[PATCH] wrapper: add workaround for open() returning EINTR
- From
Jeff King <peff@peff.net>
- Date
- Feb 24, 2021, 04:43 UTC
- Message-ID
- <YDXZY8XFRayiM1If@coredump.intra.peff.net>
On some platforms, open() reportedly returns EINTR when opening regular files and we receive a signal (usually SIGALRM from our progress meter). This shouldn't happen, as open() should be a restartable syscall, and we specify SA_RESTART when setting up the alarm handler. So it may actually be a kernel or libc bug for this to happen. But it has been reported on at least one version of Linux (on a network filesystem):
https://lore.kernel.org/git/c8061cce-71e4-17bd-a56a-a5fed93804da@neanderfunk.de/
as well as on macOS starting with Big Sur even on a regular filesystem.
We can work around it by retrying open() calls that get EINTR, just as we do for read(), etc. Since we don't ever _want_ to interrupt an open() call, we can get away with just redefining open, rather than insisting all callsites use xopen().
We actually do have an xopen() wrapper already (and it even does this retry, though there's no indication of it being an observed problem back then; it seems simply to have been lifted from xread(), etc). But it is used hardly anywhere, and isn't suitable for general use because it will die() on error. In theory we could combine the two, but it's awkward to do so because of the variable-args interface of open().
The workaround here is enabled all the time, without a Makefile knob, since it's a complete noop if open() never returns EINTR. I did push it into its own compat/ source file, though, since it has to #undef our macro redirection. Putting it in a file with other code risks confusion if more code is added after that #undef.
Reported-by: Aleksey Kliger <alklig@microsoft.com> Signed-off-by: Jeff King <peff@peff.net> --- This was reported to me off-list. Aleksey was kind enough to test the patch on a system that was reliably showing the problem during the checkout phase of git-clone (which naturally has both a progress meter and is calling open() on a ton of files).
I do still think the OS is doing the wrong thing here. But even if I'm right, it's probably prudent to work around it.
git-compat-util.h | 4 ++++ wrapper.c | 29 +++++++++++++++++++++++++++++ 2 files changed, 33 insertions(+)
diff --git a/git-compat-util.h b/git-compat-util.h index 838246289c..2be022c422 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -788,6 +788,10 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap); #endif +#undef open +#define open git_open_with_retry +int git_open_with_retry(const char *path, int flag, ...); + #ifdef __GLIBC_PREREQ #if __GLIBC_PREREQ(2, 1) #define HAVE_STRCHRNUL diff --git a/wrapper.c b/wrapper.c index bcda41e374..130f441064 100644 --- a/wrapper.c +++ b/wrapper.c @@ -678,3 +678,32 @@ int is_empty_or_missing_file(const char *filename) return !st.st_size; } + +/* + * Do not add other callers of open() after this. Our redefinition + * below will ensure the compiler catches any. + */ +#undef open +int git_open_with_retry(const char *path, int flags, ...) +{ + mode_t mode = 0; + int ret; + + /* + * Also O_TMPFILE would take a mode, but it isn't defined everywhere. + * And anyway, we don't use it in our code base. + */ + if (flags & O_CREAT) { + va_list ap; + va_start(ap, flags); + mode = va_arg(ap, int); + va_end(ap); + } + + do { + ret = open(path, flags, mode); + } while (ret < 0 && errno == EINTR); + + return ret; +} +#define open do_not_allow_use_of_bare_open
-- 2.30.1.1095.g03347429ea