On 06/03/2020 22:04, Eryk Sun wrote:
On 3/6/20, Robin Becker <ro...@reportlab.com> wrote:
OK I want to read the (small) file completely. The other process may try to
re-write the file while I am reading it.
I thought the other process already had the file open for writing, but
apparently you just want to deny anyone the right to open the file
with write access while you're reading it. If the file is already open
with write access, you'll have to periodically retry opening it until
you can get read access without sharing write access.
yes we poll periodically for late mod time files and try to post these to a web server. If there are misreads we hope
that the next poll will find them not being written. After some of the thoughts posted here I suspect we probably ought
not to try to prevent the other process writing. What we are really trying to do is get a snapshot of files which have
changed since our last poll read. The real problem is to ensure that the content read is not partial ie was not being
written at the time of read.
Alternatively, you can use regular read-write sharing, but lock the
file data via msvcrt.locking. This doesn't deny opening the file for
write access, but any attempt to write to a locked region will fail
with a locking violation. The locked region can encompass the entire
file. (File locking in Windows doesn't prevent deleting or renaming a
file, but the default share mode already prevents that.)
unfortunately I cannot assume they are appending so partial locking is probably
not useful.
Of course the other process may adopt a completely orthogonal scheme of
opening with a different name and then renaming,
Normally, open files in Python cannot be deleted or renamed until
they're closed. This is the case as long as you have the file opened
with just read or read-write sharing. Within the standard library,
only the low-level open flag O_TEMPORARY uses shared delete/rename
access.
--
Robin Becker
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