Makefile: add OPEN_RETURNS_EINTR knob

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().

This patch adds a Makefile knob for enabling the workaround. It's not
enabled by default for any platforms in config.mak.uname yet, as we
don't have enough data to decide how common this is (I have not been
able to reproduce on either Linux or Big Sur myself). It may be worth
enabling preemptively anyway, since the cost is pretty low (if we don't
see an EINTR, it's just an extra conditional).

However, note that we must not enable this on Windows. It doesn't do
anything there, and the macro overrides the existing mingw_open()
redirection. I've added a preemptive #undef here in the mingw header
(which is processed first) to just quietly disable it (we could also
make it an #error, but there is little point in being so aggressive).

Reported-by: Aleksey Kliger <alklig@microsoft.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 4128b457e1..08aa57f1db 100644
--- a/Makefile
+++ b/Makefile
@@ -22,6 +22,9 @@ all::
 # when attempting to read from an fopen'ed directory (or even to fopen
 # it at all).
 #
+# Define OPEN_RETURNS_EINTR if your open() system call may return EINTR
+# when a signal is received (as opposed to restarting).
+#
 # Define NO_OPENSSL environment variable if you do not have OpenSSL.
 #
 # Define USE_LIBPCRE if you have and want to use libpcre. Various
@@ -1551,6 +1554,10 @@ ifdef FREAD_READS_DIRECTORIES
 	COMPAT_CFLAGS += -DFREAD_READS_DIRECTORIES
 	COMPAT_OBJS += compat/fopen.o
 endif
+ifdef OPEN_RETURNS_EINTR
+	COMPAT_CFLAGS += -DOPEN_RETURNS_EINTR
+	COMPAT_OBJS += compat/open.o
+endif
 ifdef NO_SYMLINK_HEAD
 	BASIC_CFLAGS += -DNO_SYMLINK_HEAD
 endif