Defang MacOS & iOS crashing text sequences

[Manish Goregaokar](https://manishearth.github.io/blog/2018/02/15/picking-apart-the-crashing-ios-string/) says
> So, ultimately, the full set of cases that cause the crash are:
>   Any sequence <consonant1, virama, consonant2, ZWNJ, vowel>
> in Devanagari, Bengali, and Telugu, where: ...

We eliminate the ZWNJ which seems the minimally damaging thing to do to

Telugu rendering per the article above:
> a ZWNJ before a vowel doesn’t really do anything for most Indic scripts.

This is needed as of February 2018, but hopefully not long after that.

Author: mikesamuel

src/main/java/org/owasp/html/Encoding.java

@@ -203,6 +203,43 @@ private static void encodeHtmlOnto(
           output.append(plainText, pos, i).append(repl);
           pos = i + 1;
         }
+      } else if ((0x93A <= ch && ch <= 0xC4C)
+          && (
+              // Devanagari vowel
+              ch <= 0x94F
+              || 0x93A <= ch && ch <= 0x94F
+              // Benagli vowels
+              || 0x985 <= ch && ch <= 0x994
+              || 0x9BE <= ch && ch < 0x9CC  // 0x9CC (Bengali AU) is ok
+              || 0x9E0 <= ch && ch <= 0x9E3
+              // Telugu vowels
+              || 0xC05 <= ch && ch <= 0xC14
+              || 0xC3E <= ch && ch != 0xC48 /* 0xC48 (Telugu AI) is ok */)) {
+        // https://manishearth.github.io/blog/2018/02/15/picking-apart-the-crashing-ios-string/
+        // > So, ultimately, the full set of cases that cause the crash are:
+        // >   Any sequence <consonant1, virama, consonant2, ZWNJ, vowel>
+        // > in Devanagari, Bengali, and Telugu, where: ...
+
+        // TODO: This is needed as of February 2018, but hopefully not long after that.
+        // We eliminate the ZWNJ which seems the minimally damaging thing to do to
+        // Telugu rendering per the article above:
+        // > a ZWNJ before a vowel doesn’t really do anything for most Indic scripts.
+
+        if (pos < i) {
+          if (plainText.charAt(i - 1) == 0x200C /* ZWNJ */) {
+            output.append(plainText, pos, i - 1);
+            // Drop the ZWNJ on the floor.
+            pos = i;
+          }
+        } else if (output instanceof StringBuilder) {
+          StringBuilder sb = (StringBuilder) output;
+          int len = sb.length();
+          if (len != 0) {
+            if (sb.charAt(len - 1) == 0x200C /* ZWNJ */) {
+              sb.setLength(len - 1);
+            }
+          }
+        }
       } else if (((char) 0xd800) <= ch) {
         if (ch <= ((char) 0xdfff)) {
           char next;

src/test/java/org/owasp/html/HtmlSanitizerTest.java

@@ -366,6 +366,54 @@ public static final void testDuplicateAttributes() {
 
   }
 
+  @Test
+  public static final void testMacOSAndIOSQueryOfDeath() {
+    // https://manishearth.github.io/blog/2018/02/15/picking-apart-the-crashing-ios-string/
+    String[][] tests = {
+        {
+          "\u0C1C\u0C4D\u0C1E\u200C\u0C3E",
+          "\u0C1C\u0C4D\u0C1E\u0C3E",
+        },
+        {
+          "\u09B8\u09CD\u09B0<interrupted>\u200C\u09C1",
+          "\u09B8\u09CD\u09B0\u09C1",
+        },
+        {
+          "\u0C1C\u0C4D\u0C1E\u200C\u0C3E",
+          "\u0C1C\u0C4D\u0C1E\u0C3E",
+        },
+        {
+          "\u09B8\u09CD\u09B0\u200C<interrupted>\u09C1",
+          "\u09B8\u09CD\u09B0\u09C1",
+        },
+        {
+          "&#x0C1C;&#x0C4D;&#x0C1E;&#x200C;&#x0C3E;",
+          "\u0C1C\u0C4D\u0C1E\u0C3E",
+        },
+        {
+          "&#x0C1C;&#x0C4D;&#x0C1E;<interrupted>&#x200C;&#x0C3E;",
+          "\u0C1C\u0C4D\u0C1E\u0C3E",
+        },
+        {
+          "&#x09B8;&#x09CD;&#x09B0;&#x200C;&#x09C1;",
+          "\u09B8\u09CD\u09B0\u09C1",
+        },
+        {
+          "&#x09B8;&#x09CD;&#x09B0;&#x200C;<interrupted>&#x09C1;",
+          "\u09B8\u09CD\u09B0\u09C1",
+        },
+        {
+          "\u0915\u094D\u0930\u200C\u093E",
+          "\u0915\u094D\u0930\u093E",
+        },
+    };
+
+    for (int i = 0, n = tests.length; i < n; ++i) {
+      String[] test = tests[i];
+      assertEquals(i + " : " + test[0], test[1], sanitize(test[0]));
+    }
+  }
+
   private static String sanitize(@Nullable String html) {
     StringBuilder sb = new StringBuilder();
     HtmlStreamRenderer renderer = HtmlStreamRenderer.create(