shared/logs-show: strip trailing carriage returns at EOL/EOF

When showing logs from a container, we would fail to show various lines:
Oct 29 09:50:51 krowka systemd-nspawn[61376]: Detected architecture x86-64.
Oct 29 09:50:51 krowka systemd-nspawn[61376]: [1B blob data]
Oct 29 09:50:51 krowka systemd-nspawn[61376]: Welcome to Fedora 32 (Rawhide)!
Oct 29 09:50:51 krowka systemd-nspawn[61376]: [1B blob data]

Those are only harmless \r characters that trail the line. We already replace
tabs and strip various ansi characters that we deem inconsequential, so let's
also strip trailing carriage returns. Non-trailing ones are different, because
they change what would be displayed.

src/basic/string-util.c

@@ -743,7 +743,7 @@ static void advance_offsets(
 }
 
 char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
-        const char *i, *begin = NULL;
+        const char *begin = NULL;
         enum {
                 STATE_OTHER,
                 STATE_ESCAPE,
@@ -751,7 +751,7 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                 STATE_CSO,
         } state = STATE_OTHER;
         char *obuf = NULL;
-        size_t osz = 0, isz, shift[2] = {};
+        size_t osz = 0, isz, shift[2] = {}, n_carriage_returns = 0;
         FILE *f;
 
         assert(ibuf);
@@ -762,6 +762,8 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
          * 1. Replaces TABs by 8 spaces
          * 2. Strips ANSI color sequences (a subset of CSI), i.e. ESC '[' … 'm' sequences
          * 3. Strips ANSI operating system sequences (CSO), i.e. ESC ']' … BEL sequences
+         * 4. Strip trailing \r characters (since they would "move the cursor", but have no
+         *    other effect).
          *
          * Everything else will be left as it is. In particular other ANSI sequences are left as they are, as
          * are any other special characters. Truncated ANSI sequences are left-as is too. This call is
@@ -777,14 +779,24 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
         if (!f)
                 return NULL;
 
-        for (i = *ibuf; i < *ibuf + isz + 1; i++) {
+        for (const char *i = *ibuf; i < *ibuf + isz + 1; i++) {
 
                 switch (state) {
 
                 case STATE_OTHER:
                         if (i >= *ibuf + isz) /* EOT */
                                 break;
-                        else if (*i == '\x1B')
+
+                        if (*i == '\r') {
+                                n_carriage_returns++;
+                                break;
+                        } else if (*i == '\n')
+                                /* Ignore carriage returns before new line */
+                                n_carriage_returns = 0;
+                        for (; n_carriage_returns > 0; n_carriage_returns--)
+                                fputc('\r', f);
+
+                        if (*i == '\x1B')
                                 state = STATE_ESCAPE;
                         else if (*i == '\t') {
                                 fputs("        ", f);
@@ -795,6 +807,8 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                         break;
 
                 case STATE_ESCAPE:
+                        assert(n_carriage_returns == 0);
+
                         if (i >= *ibuf + isz) { /* EOT */
                                 fputc('\x1B', f);
                                 advance_offsets(i - *ibuf, highlight, shift, 1);
@@ -815,6 +829,7 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                         break;
 
                 case STATE_CSI:
+                        assert(n_carriage_returns == 0);
 
                         if (i >= *ibuf + isz || /* EOT … */
                             !strchr("01234567890;m", *i)) { /* … or invalid chars in sequence */
@@ -829,6 +844,7 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                         break;
 
                 case STATE_CSO:
+                        assert(n_carriage_returns == 0);
 
                         if (i >= *ibuf + isz || /* EOT … */
                             (*i != '\a' && (uint8_t) *i < 32U) || (uint8_t) *i > 126U) { /* … or invalid chars in sequence */
@@ -848,7 +864,6 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                 fclose(f);
                 return mfree(obuf);
         }
-
         fclose(f);
 
         free_and_replace(*ibuf, obuf);

src/test/test-strip-tab-ansi.c

@@ -34,6 +34,21 @@ int main(int argc, char *argv[]) {
         assert_se(streq(p, "\x1B[waldo"));
         free(p);
 
+        assert_se(p = strdup("\r\rwaldo"));
+        assert_se(strip_tab_ansi(&p, NULL, NULL));
+        assert_se(streq(p, "\r\rwaldo"));
+        free(p);
+
+        assert_se(p = strdup("waldo\r\r"));
+        assert_se(strip_tab_ansi(&p, NULL, NULL));
+        assert_se(streq(p, "waldo"));
+        free(p);
+
+        assert_se(p = strdup("waldo\r\r\n\r\n"));
+        assert_se(strip_tab_ansi(&p, NULL, NULL));
+        assert_se(streq(p, "waldo\n\n"));
+        free(p);
+
         assert_se(terminal_urlify_path("/etc/fstab", "i am a fabulous link", &urlified) >= 0);
         assert_se(p = strjoin("something ", urlified, " something-else"));
         assert_se(q = strdup(p));

src/test/test-utf8.c

@@ -13,6 +13,9 @@ static void test_utf8_is_printable(void) {
         assert_se(utf8_is_printable("\342\204\242", 3));
         assert_se(!utf8_is_printable("\341\204", 2));
         assert_se(utf8_is_printable("ąę", 4));
+        assert_se(!utf8_is_printable("\r", 1));
+        assert_se(utf8_is_printable("\n", 1));
+        assert_se(utf8_is_printable("\t", 1));
 }
 
 static void test_utf8_is_valid(void) {