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List: cgit
Subject: Re: [PATCH] cache: Tolerate short writes in print_slot
From: Hristo Venev <hristo () venev ! name>
Date: 2021-10-07 15:44:35
Message-ID: 50038a48ec126969331acfa1df13b4ae6bdd3814.camel () venev ! name
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On Thu, 2021-10-07 at 10:35 +0100, John Keeping wrote:
> Have you seen these errors in practice, or is this just theoretical?
>
> In recent (since v2.6.33) versions of Linux, all files should support
> sendfile(), especially since we expect out_fd to be a socket or pipe.
Even though I haven't seen any errors from sendfile, I'm not sure if it
always works with fuse filesystems and with third-party filesystems
like zfs. The sendfile(2) man page says this:
"Applications may wish to fall back to read(2)/write(2) in the case
where sendfile() fails with EINVAL or ENOSYS."
> > On the read/write path, use write_in_full which deals with short
> > writes.
> >
> > Signed-off-by: Hristo Venev <hristo@venev.name>
> > ---
> > cache.c | 46 ++++++++++++++++++++++++----------------------
> > 1 file changed, 24 insertions(+), 22 deletions(-)
> >
> > diff --git a/cache.c b/cache.c
> > index 55199e8..85cfbd9 100644
> > --- a/cache.c
> > +++ b/cache.c
> > @@ -85,40 +85,42 @@ static int close_slot(struct cache_slot *slot)
> > /* Print the content of the active cache slot (but skip the key). */
> > static int print_slot(struct cache_slot *slot)
> > {
> > -#ifdef HAVE_LINUX_SENDFILE
> > - off_t start_off;
> > - int ret;
> > + off_t off;
> > + ssize_t i;
> > +
> > + off = slot->keylen + 1;
> >
> > - start_off = slot->keylen + 1;
> > +#ifdef HAVE_LINUX_SENDFILE
> > + off_t size;
>
> decl-after-stmt if HAVE_LINUX_SENDFILE is set.
I didn't know that compilers that don't support that exist outside
museums. I will fix it in v2.
> > + size = slot->cache_st.st_size;
> >
> > do {
> > - ret = sendfile(STDOUT_FILENO, slot->cache_fd,
> > &start_off,
> > - slot->cache_st.st_size - start_off);
> > - if (ret < 0) {
> > + i = sendfile(STDOUT_FILENO, slot->cache_fd, &off,
> > size - off);
>
> Why is ret renamed? i is normally a loop index variable, using it for
> the return value here is strange, please stick with "ret".
I will fix this in v2.
> > + if (i < 0) {
> > if (errno == EAGAIN || errno == EINTR)
> > continue;
> > + /* Fall back to read/write on EINVAL */
> > + if (errno == EINVAL)
> > + break;
> > return errno;
> > }
> > - return 0;
> > + if (off == size)
> > + return 0;
> > } while (1);
> > -#else
> > - ssize_t i, j;
> > +#endif
> >
> > - i = lseek(slot->cache_fd, slot->keylen + 1, SEEK_SET);
> > - if (i != slot->keylen + 1)
> > + if (lseek(slot->cache_fd, off, SEEK_SET) != off)
> > return errno;
> >
> > do {
> > - i = j = xread(slot->cache_fd, slot->buf, sizeof(slot-
> > >buf));
> > - if (i > 0)
> > - j = xwrite(STDOUT_FILENO, slot->buf, i);
> > - } while (i > 0 && j == i);
> > -
> > - if (i < 0 || j != i)
> > - return errno;
> > - else
> > - return 0;
> > -#endif
> > + i = xread(slot->cache_fd, slot->buf, sizeof(slot-
> > >buf));
> > + if (i < 0)
> > + return errno;
> > + if (i == 0)
> > + return 0;
> > + if (write_in_full(STDOUT_FILENO, slot->buf, i) < 0)
> > + return errno;
> > + } while (1);
> > }
> >
> > /* Check if the slot has expired */
> > --
> > 2.31.1
> >
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