Ensure target exists calling FileSystemResourceManager.write() #103

The FileSystemResourceManager.write() method assumes the passed target
to exist. Some operations in the File class do not properly ensure that.
They check the file existence before starting a workspace operation,
which allows some other operation, such as a refresh, to remove the file
from the workspace between the check and the start of the operation.

With this change, the existence check in the File class is performed
after starting a workspace operation, such that no other operation is
able to remove the file concurrently and make the write operation file.

Contributes to
#103.

resources/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/resources/File.java

@@ -70,8 +70,8 @@ public void appendContents(InputStream content, int updateFlags, IProgressMonito
 			try {
 				workspace.prepareOperation(rule, newChild);
 				ResourceInfo info = getResourceInfo(false, false);
-				checkAccessible(getFlags(info));
 				workspace.beginOperation(true);
+				checkAccessible(getFlags(info));
 				IFileInfo fileInfo = getStore().fetchInfo();
 				internalSetContents(content, fileInfo, updateFlags, true, subMonitor.newChild(99));
 			} catch (OperationCanceledException e) {
@@ -354,8 +354,8 @@ public void setContents(InputStream content, int updateFlags, IProgressMonitor m
 			try {
 				workspace.prepareOperation(rule, newChild);
 				ResourceInfo info = getResourceInfo(false, false);
-				checkAccessible(getFlags(info));
 				workspace.beginOperation(true);
+				checkAccessible(getFlags(info));
 				IFileInfo fileInfo = getStore().fetchInfo();
 				internalSetContents(content, fileInfo, updateFlags, false, subMonitor.newChild(99));
 			} catch (OperationCanceledException e) {