avoid a memcpy when constructing output of tools::JsonWriter

we can store the data in an rtl::OString object and then return
that directly

Change-Id: I65af6820bfd3135b38d437cf9736fffff8924e88
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/164291
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
(cherry picked from commit b1c081196979188bd5502778617a2fa694830bff)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/164271
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>

2 files changed, 15 insertions, 12 deletions

diff --git a/include/tools/json_writer.hxx b/include/tools/json_writer.hxx
index c5faf542f1d8..8bb8d192d2ff 100644
--- a/include/tools/json_writer.hxx
+++ b/include/tools/json_writer.hxx
@@ -34,7 +34,7 @@ class TOOLS_DLLPUBLIC JsonWriter
     friend class ScopedJsonWriterArray;
     friend class ScopedJsonWriterStruct;
 
-    char* mpBuffer;
+    rtl_String* mpBuffer;
     char* mPos;
     int mSpaceAllocated;
     int mStartNodeCount;
@@ -99,14 +99,14 @@ private:
     inline void addValidationMark()
     {
 #ifndef NDEBUG
-        *(mpBuffer + mSpaceAllocated - 1) = JSON_WRITER_DEBUG_MARKER;
+        *(mpBuffer->buffer + mSpaceAllocated - 1) = JSON_WRITER_DEBUG_MARKER;
 #endif
     }
 
     inline void validate()
     {
 #ifndef NDEBUG
-        unsigned char c = *(mpBuffer + mSpaceAllocated - 1);
+        unsigned char c = *(mpBuffer->buffer + mSpaceAllocated - 1);
         assert(c == JSON_WRITER_DEBUG_MARKER);
 #endif
     }
diff --git a/tools/source/misc/json_writer.cxx b/tools/source/misc/json_writer.cxx
index e3e27bf756b4..e091ed8b8b76 100644
--- a/tools/source/misc/json_writer.cxx
+++ b/tools/source/misc/json_writer.cxx
@@ -19,8 +19,8 @@ namespace tools
 constexpr int DEFAULT_BUFFER_SIZE = 2048;
 
 JsonWriter::JsonWriter()
-    : mpBuffer(static_cast<char*>(malloc(DEFAULT_BUFFER_SIZE)))
-    , mPos(mpBuffer)
+    : mpBuffer(rtl_string_alloc(DEFAULT_BUFFER_SIZE))
+    , mPos(mpBuffer->buffer)
     , mSpaceAllocated(DEFAULT_BUFFER_SIZE)
     , mStartNodeCount(0)
     , mbFirstFieldInNode(true)
@@ -37,7 +37,7 @@ JsonWriter::JsonWriter()
 JsonWriter::~JsonWriter()
 {
     assert(mbClosed && "forgot to extract data?");
-    free(mpBuffer);
+    rtl_string_release(mpBuffer);
 }
 
 ScopedJsonWriterNode JsonWriter::startNode(std::string_view pNodeName)
@@ -344,12 +344,15 @@ void JsonWriter::addCommaBeforeField()
 void JsonWriter::ensureSpace(int noMoreBytesRequired)
 {
     assert(!mbClosed && "already extracted data");
-    int currentUsed = mPos - mpBuffer;
+    int currentUsed = mPos - mpBuffer->buffer;
     if (currentUsed + noMoreBytesRequired >= mSpaceAllocated)
     {
         auto newSize = (currentUsed + noMoreBytesRequired) * 2;
-        mpBuffer = static_cast<char*>(realloc(mpBuffer, newSize));
-        mPos = mpBuffer + currentUsed;
+        rtl_String* pNewBuffer = rtl_string_alloc(newSize);
+        memcpy(pNewBuffer->buffer, mpBuffer->buffer, currentUsed);
+        rtl_string_release(mpBuffer);
+        mpBuffer = pNewBuffer;
+        mPos = mpBuffer->buffer + currentUsed;
         mSpaceAllocated = newSize;
 
         addValidationMark();
@@ -392,13 +395,13 @@ OString JsonWriter::finishAndGetAsOString()
     *mPos = 0;
     mbClosed = true;
 
-    OString ret(mpBuffer, mPos - mpBuffer);
-    return ret;
+    mpBuffer->length = mPos - mpBuffer->buffer;
+    return mpBuffer;
 }
 
 bool JsonWriter::isDataEquals(std::string_view s) const
 {
-    return std::string_view(mpBuffer, static_cast<size_t>(mPos - mpBuffer)) == s;
+    return std::string_view(mpBuffer->buffer, static_cast<size_t>(mPos - mpBuffer->buffer)) == s;
 }
 
 } // namespace tools