Guard against calling malloc(0) when create ByteBuffer. (#7948)

Motivation:

We did not guard against the case of calling malloc(0) when creating a ByteBuffer without a Cleaner. The problem is that malloc(0) can have different behaviour, it either return a null-pointer or a valid pointer that you can pass to free.

The real problem arise if Unsafe.allocateMemory(0) returns 0 and we use it as the memoryAddress of the ByteBuffer. The problem here is that native libraries test for 0 and handle it as a null-ptr. This is for example true in SSL.bioSetByteBuffer(...) which would throw a NPE when 0 is used as memoryAddress and so produced errors during SSL usage.

Modifications:

- Always allocate 1 byte as minimum (even if we ask for an empty buffer).
- Add unit test.

Result:

No more errors possible because of malloc(0).

common/src/main/java/io/netty/util/internal/PlatformDependent0.java

@@ -427,7 +427,10 @@ final class PlatformDependent0 {
     }
 
     static ByteBuffer allocateDirectNoCleaner(int capacity) {
-        return newDirectBuffer(UNSAFE.allocateMemory(capacity), capacity);
+        // Calling malloc with capacity of 0 may return a null ptr or a memory address that can be used.
+        // Just use 1 to make it safe to use in all cases:
+        // See: http://pubs.opengroup.org/onlinepubs/009695399/functions/malloc.html
+        return newDirectBuffer(UNSAFE.allocateMemory(Math.max(1, capacity)), capacity);
     }
 
     static boolean hasAllocateArrayMethod() {

common/src/test/java/io/netty/util/internal/PlatformDependentTest.java

@@ -17,14 +17,13 @@ package io.netty.util.internal;
 
 import org.junit.Test;
 
+import java.nio.ByteBuffer;
 import java.util.Random;
 
 import static io.netty.util.internal.PlatformDependent.hashCodeAscii;
 import static io.netty.util.internal.PlatformDependent.hashCodeAsciiSafe;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNotSame;
-import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.*;
+import static org.junit.Assume.assumeTrue;
 
 public class PlatformDependentTest {
     private static final Random r = new Random();
@@ -146,4 +145,13 @@ public class PlatformDependentTest {
                     hashCodeAscii(string));
         }
     }
+
+    @Test
+    public void testAllocateWithCapacity0() {
+        assumeTrue(PlatformDependent.hasDirectBufferNoCleanerConstructor());
+        ByteBuffer buffer = PlatformDependent.allocateDirectNoCleaner(0);
+        assertNotEquals(0, PlatformDependent.directBufferAddress(buffer));
+        assertEquals(0, buffer.capacity());
+        PlatformDependent.freeDirectNoCleaner(buffer);
+    }
 }