pretty-print: introduce WITH_BUFFERED_STDERR macro to enable buffering

Then, the buffer will be flushed and buffering will be disabled on
exiting from the scope.

src/shared/pretty-print.c

@@ -531,28 +531,14 @@ void clear_progress_bar_impl(const char *prefix) {
 }
 
 void draw_progress_bar(const char *prefix, double percentage) {
-
         /* We are going output a bunch of small strings that shall appear as a single line to STDERR which is
          * unbuffered by default. Let's temporarily turn on full buffering, so that this is passed to the tty
          * as a single buffer, to make things more efficient. */
-        char buffer[LONG_LINE_MAX];
-        setvbuf(stderr, buffer, _IOFBF, sizeof(buffer));
-
+        WITH_BUFFERED_STDERR;
         draw_progress_bar_impl(prefix, percentage);
-
-        fflush(stderr);
-
-        /* Disable buffering again */
-        setvbuf(stderr, NULL, _IONBF, 0);
 }
 
 void clear_progress_bar(const char *prefix) {
-        char buffer[LONG_LINE_MAX];
-        setvbuf(stderr, buffer, _IOFBF, sizeof(buffer));
-
+        WITH_BUFFERED_STDERR;
         clear_progress_bar_impl(prefix);
-
-        fflush(stderr);
-
-        setvbuf(stderr, NULL, _IONBF, 0);
 }

src/shared/pretty-print.h

@@ -57,3 +57,32 @@ void draw_progress_bar(const char *prefix, double percentage);
 void clear_progress_bar(const char *prefix);
 void draw_progress_bar_impl(const char *prefix, double percentage);
 void clear_progress_bar_impl(const char *prefix);
+
+static inline FILE* enable_buffering(FILE *f, char *buffer, size_t size) {
+        assert(f);
+        assert(buffer);
+        assert(size > 0);
+
+        if (setvbuf(f, buffer, _IOFBF, size) != 0)
+                return NULL;
+
+        return f;
+}
+
+static inline void fflush_and_disable_bufferingp(FILE **p) {
+        assert(p);
+
+        if (*p) {
+                fflush(*p);
+                setvbuf(*p, NULL, _IONBF, 0); /* Disable buffering again. */
+        }
+}
+
+/* Even though the macro below is slightly generic, but it may not work most streams except for stderr,
+ * as stdout is buffered and fopen() enables buffering by default. */
+#define _WITH_BUFFERED_STREAM(f, size, p)                               \
+        _unused_ _cleanup_(fflush_and_disable_bufferingp) FILE *p =     \
+                enable_buffering(f, (char[size]) {}, size)
+
+#define WITH_BUFFERED_STDERR                                            \
+        _WITH_BUFFERED_STREAM(stderr, LONG_LINE_MAX, UNIQ_T(p, UNIQ))

src/sysupdate/updatectl.c

@@ -798,8 +798,7 @@ static int update_render_progress(sd_event_source *source, void *userdata) {
 
         /* We're outputting lots of small strings to STDERR, which is unbuffered by default. So let's turn
          * on full buffering, so we pass this all to the TTY in one go, to make things more efficient */
-        char buffer[LONG_LINE_MAX];
-        setvbuf(stderr, buffer, _IOFBF, sizeof(buffer));
+        WITH_BUFFERED_STDERR;
 
         if (!terminal_is_dumb()) {
                 for (size_t i = 0; i <= n; i++)
@@ -855,8 +854,6 @@ static int update_render_progress(sd_event_source *source, void *userdata) {
         } else if (!exiting)
                 fputs("------\n", stderr);
 
-        fflush(stderr);
-        setvbuf(stderr, NULL, _IONBF, 0); /* Disable buffering again */
         return 0;
 }