Fix escaping of % in strftime

Fixes: #16155

Author: sbc100

src/library.js

@@ -1024,11 +1024,16 @@ LibraryManager.library = {
         return '%';
       }
     };
+
+    // Replace %% with a pair of NULLs (which cannot occur in a C string), then
+    // re-inject them after processing.
+    pattern = pattern.replace(/%%/g, '\0\0')
     for (var rule in EXPANSION_RULES_2) {
       if (pattern.includes(rule)) {
         pattern = pattern.replace(new RegExp(rule, 'g'), EXPANSION_RULES_2[rule](date));
       }
     }
+    pattern = pattern.replace(/\0\0/g, '%')
 
     var bytes = intArrayFromString(pattern, false);
     if (bytes.length > maxsize) {

tests/core/test_strftime.cpp

@@ -252,5 +252,10 @@ int main() {
   size = strftime(s, 10, "%I %M %p", &tm);
   test(!cmp(s, "12 01 PM"), "strftime test #35", s);
 
+  // Test %% escaping
+  const char *fmt = "%%H=%H %%M=%m %%z=%z";
+  strftime(s, sizeof(s), fmt, &tm);
+  printf("%s -> %s\n", fmt, s);
+
   return 0;
 }

tests/core/test_strftime.out

@@ -60,3 +60,4 @@ strftime test #32: 1
 strftime test #33: 1
 strftime test #34: 1
 strftime test #35: 1
+%%H=%H %%M=%m %%z=%z -> %H=12 %M=12 %z=-0800