PrismFileLog: make more robust (multiple close, print after close)

On first close, we set the file pointer to 0 to indicate that it
is not valid anymore. As PrismLog implements Close/AutoCloseable, multiple
close calls have to be supported.

Before, calling close multiple times could result in double free
crashes on Linux (seen in development branches, not master).

We also check that we don't write on a closed FILE pointer, but raise
an exception instead. Here, we use a RuntimeException
(IllegalStateException) instead of a PrismException as changing the
PrismLog interface would be quite disruptive to the rest of the code.

prism/src/prism/PrismFileLog.java

@@ -132,13 +132,23 @@ public class PrismFileLog extends PrismLog
 	@Override
 	public void flush()
 	{
+		if (fp == 0) {
+			throw new IllegalStateException("Trying to flush an invalid file handle (already closed?)");
+		}
 		PrismNative.PN_FlushFile(fp);
 	}
 
 	@Override
 	public void close()
 	{
+		if (fp == 0) {
+			// already closed, ignore (as specified by Closable contract)
+			return;
+		}
+
 		if (!stdout) PrismNative.PN_CloseFile(fp);
+		// set fp to zero to indicate that the file handle is not valid anymore
+		fp = 0;
 	}
 	
 	@Override
@@ -200,6 +210,10 @@ public class PrismFileLog extends PrismLog
 	 */
 	private void printToLog(String s)
 	{
+		if (fp == 0) {
+			throw new IllegalStateException("Trying to write to an invalid file handle (already closed?)");
+		}
+
 		PrismNative.PN_PrintToFile(fp, s);
 	}
 }

prism/src/prism/PrismLog.java

@@ -92,6 +92,7 @@ public abstract class PrismLog implements Closeable, AutoCloseable
 
 	public abstract void flush();
 
+	@Override
 	public abstract void close();
 
 	public abstract void print(boolean b);