Check whether data with a BOM actually decoded instead of blindly continuing.

Noticed this while studying the code around issue #86. Hard to say whether this is the observed issue, as we can't reliably reproduce that crash.

Author: nolanw

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## [Unreleased]
 
+* Fix crash when given data that starts with a UTF-8 BOM but is not valid UTF-8.
+
 ## [2.1.2][]
 
 * Avoid symbol collision for `parseNumber` with Tapjoy SDK by marking it and other helper functions in `HTMLSelector.m` as `static`. (Fixes #85.)

HTMLReaderTests/HTMLDocumentTests.m

@@ -37,4 +37,13 @@ - (void)testParsedStringEncodingContentTypeUTF8
     XCTAssertEqual(document.parsedStringEncoding, (NSStringEncoding)NSUTF8StringEncoding);
 }
 
+- (void)testInvalidUTF8WithBOM
+{
+    char buffer[] = "\xEF\xBB\xBF" // UTF-8 BOM
+                    "\xF5";        // invalid byte in UTF-8
+    NSData *data = [NSData dataWithBytesNoCopy:buffer length:(sizeof(buffer) - 1) freeWhenDone:NO];
+    HTMLDocument *document = [HTMLDocument documentWithData:data contentTypeHeader:nil];
+    XCTAssertEqual(document.parsedStringEncoding, (NSStringEncoding)NSWindowsCP1252StringEncoding);
+}
+
 @end

Sources/HTMLEncoding.m

@@ -21,23 +21,32 @@ HTMLStringEncoding DeterminedStringEncodingForData(NSData *data, NSString *conte
     unsigned char buffer[3] = {0};
     [data getBytes:buffer length:MIN(data.length, 3U)];
     if (buffer[0] == 0xFE && buffer[1] == 0xFF) {
-        *outDecodedString = [[NSString alloc] initWithData:data encoding:NSUTF16BigEndianStringEncoding];
-        return (HTMLStringEncoding){
-            .encoding = NSUTF16BigEndianStringEncoding,
-            .confidence = Certain
-        };
+        NSString *decodedString = [[NSString alloc] initWithData:data encoding:NSUTF16BigEndianStringEncoding];
+        if (decodedString) {
+            *outDecodedString = decodedString;
+            return (HTMLStringEncoding){
+                .encoding = NSUTF16BigEndianStringEncoding,
+                .confidence = Certain
+            };
+        }
     } else if (buffer[0] == 0xFF && buffer[1] == 0xFE) {
-        *outDecodedString = [[NSString alloc] initWithData:data encoding:NSUTF16LittleEndianStringEncoding];
-        return (HTMLStringEncoding){
-            .encoding = NSUTF16LittleEndianStringEncoding,
-            .confidence = Certain
-        };
+        NSString *decodedString = [[NSString alloc] initWithData:data encoding:NSUTF16LittleEndianStringEncoding];
+        if (decodedString) {
+            *outDecodedString = decodedString;
+            return (HTMLStringEncoding){
+                .encoding = NSUTF16LittleEndianStringEncoding,
+                .confidence = Certain
+            };
+        }
     } else if (buffer[0] == 0xEF && buffer[1] == 0xBB && buffer[2] == 0xBF) {
-        *outDecodedString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
-        return (HTMLStringEncoding){
-            .encoding = NSUTF8StringEncoding,
-            .confidence = Certain
-        };
+        NSString *decodedString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
+        if (decodedString) {
+            *outDecodedString = decodedString;
+            return (HTMLStringEncoding){
+                .encoding = NSUTF8StringEncoding,
+                .confidence = Certain
+            };
+        }
     }
 
     if (contentType) {