[bug #13143] "quick save" may clobber files


                 Summary: "quick save" may clobber files
                 Project: GNU Midnight Commander
            Submitted by: ossi
            Submitted on: Sat 05/21/2005 at 17:24
                Category: Editor
                Severity: 3 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
                 Release: current (CVS or snapshot)
        Platform Version: All



if we run out of disk space, some i/o error occurs, or we are just killed
while saving a file, the old contents will be lost.
that's because the old file is opened with O_TRUNC and rewritten.
the "solution" would be not to use O_TRUNC at all. instead, first write the
part that extends beyond the end of the old file (note that we cannot just
seek to the new end and write one byte because of sparse files) - if that
fails to complete, truncate to the old length (if we crash in between, the
old file will have garbage appended, but at least it's not clobbered
alltogether).  after the "extension" was written, overwrite the start of the
file with the new content (if we crash in during this, the file is clobbered,
but that's what you get for using "quick save") and possibly truncate the file
to the new length.
some VFS inherently don't support seeking, so this "slightly safer" mode is
not universally applicable.

a related note: for non-local files, "quick save" is used independently of
the configured save mode. i think the VFS should tell the editor whether it
can do remote renames, so the safe save modes we can be used in as many cases
as possible.


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