Revise Utils::Gzip::compress code

Change signature
Add ZLIB_CONST define to make  z_stream.next_in const
Cast to zlib defined type Bytef*
Set memLevel to 9 in deflateInit2() for maximum performance
Revise compression loop
On returning false, free memory correctly by calling deflateEnd()
Reserve space by the estimation of deflateBound()

src/base/http/responsegenerator.cpp

@@ -93,14 +93,15 @@ void Http::compressContent(Response &response)
         return;
 
     // try compressing
-    QByteArray buf;
-    if (!Utils::Gzip::compress(response.content, buf))
+    bool ok = false;
+    const QByteArray compressedData = Utils::Gzip::compress(response.content, 6, &ok);
+    if (!ok)
         return;
 
     // "Content-Encoding: gzip\r\n" is 24 bytes long
-    if ((buf.size() + 24) >= contentSize)
+    if ((compressedData.size() + 24) >= contentSize)
         return;
 
-    response.content = buf;
+    response.content = compressedData;
     response.headers[HEADER_CONTENT_ENCODING] = QLatin1String("gzip");
 }

src/base/utils/gzip.cpp

@@ -27,66 +27,71 @@
  * exception statement from your version.
  */
 
+#include "gzip.h"
+
 #include <QByteArray>
-#include <zlib.h>
 
-#include "gzip.h"
+#ifndef ZLIB_CONST
+#define ZLIB_CONST  // make z_stream.next_in const
+#endif
+#include <zlib.h>
 
-bool Utils::Gzip::compress(QByteArray src, QByteArray &dest)
+QByteArray Utils::Gzip::compress(const QByteArray &data, const int level, bool *ok)
 {
-    static const int BUFSIZE = 128 * 1024;
-    char tmpBuf[BUFSIZE];
-    int ret;
+    if (ok) *ok = false;
 
-    dest.clear();
+    if (data.isEmpty())
+        return {};
+
+    const int BUFSIZE = 128 * 1024;
+    char tmpBuf[BUFSIZE] = {0};
 
     z_stream strm;
     strm.zalloc = Z_NULL;
     strm.zfree = Z_NULL;
     strm.opaque = Z_NULL;
-    strm.next_in = reinterpret_cast<uchar *>(src.data());
-    strm.avail_in = src.length();
-    strm.next_out = reinterpret_cast<uchar *>(tmpBuf);
+    strm.next_in = reinterpret_cast<const Bytef *>(data.constData());
+    strm.avail_in = uInt(data.size());
+    strm.next_out = reinterpret_cast<Bytef *>(tmpBuf);
     strm.avail_out = BUFSIZE;
 
     // windowBits = 15 + 16 to enable gzip
     // From the zlib manual: windowBits can also be greater than 15 for optional gzip encoding. Add 16 to windowBits
     // to write a simple gzip header and trailer around the compressed data instead of a zlib wrapper.
-    ret = deflateInit2(&strm, Z_BEST_COMPRESSION, Z_DEFLATED, 15 + 16, 8, Z_DEFAULT_STRATEGY);
+    int result = deflateInit2(&strm, level, Z_DEFLATED, (15 + 16), 9, Z_DEFAULT_STRATEGY);
+    if (result != Z_OK)
+        return {};
 
-    if (ret != Z_OK)
-        return false;
+    QByteArray output;
+    output.reserve(deflateBound(&strm, data.size()));
 
-    while (strm.avail_in != 0) {
-        ret = deflate(&strm, Z_NO_FLUSH);
-        if (ret != Z_OK)
-            return false;
+    // feed to deflate
+    while (strm.avail_in > 0) {
+        result = deflate(&strm, Z_NO_FLUSH);
 
-        if (strm.avail_out == 0) {
-            dest.append(tmpBuf, BUFSIZE);
-            strm.next_out = reinterpret_cast<uchar *>(tmpBuf);
-            strm.avail_out = BUFSIZE;
-        }
+        if (result != Z_OK) {
+            deflateEnd(&strm);
+            return {};
         }
 
-    int deflateRes = Z_OK;
-    while (deflateRes == Z_OK) {
-        if (strm.avail_out == 0) {
-            dest.append(tmpBuf, BUFSIZE);
-            strm.next_out = reinterpret_cast<uchar *>(tmpBuf);
+        output.append(tmpBuf, (BUFSIZE - strm.avail_out));
+        strm.next_out = reinterpret_cast<Bytef *>(tmpBuf);
         strm.avail_out = BUFSIZE;
     }
 
-        deflateRes = deflate(&strm, Z_FINISH);
-    }
+    // flush the rest from deflate
+    while (result != Z_STREAM_END) {
+        result = deflate(&strm, Z_FINISH);
 
-    if (deflateRes != Z_STREAM_END)
-        return false;
+        output.append(tmpBuf, (BUFSIZE - strm.avail_out));
+        strm.next_out = reinterpret_cast<Bytef *>(tmpBuf);
+        strm.avail_out = BUFSIZE;
+    }
 
-    dest.append(tmpBuf, BUFSIZE - strm.avail_out);
     deflateEnd(&strm);
 
-    return true;
+    if (ok) *ok = true;
+    return output;
 }
 
 bool Utils::Gzip::uncompress(QByteArray src, QByteArray &dest)

src/base/utils/gzip.h

@@ -36,7 +36,7 @@ namespace Utils
 {
     namespace Gzip
     {
-        bool compress(QByteArray src, QByteArray &dest);
+        QByteArray compress(const QByteArray &data, int level = 6, bool *ok = nullptr);
         bool uncompress(QByteArray src, QByteArray &dest);
     }
 }