rsync 3.4.3 and later won't build for Linux < 5.6 out of the box due to openat2()

[mmayer]

While "configure" provides a command line option for disabling the use of openat2(), it doesn't try to detect on its own whether the target system supports the sys call. This means that just running ./configure && make against an older kernel will fail

gcc -I. -I. -I./popt -I./zlib -g -O2 -DHAVE_CONFIG_H -Wall -W  -c syscall.c -o syscall.o
syscall.c:38:10: fatal error: linux/openat2.h: No such file or directory
 #include <linux/openat2.h>
          ^~~~~~~~~~~~~~~~~
compilation terminated.
Makefile:72: recipe for target 'syscall.o' failed
make: *** [syscall.o] Error 1

This can be remedied by manually configuring rsync with "--disable-openat2", but I believe there is a better way. Ensuring SYS_openat2 is supported is a quick check that can be easily added to configure, making the experience automatic and seamless.

[mmayer]

PS: I am preparing a pull request with a proposed implementation.

[buhtz]

Duplicate of #905?

[mmayer]

Looks like a duplicate. But this one comes with a solution (pull request).