Re: [PATCH v2 1/3] core.fsyncmethod: add writeout-only mode

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, Dec 07 2021, Neeraj Singh via GitGitGadget wrote:

> From: Neeraj Singh <neerajsi@microsoft.com>
>
> This commit introduces the `core.fsyncmethod` configuration

Just a commit msg nit: core.fsyncMethod (I see the docs etc. are using
it camelCased, good..

> diff --git a/compat/win32/flush.c b/compat/win32/flush.c
> new file mode 100644
> index 00000000000..75324c24ee7
> --- /dev/null
> +++ b/compat/win32/flush.c
> @@ -0,0 +1,28 @@
> +#include "../../git-compat-util.h"

nit: Just FWIW I think the better thing is '#include
"git-compat-util.h"', i.e. we're compiling at the top-level and have
added it to -I.

(I know a lot of compat/ and contrib/ and even main-tree stuff does
that, but just FWIW it's not needed).

> +	if (!strcmp(var, "core.fsyncmethod")) {
> +		if (!value)
> +			return config_error_nonbool(var);
> +		if (!strcmp(value, "fsync"))
> +			fsync_method = FSYNC_METHOD_FSYNC;
> +		else if (!strcmp(value, "writeout-only"))
> +			fsync_method = FSYNC_METHOD_WRITEOUT_ONLY;
> +		else

As a non-nit comment I think this config schema looks great so far.

> +			warning(_("unknown %s value '%s'"), var, value);

Just a suggestion maybe something slightly scarier like:

    "unknown core.fsyncMethod value '%s'; config from future git version? ignoring requested fsync strategy"

Also using the nicer camelCased version instead of "var" (also helps
translators with context...)

> +int git_fsync(int fd, enum fsync_action action)
> +{
> +	switch (action) {
> +	case FSYNC_WRITEOUT_ONLY:
> +
> +#ifdef __APPLE__
> +		/*
> +		 * on macOS, fsync just causes filesystem cache writeback but does not
> +		 * flush hardware caches.
> +		 */
> +		return fsync(fd);
> +#endif
> +
> +#ifdef HAVE_SYNC_FILE_RANGE
> +		/*
> +		 * On linux 2.6.17 and above, sync_file_range is the way to issue
> +		 * a writeback without a hardware flush. An offset of 0 and size of 0
> +		 * indicates writeout of the entire file and the wait flags ensure that all
> +		 * dirty data is written to the disk (potentially in a disk-side cache)
> +		 * before we continue.
> +		 */
> +
> +		return sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WAIT_BEFORE |
> +						 SYNC_FILE_RANGE_WRITE |
> +						 SYNC_FILE_RANGE_WAIT_AFTER);
> +#endif
> +
> +#ifdef fsync_no_flush
> +		return fsync_no_flush(fd);
> +#endif
> +
> +		errno = ENOSYS;
> +		return -1;
> +
> +	case FSYNC_HARDWARE_FLUSH:
> +		/*
> +		 * On some platforms fsync may return EINTR. Try again in this
> +		 * case, since callers asking for a hardware flush may die if
> +		 * this function returns an error.
> +		 */
> +		for (;;) {
> +			int err;
> +#ifdef __APPLE__
> +			err = fcntl(fd, F_FULLFSYNC);
> +#else
> +			err = fsync(fd);
> +#endif
> +			if (err >= 0 || errno != EINTR)
> +				return err;
> +		}
> +
> +	default:
> +		BUG("unexpected git_fsync(%d) call", action);

Don't include such "default" cases, you have an exhaustive "enum", if
you skip it the compiler will check this for you.

> +	}
> +}
> +
>  static int warn_if_unremovable(const char *op, const char *file, int rc)

Just a code nit: I think it's very much preferred if possible to have as
much of code like this compile on all platforms. See the series at
4002e87cb25 (grep: remove #ifdef NO_PTHREADS, 2018-11-03) is part of for
a good example.

Maybe not worth it in this case since they're not nested ifdef's.

I'm basically thinking of something (also re Patrick's comment on the
2nd patch) where we have a platform_fsync() whose return
value/arguments/whatever capture this "I want to return now" or "you
should be looping" and takes the enum_fsync_action" strategy.

Then the git_fsync() would be the platform-independent looping etc., and
another funciton would do the "one fsync at a time, maybe call me
again".

Maybe it would suck more, just food for thought... :)