Re: [PATCH 1/2] xread: retry after poll on EAGAIN/EWOULDBLOCK
- From
- Eric Wong <e@80x24.org>
- Date
- Jun 27, 2016, 20:13 UTC
- Message-ID
- <20160627201311.GA7039@dcvr.yhbt.net>
- In-Reply-To
- <CAGZ79kZ94PaOfq3GimWiHULbTE7ihMzL9S=Y+npQ4F5gGwFrsA@mail.gmail.com>
Stefan Beller <sbeller@google.com> wrote:
Show 7 quoted lines
> On Mon, Jun 27, 2016 at 7:36 AM, Jeff King <peff@peff.net> wrote: > > It's also true that our error rate will never be 0%. So some bugs will > > always slip through, some review comments will be forgotten, etc. Eric > > did find and fix the bug just now, so the "many eyes" theory did work > > here eventually. > > Eric, thanks for catching and fixing the bug!
No problem :) I only noticed it because I was scanning emails randomly and Duy and David's index-helper thread turned up.
> Quite a while ago, when I started doing code reviews professionally, I wondered > if the code review procedure can be semi-automated, as automation helps keeping > the error rate low. By that I mean having a check list which I can > check off each point
Maybe a test case or even a small unit test would've helped. I didn't notice the problem in xread until:
1) I copied the code into xwrite 2) s/POLLIN/POLLOUT/; 3) forced EAGAIN using a patched, home-baked HTTP server
The biggish comment before the poll() obscured the missing "continue" for me. I read xread() before and did not notice the missing "continue".
Maybe the following optional patch on top of this series improves readability:
----------8<-------- Subject: [PATCH 3/2] hoist out io_wait function for xread and xwrite
At least for me, this improves the readability of xread and xwrite; hopefully allowing missing "continue" statements to be spotted more easily.
Signed-off-by: Eric Wong <e@80x24.org> --- wrapper.c | 40 ++++++++++++++++------------------------ 1 file changed, 16 insertions(+), 24 deletions(-)
diff --git a/wrapper.c b/wrapper.c index d973f86..04bb952 100644 --- a/wrapper.c +++ b/wrapper.c @@ -227,6 +227,20 @@ int xopen(const char *path, int oflag, ...) } } +static void io_wait(int fd, short poll_events) +{ + struct pollfd pfd; + + pfd.fd = fd; + pfd.events = poll_events; + + /* + * no need to check for errors, here; + * a subsequent read/write will detect unrecoverable errors + */ + poll(&pfd, 1, -1); +} + /* * xread() is the same a read(), but it automatically restarts read() * operations with a recoverable error (EAGAIN and EINTR). xread() @@ -243,18 +257,7 @@ ssize_t xread(int fd, void *buf, size_t len) if (errno == EINTR) continue; if (errno == EAGAIN || errno == EWOULDBLOCK) { - struct pollfd pfd; - pfd.events = POLLIN; - pfd.fd = fd; - /* - * it is OK if this poll() failed; we - * want to leave this infinite loop - * only when read() returns with - * success, or an expected failure, - * which would be checked by the next - * call to read(2). - */ - poll(&pfd, 1, -1); + io_wait(fd, POLLIN); continue; } } @@ -278,18 +281,7 @@ ssize_t xwrite(int fd, const void *buf, size_t len) if (errno == EINTR) continue; if (errno == EAGAIN || errno == EWOULDBLOCK) { - struct pollfd pfd; - pfd.events = POLLOUT; - pfd.fd = fd; - /* - * it is OK if this poll() failed; we - * want to leave this infinite loop - * only when write() returns with - * success, or an expected failure, - * which would be checked by the next - * call to write(2). - */ - poll(&pfd, 1, -1); + io_wait(fd, POLLOUT); continue; } }
-- EW