On win9x systems, random data seems to be added to the end of the file

on system crashes, causing apparent file corruption.  Code has been
added to check for this case and recover by truncating the file with a
log message.

Fixed bug in file-layout doc.

Added quota option.

src/ZODB/FileStorage.py

@@ -104,7 +104,7 @@
 #   
 #   - 2-byte length of description 
 #   
-#   - 4-byte length of extension attributes 
+#   - 2-byte length of extension attributes 
 #   
 #   -   user name
 #   
@@ -184,9 +184,9 @@
 #   may have a back pointer to a version record or to a non-version
 #   record.
 #
-__version__='$Revision: 1.22 $'[11:-2]
+__version__='$Revision: 1.23 $'[11:-2]
 
-import struct, time, os, bpthread, string, base64
+import struct, time, os, bpthread, string, base64, sys
 from struct import pack, unpack
 from cPickle import loads
 import POSException
@@ -205,6 +205,9 @@ def warn(message, *data):
 def error(message, *data):
     LOG('ZODB FS',ERROR,"%s ERROR: %s\n" % (packed_version, (message % data)))
 
+def nearPanic(message, *data):
+    LOG('ZODB FS',PANIC,"%s ERROR: %s\n" % (packed_version, (message % data)))
+
 def panic(message, *data):
     message=message%data
     LOG('ZODB FS',PANIC,"%s ERROR: %s\n" % (packed_version, message))
@@ -226,12 +229,18 @@ class CorruptedFileStorageError(FileStorageError,
 class CorruptedTransactionError(CorruptedFileStorageError): pass
 class CorruptedDataError(CorruptedFileStorageError): pass
 
+class FileStorageQuotaError(FileStorageError,
+                                POSException.StorageSystemError):
+    """File storage quota exceeded
+    """
+
 packed_version='FS21'
 
 class FileStorage(BaseStorage.BaseStorage):
     _packt=z64
 
-    def __init__(self, file_name, create=0, read_only=0, stop=None):
+    def __init__(self, file_name, create=0, read_only=0, stop=None,
+                 quota=None):
 
         if not os.path.exists(file_name): create = 1
 
@@ -287,6 +296,8 @@ class FileStorage(BaseStorage.BaseStorage):
                 # a month in the future? This is bogus, use current time
                 self._ts=t
 
+        self._quota=quota
+            
 
     def _initIndex(self, index, vindex, tindex, tvindex):
         self._index=index
@@ -515,6 +526,12 @@ class FileStorage(BaseStorage.BaseStorage):
 
             write(data)
 
+            # Check quota
+            quota=self._quota
+            if quota is not None and tfile.tell()+pos+self._thl > quota:
+                raise FileStorageQuotaError, (
+                    'The storage quota has been exceeded.')
+
             return serial
         
         finally: self._lock_release()
@@ -544,6 +561,8 @@ class FileStorage(BaseStorage.BaseStorage):
         file.seek(pos)
         tl=tlen+dlen
         stl=p64(tl)
+
+        try:
             # Note that we use a status of 'c', for checkpoint.
             # If this flag isn't cleared, anything after this is
             # suspect.
@@ -563,6 +582,10 @@ class FileStorage(BaseStorage.BaseStorage):
             file.seek(pos+16)
             write(' ')        
             file.flush()
+        except:
+            # Hm, an error occured writing out the data. Maybe the
+            # disk is full. We don't want any turd at the end.
+            file.truncate(pos)
         
         self._pos=pos+tl+8
 
@@ -1105,30 +1128,31 @@ def read_index(file, name, index, vindex, tindex, stop='\377'*8):
             # in which case, we don't want to totally lose the data.
             warn("%s truncated, possibly due to damaged records at %s",
                  name, pos)
-            try:
-                i=0
-                while 1:
-                    if os.path.exists('%s.tr%s' % (name, i)):
-                        i=i+1
-                    else:
-                        o=open('%s.tr%s' % (name, i),'wb')
-                        seek(pos)
-                        cp(file, o, file_size-pos)
-                        o.close()
-                        break
-            except:
-                error("couldn\'t write truncated data for %s", name)
-                raise POSException.StorageSystemError, (
-                    "Couldn't save truncated data")
-            
-            seek(pos)
-            file.truncate()
+            _truncate(file, name, pos)
             break
 
         if status not in ' up':
             warn('%s has invalid status, %s, at %s', name, status, pos)
 
-        if ul > tl or dl > tl or el > tl:
+        if ul > tl or dl > tl or el > tl or tl < (23+ul+dl+el):
+            # We're in trouble. Find out if this is bad data in the
+            # middle of the file, or just a turd that Win 9x dropped
+            # at the end when the system crashed.
+            # Skip to the end and read what should be the transaction length
+            # of the last transaction.
+            seek(-8, 2)
+            rtl=u64(read(8))
+            # Now check to see if the redundant transaction length is
+            # reasonable:
+            if file_size - rtl < pos or rtl < 23:
+                nearPanic('%s has invalid transaction header at %s', name, pos)
+                warn("It appears that there is invalid data at the end of the "
+                     "file, possibly due to a system crash.  %s truncated "
+                     "to recover from bad data at end."
+                     % name)
+                _truncate(file, name, pos)
+                break
+            else:
                 panic('%s has invalid transaction header at %s', name, pos)
 
         if tid >= stop: break
@@ -1230,3 +1254,28 @@ def _loadBackPOS(file, oid, back):
         if vlen: seek(vlen+16,1)
         if plen != z64: return old
         back=read(8) # We got a back pointer!
+
+def _truncate(file, name, pos):
+    seek=file.seek
+    seek(0,2)
+    file_size=file.tell()
+    try:
+        i=0
+        while 1:
+            oname='%s.tr%s' % (name, i)
+            if os.path.exists(oname):
+                i=i+1
+            else:
+                warn("Writing truncated data from %s to %s", name, oname)
+                o=open(oname,'wb')
+                seek(pos)
+                cp(file, o, file_size-pos)
+                o.close()
+                break
+    except:
+        error("couldn\'t write truncated data for %s", name)
+        raise POSException.StorageSystemError, (
+            "Couldn't save truncated data")
+            
+    seek(pos)
+    file.truncate()