DAOS-17688 ddb: Prevent C stdout output truncation when piped or redi…

src/control/cmd/ddb/main.go

@@ -366,6 +366,11 @@ func parseOpts(args []string, opts *cliOptions) error {
 func main() {
 	var opts cliOptions
 
+	// Must be called before any write to stdout.
+	if err := logging.DisableCStdoutBuffering(); err != nil {
+		exitWithError(err)
+	}
+
 	if err := parseOpts(os.Args[1:], &opts); err != nil {
 		exitWithError(err)
 	}

src/control/logging/cstdio.go

@@ -0,0 +1,33 @@
+package logging
+
+/*
+ #include <stdio.h>
+*/
+import "C"
+import "github.com/pkg/errors"
+
+// DisableCStdoutBuffering disables C code write to stdout buffering and must be called before any write to stdout.
+//
+// When a Go program's stdout is connected to a pipe (e.g. redirected or piped to another
+// process), C stdout becomes fully-buffered by Unix convention instead of line-buffered,
+// which can cause output from C functions such as printf() to be lost or delayed.
+// Disabling buffering ensures that C stdout output is flushed immediately, consistent
+// with the behavior of Go's own stdout.
+//
+// References:
+//   - https://stackoverflow.com/questions/42634640/using-cgo-why-does-c-output-not-survive-piping-when-golangs-does
+//   - https://stackoverflow.com/questions/1716296/why-does-printf-not-flush-after-the-call-unless-a-newline-is-in-the-format-strin
+//   - https://stackoverflow.com/questions/3723795/is-stdout-line-buffered-unbuffered-or-indeterminate-by-default
+//   - https://groups.google.com/g/comp.lang.c/c/dvRKt-iuO40#
+func DisableCStdoutBuffering() error {
+	rc, err := C.setvbuf(C.stdout, nil, C._IONBF, 0)
+	if rc == 0 {
+		return nil
+	}
+
+	if err != nil {
+		return errors.Wrap(err, "failed to disable C stdout buffering")
+	}
+
+	return errors.Errorf("failed to disable C stdout buffering: rc=%d", int(rc))
+}