reftable: add OutputStream argument to ReftableWriter constructor

This lets us write reftables generically with functions that take
just ReftableWriter argument

Change-Id: I7285951f62f9bd4c78e8f0de194c077d51fa4e51
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java

@@ -765,12 +765,10 @@ public class DfsGarbageCollector {
 			throws IOException {
 		try (DfsOutputStream out = objdb.writeFile(pack, REFTABLE)) {
 			ReftableConfig cfg = configureReftable(reftableConfig, out);
-			ReftableWriter writer = new ReftableWriter(cfg)
+			ReftableWriter writer = new ReftableWriter(cfg, out)
 					.setMinUpdateIndex(reftableInitialMinUpdateIndex)
-					.setMaxUpdateIndex(reftableInitialMaxUpdateIndex)
-					.begin(out)
-					.sortAndWriteRefs(refs)
-					.finish();
+					.setMaxUpdateIndex(reftableInitialMaxUpdateIndex).begin()
+					.sortAndWriteRefs(refs).finish();
 			pack.addFileExt(REFTABLE);
 			pack.setReftableStats(writer.getStats());
 		}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableWriter.java

@@ -108,6 +108,7 @@ public class ReftableWriter {
 	private long minUpdateIndex;
 	private long maxUpdateIndex;
 
+	private OutputStream outputStream;
 	private ReftableOutputStream out;
 	private ObjectIdSubclassMap<RefList> obj2ref;
 
@@ -136,7 +137,20 @@ public class ReftableWriter {
 	 *            configuration for the writer.
 	 */
 	public ReftableWriter(ReftableConfig cfg) {
+		this(cfg, null);
+	}
+
+	/**
+	 * Initialize a writer with a configuration.
+	 *
+	 * @param cfg
+	 *            configuration for the writer
+	 * @param os
+	 *            output stream. Do not supply a stream to begin() on this writer.
+	 */
+	public ReftableWriter(ReftableConfig cfg, @Nullable OutputStream os) {
 		config = cfg;
+		outputStream = os;
 	}
 
 	/**
@@ -183,16 +197,32 @@ public class ReftableWriter {
 	}
 
 	/**
-	 * Begin writing the reftable.
+	 * Begin writing the reftable. Should be called only once.
 	 *
 	 * @param os
 	 *            stream to write the table to. Caller is responsible for
 	 *            closing the stream after invoking {@link #finish()}.
 	 * @return {@code this}
-	 * @throws java.io.IOException
-	 *             if reftable header cannot be written.
 	 */
-	public ReftableWriter begin(OutputStream os) throws IOException {
+	public ReftableWriter begin(OutputStream os) {
+		if (outputStream != null) {
+			throw new IllegalStateException("begin() called twice.");//$NON-NLS-1$
+		}
+		outputStream = os;
+		return begin();
+	}
+
+	/**
+	 * Begin writing the reftable. Should be called only once. Call this
+	 * if a stream was passed to the constructor.
+	 *
+	 * @return {@code this}
+	 */
+	public ReftableWriter begin() {
+		if (out != null) {
+			throw new IllegalStateException("begin() called twice.");//$NON-NLS-1$
+		}
+
 		refBlockSize = config.getRefBlockSize();
 		logBlockSize = config.getLogBlockSize();
 		restartInterval = config.getRestartInterval();
@@ -212,7 +242,7 @@ public class ReftableWriter {
 			restartInterval = refBlockSize < (60 << 10) ? 16 : 64;
 		}
 
-		out = new ReftableOutputStream(os, refBlockSize, alignBlocks);
+		out = new ReftableOutputStream(outputStream, refBlockSize, alignBlocks);
 		refs = new Section(REF_BLOCK_TYPE);
 		if (indexObjects) {
 			obj2ref = new ObjectIdSubclassMap<>();