lib/zlib/GzipOutputStream: add SyncFlush()

src/lib/zlib/GzipOutputStream.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2014-2021 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -54,6 +54,29 @@ GzipOutputStream::~GzipOutputStream()
 	deflateEnd(&z);
 }
 
+void
+GzipOutputStream::SyncFlush()
+{
+	/* no more input */
+	z.next_in = nullptr;
+	z.avail_in = 0;
+
+	do {
+		Bytef output[16384];
+		z.next_out = output;
+		z.avail_out = sizeof(output);
+
+		int result = deflate(&z, Z_SYNC_FLUSH);
+		if (result != Z_OK)
+			throw ZlibError(result);
+
+		if (z.next_out == output)
+			break;
+
+		next.Write(output, z.next_out - output);
+	} while (z.avail_out == 0);
+}
+
 void
 GzipOutputStream::Finish()
 {

src/lib/zlib/GzipOutputStream.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2014-2021 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -54,6 +54,11 @@ public:
 	explicit GzipOutputStream(OutputStream &_next);
 	~GzipOutputStream();
 
+	/**
+	 * Throws on error.
+	 */
+	void SyncFlush();
+
 	/**
 	 * Finish the file and write all data remaining in zlib's
 	 * output buffer.