Avoid cachedir races

1. Add a Py3-only version of the cachedir config read,
using exclusive open to avoid races.
2. In the Py2-only version, add the hack from issue #3351
to dodge one source of races.

Signed-off-by: Mats Wichmann <mats@linux.com>

1 files changed, 69 insertions, 6 deletions

diff --git a/src/engine/SCons/CacheDir.py b/src/engine/SCons/CacheDir.py
index dabda8e..f2d3810 100644
--- a/src/engine/SCons/CacheDir.py
+++ b/src/engine/SCons/CacheDir.py
@@ -139,16 +139,78 @@ warned = dict()
 class CacheDir(object):
 
     def __init__(self, path):
+        """
+        Initialize a CacheDir object.
+
+        The cache configuration is stored in the object. It
+        is read from the config file in the supplied path if
+        one exists,  if not the config file is created and
+        the default config is written, as well as saved in the object.
+        """
         self.path = path
         self.current_cache_debug = None
         self.debugFP = None
         self.config = dict()
         if path is None:
             return
-        # See if there's a config file in the cache directory. If there is,
-        # use it. If there isn't, and the directory exists and isn't empty,
-        # produce a warning. If the directory doesn't exist or is empty,
-        # write a config file.
+
+        from SCons.Util import PY3
+        if PY3:
+            self._readconfig3(path)
+        else:
+            self._readconfig2(path)
+
+
+    def _readconfig3(self, path):
+        """
+        Python3 version of reading the cache config.
+
+        If directory or config file do not exist, create.  Take advantage
+        of Py3 capability in os.makedirs() and in file open(): just try
+        the operation and handle failure appropriately.
+
+        Omit the check for old cache format, assume that's old enough
+        there will be none of those left to worry about.
+
+        :param path: path to the cache directory
+        """
+        config_file = os.path.join(path, 'config')
+        try:
+            os.makedirs(path, exist_ok=True)
+        except FileExistsError:
+            pass
+        except OSError:
+            msg = "Failed to create cache directory " + path
+            raise SCons.Errors.EnvironmentError(msg)
+
+        try:
+            with open(config_file, 'x') as config:
+                self.config['prefix_len'] = 2
+                try:
+                    json.dump(self.config, config)
+                except:
+                    msg = "Failed to write cache configuration for " + path
+                    raise SCons.Errors.EnvironmentError(msg)
+        except FileExistsError:
+            try:
+                with open(config_file) as config:
+                    self.config = json.load(config)
+            except ValueError:
+                msg = "Failed to read cache configuration for " + path
+                raise SCons.Errors.EnvironmentError(msg)
+
+
+    def _readconfig2(self, path):
+        """
+        Python2 version of reading cache config.
+
+        See if there's a config file in the cache directory. If there is,
+        use it. If there isn't, and the directory exists and isn't empty,
+        produce a warning. If the directory doesn't exist or is empty,
+        write a config file.
+
+        :param path: path to the cache directory
+        """
         config_file = os.path.join(path, 'config')
         if not os.path.exists(config_file):
             # A note: There is a race hazard here, if two processes start and
@@ -159,14 +221,15 @@ class CacheDir(object):
             # as an attempt to alleviate this, on the basis that it's a pretty
             # unlikely occurence (it'd require two builds with a brand new cache
             # directory)
-            if os.path.isdir(path) and len(os.listdir(path)) != 0:
+            #if os.path.isdir(path) and len(os.listdir(path)) != 0:
+            if os.path.isdir(path) and len([f for f in os.listdir(path) if os.path.basename(f) != "config"]) != 0:
                 self.config['prefix_len'] = 1
                 # When building the project I was testing this on, the warning
                 # was output over 20 times. That seems excessive
                 global warned
                 if self.path not in warned:
                     msg = "Please upgrade your cache by running " +\
-                          " scons-configure-cache.py " +  self.path
+                          "scons-configure-cache.py " +  self.path
                     SCons.Warnings.warn(SCons.Warnings.CacheVersionWarning, msg)
                     warned[self.path] = True
             else: