Re: The "Unix Philosophy 2020" document

["Casper Ti. Vector" <caspervector@gmail.com>]

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On Sun, Nov 24, 2019 at 05:13:15PM -0300, Guillermo wrote:
> Those are just two if statements 'sharing' a body for brevity. That
> deals with errors in the openat() and subsequent write() calls, for
> the file that controls cgroup membership. By displaying a message
> constructed in the same way in both cases, and then throwing an
> exception. *Shrug*

That particular piece of code
> if (0 > cgroup_procs_fd.get()) {
>     procs_file_error: ...
> }
> if (0 > write(cgroup_procs_fd.get(), "0\n", 2)) goto procs_file_error;
seems equivalent to
> if (0 > cgroup_procs_fd.get() ||
>     0 > write(cgroup_procs_fd.get(), "0\n", 2)) {
>     ...
> }

If the error handling branches that need reuse become more complex, the
following way can also be considered (cf. [1]):
> ...
> if (...) goto err;
> ...
> if (...) goto err;
> ...
> return;
> err:
> ...
> return;

Macros and/or helper functions (again cf. [1]; they can be factored into
a mini-library in nosh) can also be used to reduce boilerplate like
> const int error(errno);
> std::fprintf(stderr, ..., std::strerror(error));
> throw EXIT_FAILURE;
which can be easily observed after the attached patch is applied.

BTW, it seems that the value of errno is passed to std::strerror()
before anything can change the errno, which implies that `const int
error(errno);' can be left out and `errno' can be directly used as the
argument of std::strerror().

[1] <https://gitea.com/CasperVector/decryst/src/branch/master/src/decr_lsa.c>.

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--- move-to-control-group.cpp	2018-09-14 21:48:55.000000000 +0800
+++ move-to-control-group.new.cpp	2019-11-25 10:50:20.518459398 +0800
@@ -50,9 +50,8 @@
 	next_prog = arg0_of(args);
 
 	FileStar self_cgroup(open_my_control_group_info("/proc/self/cgroup"));
-	if (!self_cgroup) {
+	if (!self_cgroup && ENOENT != errno) {  // `ENOENT == error' is what we'll see on a BSD.
 		const int error(errno);
-		if (ENOENT == error) return;	// This is what we'll see on a BSD.
 		std::fprintf(stderr, "%s: FATAL: %s: %s\n", prog, "/proc/self/cgroup", std::strerror(error));
 		throw EXIT_FAILURE;
 	}
@@ -73,20 +72,16 @@
 
 	current = prefix + current;
 
-	if (0 > mkdirat(AT_FDCWD, current.c_str(), 0755)) {
+	if (0 > mkdirat(AT_FDCWD, current.c_str(), 0755) && EEXIST != error) {
 		const int error(errno);
-		if (EEXIST != error) {
-			std::fprintf(stderr, "%s: FATAL: %s: %s\n", prog, current.c_str(), std::strerror(error));
-			throw EXIT_FAILURE;
-		}
+		std::fprintf(stderr, "%s: FATAL: %s: %s\n", prog, current.c_str(), std::strerror(error));
+		throw EXIT_FAILURE;
 	}
 
 	const FileDescriptorOwner cgroup_procs_fd(open_appendexisting_at(AT_FDCWD, (current + "/cgroup.procs").c_str()));
-	if (0 > cgroup_procs_fd.get()) {
-procs_file_error:
+	if (0 > cgroup_procs_fd.get() || 0 > write(cgroup_procs_fd.get(), "0\n", 2)) {
 		const int error(errno);
 		std::fprintf(stderr, "%s: FATAL: %s%s: %s\n", prog, current.c_str(), "/cgroup.procs", std::strerror(error));
 		throw EXIT_FAILURE;
 	}
-	if (0 > write(cgroup_procs_fd.get(), "0\n", 2)) goto procs_file_error;
 }