systemd/0138-io-util-document-EINTR-situation-a-bit.patch

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From 3850d27f47a887a958ded828f6ce8de4e791037c Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Tue, 22 Nov 2022 15:23:34 +0100
Subject: [PATCH] io-util: document EINTR situation a bit
(cherry picked from commit ffbcc8d423671ad2fe827e4823a8032dc1f0a8b3)
Related: #2137584
---
src/basic/io-util.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/src/basic/io-util.c b/src/basic/io-util.c
index cdad939aa6..f642beca3a 100644
--- a/src/basic/io-util.c
+++ b/src/basic/io-util.c
@@ -161,6 +161,21 @@ int ppoll_usec(struct pollfd *fds, size_t nfds, usec_t timeout) {
assert(fds || nfds == 0);
+ /* This is a wrapper around ppoll() that does primarily two things:
+ *
+ * ✅ Takes a usec_t instead of a struct timespec
+ *
+ * ✅ Guarantees that if an invalid fd is specified we return EBADF (i.e. converts POLLNVAL to
+ * EBADF). This is done because EBADF is a programming error usually, and hence should bubble up
+ * as error, and not be eaten up as non-error POLLNVAL event.
+ *
+ * ⚠️ ⚠️ ⚠️ Note that this function does not add any special handling for EINTR. Don't forget
+ * poll()/ppoll() will return with EINTR on any received signal always, there is no automatic
+ * restarting via SA_RESTART available. Thus, typically you want to handle EINTR not as an error,
+ * but just as reason to restart things, under the assumption you use a more appropriate mechanism
+ * to handle signals, such as signalfd() or signal handlers. ⚠️ ⚠️ ⚠️
+ */
+
if (nfds == 0)
return 0;
@@ -188,6 +203,9 @@ int fd_wait_for_event(int fd, int event, usec_t timeout) {
};
int r;
+ /* ⚠️ ⚠️ ⚠️ Keep in mind you almost certainly want to handle -EINTR gracefully in the caller, see
+ * ppoll_usec() above! ⚠️ ⚠️ ⚠️ */
+
r = ppoll_usec(&pollfd, 1, timeout);
if (r <= 0)
return r;