player/screenshot: %tX template format fixes

DOCS/interface-changes/screenshot-template-time.txt

@@ -0,0 +1 @@
+change `%tX` format specifier of `--screenshot-template` option to only accept C11 strftime specifiers or `%s` specifier

DOCS/man/options.rst

@@ -5031,6 +5031,12 @@ Screenshot
         the result of passing "%X" to ``strftime``. For example, ``%tm`` will
         insert the number of the current month as number. You have to use
         multiple ``%tX`` specifiers to build a full date/time string.
+
+        .. note::
+
+            Since mpv 0.42.0, this only supports the ``strftime`` format
+            specifiers specified in C11 standard, or the ``%s`` extension for
+            UNIX timestamp. Other format specifiers are unsupported.
     ``%{prop[:fallback text]}``
         Insert the value of the input property 'prop'. E.g. ``%{filename}`` is
         the same as ``%f``. If the property does not exist or is not available,

player/screenshot.c

@@ -228,13 +228,20 @@ static char *create_fname(struct MPContext *mpctx, char *template,
         }
         case 't': {
             char tfmt = *template;
-            if (!tfmt)
+            // Translate common extensions to the closest alternative.
+            size_t i = strcspn("sklPaAbBcCdDeFgGhHIjmMnprRStTuUVwWxXyYzZ%", (char[]){tfmt, '\0'});
+            tfmt =             "sHIpaAbBcCdDeFgGhHIjmMnprRStTuUVwWxXyYzZ%"[i];
+            if (!tfmt || !local_time)
                 goto error_exit;
             template++;
-            char fmtstr[] = {'%', tfmt, '\0'};
             char buffer[80];
-            if (strftime(buffer, sizeof(buffer), fmtstr, local_time) == 0)
-                buffer[0] = '\0';
+            if (tfmt == 's') {
+                snprintf(buffer, sizeof(buffer), "%"PRId64, (int64_t)raw_time);
+            } else {
+                char fmtstr[] = {'%', tfmt, '\0'};
+                if (strftime(buffer, sizeof(buffer), fmtstr, local_time) == 0)
+                    buffer[0] = '\0';
+            }
             append_filename(&res, buffer);
             break;
         }