builtin: implement unbuffer_stdout/0 (#20662)

vlib/builtin/builtin.c.v

@@ -223,6 +223,30 @@ pub fn flush_stderr() {
 	}
 }
 
+// unbuffer_stdout will turn off the default buffering done for stdout.
+// It will affect all consequent print and println calls, effectively making them behave like
+// eprint and eprintln do. It is useful for programs, that want to produce progress bars, without
+// cluttering your code with a flush_stdout() call after every print() call. It is also useful for
+// programs (sensors), that produce small chunks of output, that you want to be able to process
+// immediately.
+// Note 1: if used, *it should be called at the start of your program*, before using
+// print or println().
+// Note 2: most libc implementations, have logic that use line buffering for stdout, when the output
+// stream is connected to an interactive device, like a terminal, and otherwise fully buffer it,
+// which is good for the output performance for programs that can produce a lot of output (like
+// filters, or cat etc), but bad for latency. Normally, it is usually what you want, so it is the
+// default for V programs too.
+// See https://www.gnu.org/software/libc/manual/html_node/Buffering-Concepts.html .
+// See https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05 .
+pub fn unbuffer_stdout() {
+	$if freestanding {
+		not_implemented := 'unbuffer_stdout is not implemented\n'
+		bare_eprint(not_implemented.str, u64(not_implemented.len))
+	} $else {
+		unsafe { C.setbuf(C.stdout, 0) }
+	}
+}
+
 // print prints a message to stdout. Note that unlike `eprint`, stdout is not automatically flushed.
 @[manualfree]
 pub fn print(s string) {

vlib/builtin/builtin_unbuffer_stdout_test.v

@@ -0,0 +1,17 @@
+import time
+
+fn test_unbuffer_stdout() {
+	// TODO: add a more extensive test, perhaps using expect,
+	// that does not require an active user to verify its validity.
+	// Currently this just tests that unbuffer_stdout/0 can be called,
+	// and its effects can be observed by commenting the call to it,
+	// on the next line, and then re-running the test:
+	unbuffer_stdout()
+	print('#---')
+	for i in 0 .. 21 {
+		print('\b\b\b> ${i:02}')
+		time.sleep(20 * time.millisecond)
+	}
+	println('\b\b\b done')
+	assert true
+}