sort: fix -s. Closes 10671

function                                             old     new   delta
sort_main                                            786     862     +76
compare_keys                                         720     794     +74
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 150/0)             Total: 150 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 57 insertions, 12 deletions

diff --git a/coreutils/sort.c b/coreutils/sort.c
index ceea24491..8ffd0cf44 100644
--- a/coreutils/sort.c
+++ b/coreutils/sort.c
@@ -62,9 +62,9 @@
 //usage:     "\n        -u      Suppress duplicate lines"
 //usage:        IF_FEATURE_SORT_BIG(
 //usage:     "\n        -z      Lines are terminated by NUL, not newline"
-////usage:     "\n      -m      Ignored for GNU compatibility"
-////usage:     "\n      -S BUFSZ Ignored for GNU compatibility"
-////usage:     "\n      -T TMPDIR Ignored for GNU compatibility"
+///////:     "\n        -m      Ignored for GNU compatibility"
+///////:     "\n        -S BUFSZ Ignored for GNU compatibility"
+///////:     "\n        -T TMPDIR Ignored for GNU compatibility"
 //usage:        )
 //usage:
 //usage:#define sort_example_usage
@@ -117,6 +117,7 @@ enum {
         FLAG_k  = 0x10000,
         FLAG_t  = 0x20000,
         FLAG_bb = 0x80000000,   /* Ignore trailing blanks  */
+        FLAG_no_tie_break = 0x40000000,
 };
 
 #if ENABLE_FEATURE_SORT_BIG
@@ -340,10 +341,34 @@ static int compare_keys(const void *xarg, const void *yarg)
 #endif
         } /* for */
 
-        /* Perform fallback sort if necessary */
-        if (!retval && !(option_mask32 & FLAG_s)) {
-                flags = option_mask32;
-                retval = strcmp(*(char **)xarg, *(char **)yarg);
+        if (retval == 0) {
+                /* So far lines are "the same" */
+
+                if (option_mask32 & FLAG_s) {
+                        /* "Stable sort": later line is "smaller",
+                         * IOW: do not allow qsort() to swap equal lines.
+                         */
+                        uint32_t *p32;
+                        uint32_t x32, y32;
+                        char *line;
+                        unsigned len;
+
+                        line = *(char**)xarg;
+                        len = (strlen(line) + 4) & (~3u);
+                        p32 = (void*)(line + len);
+                        x32 = *p32;
+                        line = *(char**)yarg;
+                        len = (strlen(line) + 4) & (~3u);
+                        p32 = (void*)(line + len);
+                        y32 = *p32;
+
+                        retval = x32 > y32;
+                } else
+                if (!(option_mask32 & FLAG_no_tie_break)) {
+                        /* fallback sort */
+                        flags = option_mask32;
+                        retval = strcmp(*(char **)xarg, *(char **)yarg);
+                }
         }
 
         if (flags & FLAG_r)
@@ -368,7 +393,7 @@ static unsigned str2u(char **str)
 int sort_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int sort_main(int argc UNUSED_PARAM, char **argv)
 {
-        char *line, **lines;
+        char **lines;
         char *str_ignored, *str_o, *str_t;
         llist_t *lst_k = NULL;
         int i;
@@ -457,7 +482,7 @@ int sort_main(int argc UNUSED_PARAM, char **argv)
                  * do not continue to next file: */
                 FILE *fp = xfopen_stdin(*argv);
                 for (;;) {
-                        line = GET_LINE(fp);
+                        char *line = GET_LINE(fp);
                         if (!line)
                                 break;
                         lines = xrealloc_vector(lines, 6, linecount);
@@ -482,15 +507,35 @@ int sort_main(int argc UNUSED_PARAM, char **argv)
                 return EXIT_SUCCESS;
         }
 #endif
+
+        /* For stable sort, store original line position beyond terminating NUL */
+        if (option_mask32 & FLAG_s) {
+                for (i = 0; i < linecount; i++) {
+                        uint32_t *p32;
+                        char *line;
+                        unsigned len;
+
+                        line = lines[i];
+                        len = (strlen(line) + 4) & (~3u);
+                        lines[i] = line = xrealloc(line, len + 4);
+                        p32 = (void*)(line + len);
+                        *p32 = i;
+                }
+                /*option_mask32 |= FLAG_no_tie_break;*/
+                /* ^^^redundant: if FLAG_s, compare_keys() does no tie break */
+        }
+
         /* Perform the actual sort */
         qsort(lines, linecount, sizeof(lines[0]), compare_keys);
 
         /* Handle -u */
         if (option_mask32 & FLAG_u) {
                 int j = 0;
-                /* coreutils 6.3 drop lines for which only key is the same */
-                /* -- disabling last-resort compare... */
-                option_mask32 |= FLAG_s;
+                /* coreutils 6.3 drop lines for which only key is the same
+                 * -- disabling last-resort compare, or else compare_keys()
+                 * will be the same only for completely identical lines.
+                 */
+                option_mask32 |= FLAG_no_tie_break;
                 for (i = 1; i < linecount; i++) {
                         if (compare_keys(&lines[j], &lines[i]) == 0)
                                 free(lines[i]);