From 4aba8a0c7a5d92e7c2e27e4f1c430f91cfc1e207 Mon Sep 17 00:00:00 2001
From: Mike Auty <mike.auty@gmail.com>
Date: Sat, 30 Jan 2021 17:18:56 +0000
Subject: [PATCH] Layers: Lock cache file, bomb out after 30 seconds

---
 volatility3/framework/constants/__init__.py |  3 ++
 volatility3/framework/layers/resources.py   | 48 +++++++++++++--------
 2 files changed, 34 insertions(+), 17 deletions(-)

diff --git a/volatility3/framework/constants/__init__.py b/volatility3/framework/constants/__init__.py
index c04fbdb46..d3cf87aa8 100644
--- a/volatility3/framework/constants/__init__.py
+++ b/volatility3/framework/constants/__init__.py
@@ -76,6 +76,9 @@ BUG_URL = "https://github.com/volatilityfoundation/volatility3/issues"
 ProgressCallback = Optional[Callable[[float, str], None]]
 """Type information for ProgressCallback objects"""
 
+URLOPEN_LOCK_TIMEOUT = 30000
+"""Number of miliseconds to wait before timing-out on file locks"""
+
 
 class Parallelism(enum.IntEnum):
     """An enumeration listing the different types of parallelism applied to
diff --git a/volatility3/framework/layers/resources.py b/volatility3/framework/layers/resources.py
index 63cfd9570..7bbbf7c0e 100644
--- a/volatility3/framework/layers/resources.py
+++ b/volatility3/framework/layers/resources.py
@@ -10,6 +10,7 @@ import logging
 import lzma
 import os
 import ssl
+import time
 import urllib.parse
 import urllib.request
 import zipfile
@@ -121,26 +122,39 @@ class ResourceAccessor(object):
                     constants.CACHE_PATH,
                     "data_" + hashlib.sha512(bytes(url, 'raw_unicode_escape')).hexdigest() + ".cache")
 
-                if not os.path.exists(temp_filename):
-                    vollog.debug("Caching file at: {}".format(temp_filename))
+                with open(temp_filename + '.lock', 'w'):
+                    if not os.path.exists(temp_filename):
+                        vollog.debug("Caching file at: {}".format(temp_filename))
 
-                    try:
-                        content_length = fp.info().get('Content-Length', -1)
-                    except AttributeError:
-                        # If our fp doesn't have an info member, carry on gracefully
-                        content_length = -1
-                    cache_file = open(temp_filename, "wb")
+                        try:
+                            content_length = fp.info().get('Content-Length', -1)
+                        except AttributeError:
+                            # If our fp doesn't have an info member, carry on gracefully
+                            content_length = -1
+                        cache_file = open(temp_filename, "wb")
 
-                    count = 0
-                    block = fp.read(block_size)
-                    while block:
-                        count += len(block)
-                        if self._progress_callback:
-                            self._progress_callback(count * 100 / max(count, int(content_length)),
-                                                    "Reading file {}".format(url))
-                        cache_file.write(block)
+                        count = 0
                         block = fp.read(block_size)
-                    cache_file.close()
+                        while block:
+                            count += len(block)
+                            if self._progress_callback:
+                                self._progress_callback(count * 100 / max(count, int(content_length)),
+                                                        "Reading file {}".format(url))
+                            cache_file.write(block)
+                            block = fp.read(block_size)
+                        cache_file.close()
+                    else:
+                        count = 0
+                        while os.path.exists(temp_filename + '.lock'):
+                            count += 1
+                            if self._progress_callback:
+                                self._progress_callback(0, "Waiting on lock to retrieve {}".format(url))
+                            time.sleep(0.1)
+                            if count >= constants.URLOPEN_LOCK_TIMEOUT // 1000:
+                                break
+                        if not os.path.exists(temp_filename):
+                            raise error.URLError("Unable to lock the cache for {}".format(url))
+
                 # Re-open the cache with a different mode
                 # Since we don't want people thinking they're able to save to the cache file,
                 # open it in read mode only and allow breakages to happen if they wanted to write