progress-bar: Add unbuffered variant

The progress_bar functions do their own buffering: they reconfigure
stderr, then print, then flush and disable buffering on their own. In
situations where multiple progress bars are being drawn at a time (for
example, in updatectl), it's even more efficient to hoist the buffering
and flushing to the call site, and avoid drawing each progress bar
individually.

To that end, new _unbuffered variants of the progress_bar functions. And
we use them in updatectl.

src/shared/pretty-print.c

@@ -460,14 +460,7 @@ bool shall_tint_background(void) {
         return cache != 0;
 }
 
-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));
-
+void draw_progress_bar_unbuffered(const char *prefix, double percentage) {
         fputc('\r', stderr);
         if (prefix) {
                 fputs(prefix, stderr);
@@ -520,17 +513,10 @@ void draw_progress_bar(const char *prefix, double percentage) {
                 fputs(ANSI_ERASE_TO_END_OF_LINE, stderr);
 
         fputc('\r', stderr);
-        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));
-
+void clear_progress_bar_unbuffered(const char *prefix) {
         fputc('\r', stderr);
 
         if (terminal_is_dumb())
@@ -542,8 +528,31 @@ void clear_progress_bar(const char *prefix) {
                 fputs(ANSI_ERASE_TO_END_OF_LINE, stderr);
 
         fputc('\r', stderr);
+}
+
+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));
+
+        draw_progress_bar_unbuffered(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));
+
+        clear_progress_bar_unbuffered(prefix);
+
+        fflush(stderr);
+
+        setvbuf(stderr, NULL, _IONBF, 0);
+}

src/shared/pretty-print.h

@@ -55,3 +55,5 @@ bool shall_tint_background(void);
 
 void draw_progress_bar(const char *prefix, double percentage);
 void clear_progress_bar(const char *prefix);
+void draw_progress_bar_unbuffered(const char *prefix, double percentage);
+void clear_progress_bar_unbuffered(const char *prefix);