Attempt to raise the file descriptor limit at startup (matrix-org#2527)

setup/base/sanity_unix.go

@@ -15,8 +15,21 @@ func platformSanityChecks() {
 	// If we run out of file descriptors, we might run into problems accessing
 	// PostgreSQL amongst other things. Complain at startup if we think the
 	// number of file descriptors is too low.
+	warn := func(rLimit *syscall.Rlimit) {
+		logrus.Warnf("IMPORTANT: Process file descriptor limit is currently %d, it is recommended to raise the limit for Dendrite to at least 65535 to avoid issues", rLimit.Cur)
+	}
 	var rLimit syscall.Rlimit
 	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit); err == nil && rLimit.Cur < 65535 {
-		logrus.Warnf("IMPORTANT: Process file descriptor limit is currently %d, it is recommended to raise the limit for Dendrite to at least 65535 to avoid issues", rLimit.Cur)
+		// The file descriptor count is too low. Let's try to raise it.
+		rLimit.Cur = 65535
+		if err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rLimit); err != nil {
+			// We failed to raise it, so log an error.
+			logrus.WithError(err).Warn("IMPORTANT: Failed to raise the file descriptor limit")
+			warn(&rLimit)
+		} else if err = syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit); err == nil && rLimit.Cur < 65535 {
+			// We think we successfully raised the limit, but a second call to
+			// get the limit told us that we didn't succeed. Log an error.
+			warn(&rLimit)
+		}
 	}
 }