Merge pull request #143 from xerial/fix-header-check

#142: Fix reading stream that happens to have MAGIC_HEADER[0]

src/main/java/org/xerial/snappy/SnappyCodec.java

@@ -120,6 +120,18 @@ public class SnappyCodec
         return Arrays.equals(MAGIC_HEADER, magic);
     }
 
+    public static boolean hasMagicHeaderPrefix(byte[] b) {
+        int limit = Math.min(MAGIC_LEN, b.length);
+        int i = 0;
+        while(i < limit) {
+            if(b[i] != MAGIC_HEADER[i]) {
+                return false;
+            }
+            ++i;
+        }
+        return true;
+    }
+
     public static SnappyCodec readHeader(InputStream in)
             throws IOException
     {

src/main/java/org/xerial/snappy/SnappyInputStream.java

@@ -93,7 +93,7 @@ public class SnappyInputStream
             // Snappy produces at least 1-byte result. So the empty input is not a valid input
             throw new SnappyIOException(SnappyErrorCode.EMPTY_INPUT, "Cannot decompress empty stream");
         }
-        if (readBytes < header.length || header[0] != SnappyCodec.MAGIC_HEADER[0]) {
+        if (readBytes < header.length || !SnappyCodec.hasMagicHeaderPrefix(header)) {
             // do the default uncompression
             // (probably) compressed by Snappy.compress(byte[])
             readFully(header, readBytes);

src/test/java/org/xerial/snappy/SnappyInputStreamTest.java

@@ -204,4 +204,37 @@ public class SnappyInputStreamTest
 
         assertArrayEquals(orig, uncompressed);
     }
+
+    @Test
+    public void checkMagicHeader()
+            throws IOException
+    {
+        ByteArrayOutputStream b = new ByteArrayOutputStream();
+        // Write uncompressed length beginning with -126 (the same with magicheader[0])
+        b.write(SnappyCodec.MAGIC_HEADER[0]);
+        b.write(0x01);
+        // uncompressed data length = 130
+
+        ByteArrayOutputStream data = new ByteArrayOutputStream();
+        for(int i=0; i<130; ++i) {
+            data.write('A');
+        }
+        byte[] dataMoreThan8Len = data.toByteArray();
+
+        // write literal (lower 2-bit of the first tag byte is 00, upper 6-bits represents data size)
+        b.write(60<<2); // 1-byte data length follows
+        b.write(dataMoreThan8Len.length-1); // subsequent data length
+        b.write(dataMoreThan8Len);
+
+        byte[] compressed = b.toByteArray();
+
+        // This should succeed
+        assertArrayEquals(dataMoreThan8Len, Snappy.uncompress(compressed));
+
+        // Reproduce error in #142
+        SnappyInputStream in = new SnappyInputStream(new ByteArrayInputStream(b.toByteArray()));
+        byte[] uncompressed = readFully(in);
+
+        assertArrayEquals(dataMoreThan8Len, uncompressed);
+    }
 }