Mapping appsink buffer as READWRITE always copies data

[Michael Gruner]

Hello all

It seems like I can never map a buffer as READWRITE that I just pulled from an appsink without the map copying the underlying data. I have a bare minimal example here:

https://pastebin.ubuntu.com/p/27b6K4VX6c/

You may run it as:
cc -o gst gst.c `pkg-config --cflags --libs gstreamer-1.0 gstreamer-app-1.0`
GST_DEBUG=2,GST_PERFORMANCE:9,GST_BUFFER:9,GST_MEMORY:9,GST_LOCKING:9 ./gst

The pipeline is simply:
"videotestsrc ! appsink name=sink enable-last-sample=false max-buffers=1 drop=false”

The process is also simple:
1. Pull the sample
2. Get the buffer from the sample
3. Make it writable
4. Map it as READWRITE  <————— Copy occurs
5. Unmap
6. Unref

Here’s my log from the minimal example:
Sample refcount: 2
Buffer 1 refcount: 1
Buffer 1 is writable: 0
Buffer 2 refcount: 1. <———— Made the buffer writable
Buffer 2 is writable: 1
Memcpy below
…
0:00:00.059691000 33882 0x7f9c55e0e000 TRACE            GST_LOCKING gstminiobject.c:221:gboolean gst_mini_object_lock(GstMiniObject *, GstLockFlags): lock 0x7f9c58000000: state 00020000, access_mode 3
0:00:00.059706000 33882 0x7f9c55e0e000 DEBUG            GST_LOCKING gstminiobject.c:256:gboolean gst_mini_object_lock(GstMiniObject *, GstLockFlags): lock failed 0x7f9c58000000: state 00020000, access_mode 3
...
0:00:00.059753000 33882 0x7f9c55e0e000 DEBUG        GST_PERFORMANCE gstallocator.c:461:GstMemorySystem *_sysmem_copy(GstMemorySystem *, gssize, gsize): memcpy 115200 memory 0x7f9c58000000 -> 0x7f9c5804b000
...
Memcpy above

I’ve tried changing the source to verify it’s not a videotestsrc thing, forcing processing in the middle, unreffing the sample earlier. Nothing seems to make a difference.

I appreciate some light on this, or if it’s an actual bug, I’d be happy to help with the fix.

Thanks!
Michael
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