terminal-util: when prompting for a choice from a list, preselect longest prefix

If all entries of a menu prompt start with the same prefix, let's
preselect the prefix to enhance user experience.

This is particularly relevant when prompting for a disk to install
things on, as typically they all start with the same prefix /dev/, and
if there's only a single target medium discoverable, then we can even
fill it out fully.

src/basic/terminal-util.c

@@ -293,17 +293,33 @@ int ask_string_full(
         assert(ret);
         assert(text);
 
+        _cleanup_free_ char *string = NULL;
+        size_t n = 0;
+
+        if (get_completions) {
+                /* Figure out what string to preselect the query with */
+                _cleanup_strv_free_ char **completions = NULL;
+                r = get_completions("", GET_COMPLETIONS_PRESELECT, &completions, userdata);
+                if (r < 0)
+                        return r;
+
+                CompletionResult cr = pick_completion(string, completions, &string);
+                if (cr < 0)
+                        return cr;
+
+                n = strlen_ptr(string);
+        }
+
         /* Output the prompt */
         fputs(ansi_highlight(), stdout);
         va_start(ap, text);
         vprintf(text, ap);
         va_end(ap);
         fputs(ansi_normal(), stdout);
+        if (string)
+                fputs(string, stdout);
         fflush(stdout);
 
-        _cleanup_free_ char *string = NULL;
-        size_t n = 0;
-
         /* Do interactive logic only if stdin + stdout are connected to the same place. And yes, we could use
          * STDIN_FILENO and STDOUT_FILENO here, but let's be overly correct for once, after all libc allows
          * swapping out stdin/stdout. */
@@ -344,7 +360,7 @@ int ask_string_full(
 
                         _cleanup_strv_free_ char **completions = NULL;
                         if (get_completions) {
-                                r = get_completions(string, &completions, userdata);
+                                r = get_completions(string, /* flags= */ 0, &completions, userdata);
                                 if (r < 0)
                                         return r;
                         }
@@ -450,6 +466,7 @@ int ask_string_full(
 
 fallback:
         /* A simple fallback without TTY magic */
+        string = mfree(string);
         r = read_line(stdin, LONG_LINE_MAX, &string);
         if (r < 0)
                 return r;

src/basic/terminal-util.h

@@ -88,7 +88,13 @@ int chvt(int vt);
 int read_one_char(FILE *f, char *ret, usec_t timeout, bool echo, bool *need_nl);
 int ask_char(char *ret, const char *replies, const char *fmt, ...) _printf_(3, 4);
 
-typedef int (*GetCompletionsCallback)(const char *key, char ***ret_list, void *userdata);
+typedef enum GetCompletionsFlags {
+        /* Only return the items subject to preselection: typically you want to suppress meta entries such as
+         * "list" or alias entries if this flag is set. */
+        GET_COMPLETIONS_PRESELECT = 1 << 0,
+} GetCompletionsFlags;
+
+typedef int (*GetCompletionsCallback)(const char *key, GetCompletionsFlags flags, char ***ret_list, void *userdata);
 int ask_string_full(char **ret, GetCompletionsCallback get_completions, void *userdata, const char *text, ...) _printf_(4, 5);
 #define ask_string(ret, text, ...) ask_string_full(ret, NULL, NULL, text, ##__VA_ARGS__)