumer to rewind to a consistent state
 	/// at the provided block hash.

store/Cargo.toml

@@ -18,6 +18,7 @@ failure = "0.1"
 failure_derive = "0.1"
 lmdb-zero = "0.4.4"
 memmap = "0.7"
+tempfile = "3.0.5"
 serde = "1"
 serde_derive = "1"
 log = "0.4"

store/src/pmmr.rs

@@ -13,7 +13,8 @@
 
 //! Implementation of the persistent Backend for the prunable MMR tree.
 
-use std::{fs, io, time};
+use std::fs::{self, File};
+use std::{io, time};
 
 use crate::core::core::hash::{Hash, Hashed};
 use crate::core::core::pmmr::{self, family, Backend};
@@ -139,6 +140,12 @@ impl<T: PMMRable> Backend<T> for PMMRBackend<T> {
 		}
 	}
 
+	fn data_as_temp_file(&self) -> Result<File, String> {
+		self.data_file
+			.as_temp_file()
+			.map_err(|_| format!("Failed to build temp data file"))
+	}
+
 	/// Rewind the PMMR backend to the given position.
 	fn rewind(&mut self, position: u64, rewind_rm_pos: &Bitmap) -> Result<(), String> {
 		// First rewind the leaf_set with the necessary added and removed positions.

store/src/types.rs

@@ -13,13 +13,14 @@
 
 //! Common storage-related types
 use memmap;
+use tempfile::tempfile;
 
 use crate::core::ser::{
 	self, BinWriter, FixedLength, Readable, Reader, StreamingReader, Writeable, Writer,
 };
 use std::fmt::Debug;
 use std::fs::{self, File, OpenOptions};
-use std::io::{self, BufReader, BufWriter, Write};
+use std::io::{self, BufReader, BufWriter, Seek, SeekFrom, Write};
 use std::marker;
 use std::path::{Path, PathBuf};
 use std::time;
@@ -144,6 +145,13 @@ where
 		self.file.path()
 	}
 
+	/// Create a new tempfile containing the contents of this data file.
+	/// This allows callers to see a consistent view of the data without
+	/// locking the data file.
+	pub fn as_temp_file(&self) -> io::Result<File> {
+		self.file.as_temp_file()
+	}
+
 	/// Drop underlying file handles
 	pub fn release(&mut self) {
 		self.file.release();
@@ -437,6 +445,22 @@ where
 		}
 	}
 
+	/// Create a new tempfile containing the contents of this append only file.
+	/// This allows callers to see a consistent view of the data without
+	/// locking the append only file.
+	pub fn as_temp_file(&self) -> io::Result<File> {
+		let mut reader = BufReader::new(File::open(&self.path)?);
+		let mut writer = BufWriter::new(tempfile()?);
+		io::copy(&mut reader, &mut writer)?;
+
+		// Remember to seek back to start of the file as the caller is likely
+		// to read this file directly without reopening it.
+		writer.seek(SeekFrom::Start(0))?;
+
+		let file = writer.into_inner()?;
+		Ok(file)
+	}
+
 	/// Saves a copy of the current file content, skipping data at the provided
 	/// prune positions. prune_pos must be ordered.
 	pub fn save_prune(&mut self, prune_pos: &[u64]) -> io::Result<()> {