Skip to content
Snippets Groups Projects
  • Colin Walters's avatar
    113c770d
    fdio: Add open_tmpfile_linkable() and link_tmpfile_at() · 113c770d
    Colin Walters authored
    We had a bug previously where we failed to clean up a temporary file
    in an error path.  This is a classic case where the new `O_TMPFILE`
    API in Linux is nicer.
    
    To implement this, as usual we start with some original bits from
    systemd.  But in this case I ended up having to heavily modify it
    because systemd doesn't support "link into place and overwrite".  They
    don't actually use their tempfile code much at all in fact - as far as
    I can tell, just in the coredump code.
    
    Whereas in many apps, ostree included, a very common use case is
    atomically updating an existing file, which is
    `glnx_file_replace_contents_at()`, including subtleties like doing an
    `fdatasync()` if the file already existed.
    
    Implementing this then is slightly weird since we need to link() the
    file into place, then rename() after.
    
    It's still better though because if we e.g. hit `ENOSPC` halfway
    through, we'll clean up the file automatically.
    
    We still do keep the mode where we error out if the file exists.
    Finally, the ostree core though does have a more unusual case where we
    want to ignore EEXIST (allow concurrent object writers), so add
    support for that now.
    
    Note: One really confusing bug I had here was that `O_TMPFILE` ignores
    the provided mode, and this caused ostree to write refs that weren't
    world readable.
    
    Rework things so we always call `fchmod()`, but as a consequence we're
    no longer honoring umask in the default case.  I doubt anyone will
    care, and if they do we should probably fix ostree to consistently use
    a mode inherited from the repo or something.
    113c770d
    History
    fdio: Add open_tmpfile_linkable() and link_tmpfile_at()
    Colin Walters authored
    We had a bug previously where we failed to clean up a temporary file
    in an error path.  This is a classic case where the new `O_TMPFILE`
    API in Linux is nicer.
    
    To implement this, as usual we start with some original bits from
    systemd.  But in this case I ended up having to heavily modify it
    because systemd doesn't support "link into place and overwrite".  They
    don't actually use their tempfile code much at all in fact - as far as
    I can tell, just in the coredump code.
    
    Whereas in many apps, ostree included, a very common use case is
    atomically updating an existing file, which is
    `glnx_file_replace_contents_at()`, including subtleties like doing an
    `fdatasync()` if the file already existed.
    
    Implementing this then is slightly weird since we need to link() the
    file into place, then rename() after.
    
    It's still better though because if we e.g. hit `ENOSPC` halfway
    through, we'll clean up the file automatically.
    
    We still do keep the mode where we error out if the file exists.
    Finally, the ostree core though does have a more unusual case where we
    want to ignore EEXIST (allow concurrent object writers), so add
    support for that now.
    
    Note: One really confusing bug I had here was that `O_TMPFILE` ignores
    the provided mode, and this caused ostree to write refs that weren't
    world readable.
    
    Rework things so we always call `fchmod()`, but as a consequence we're
    no longer honoring umask in the default case.  I doubt anyone will
    care, and if they do we should probably fix ostree to consistently use
    a mode inherited from the repo or something.