Set the sourcepath properly when compiling.

Summary:
javac will scan the classpath for any missing source files.
While this is interesting behaviour, it's not what we want.
As such, set the sourcepath to a value that will work.

This closes #244 on GitHub.

Test Plan: buck test --all

Author: shs96c

src/com/facebook/buck/java/JavacOptions.java

@@ -92,7 +92,7 @@ public Javac getJavac() {
       ProcessExecutorParams params = ImmutableProcessExecutorParams.builder()
           .setCommand(ImmutableList.of(externalJavac.get().toString(), "-version"))
           .build();
-      ProcessExecutor.Result result = null;
+      ProcessExecutor.Result result;
       try {
         result = getProcessExecutor().get().launchAndExecute(params);
       } catch (InterruptedException | IOException e) {
@@ -119,6 +119,9 @@ public void appendOptionsToList(
     optionsBuilder.add("-source", getSourceLevel());
     optionsBuilder.add("-target", getTargetLevel());
 
+    // Set the sourcepath to stop us reading source files out of jars by mistake.
+    optionsBuilder.add("-sourcepath", "");
+
     if (isDebug()) {
       optionsBuilder.add("-g");
     }

test/com/facebook/buck/java/DefaultJavaLibraryIntegrationTest.java

@@ -381,6 +381,23 @@ public void ensureChangingDepFromProvidedToTransitiveTriggersRebuild() throws IO
     workspace.getBuildLog().assertTargetBuiltLocally("//:library");
   }
 
+  @Test
+  public void ensureThatSourcePathIsSetSensibly() throws IOException {
+    ProjectWorkspace workspace = TestDataHelper.createProjectWorkspaceForScenario(
+        this,
+        "sourcepath",
+        tmp);
+    workspace.setUp();
+
+    ProcessResult result = workspace.runBuckBuild("//:b");
+
+    // This should fail, since we expect the symbol for A not to be found.
+    result.assertFailure();
+    String stderr = result.getStderr();
+
+    assertTrue(stderr, stderr.contains("cannot find symbol"));
+  }
+
   /**
    * Asserts that the specified file exists and returns its contents.
    */

test/com/facebook/buck/java/testdata/sourcepath/A.java

@@ -0,0 +1,5 @@
+// In the default package
+
+class A {
+
+}

test/com/facebook/buck/java/testdata/sourcepath/B.java

@@ -0,0 +1,5 @@
+// Default package
+
+class B extends A {
+}
+

test/com/facebook/buck/java/testdata/sourcepath/BUCK

@@ -0,0 +1,11 @@
+java_library(
+  name = 'a',
+  resources = ['A.java'],
+)
+
+java_library(
+  name = 'b',
+  srcs = ['B.java'],
+  deps = [':a'],
+)
+