In gst_base_src_default_negotiate(), if gst_caps_is_any() goto no_nego_needed, which means that gst_base_src_set_caps is never called and no GST_EVENT_CAPS is emitted from the src element.

In gst_base_transform_default_query(), when answering GST_QUERY_ALLOCATION, if !priv->negotiated, "not negotiated yet, can't answer ALLOCATION query".  

priv->negotiated only gets set in response to a GST_EVENT_CAPS, so for some pipelines the transform element is unable to query further downstream with GST_QUERY_ALLOCATION and instead allocates buffers itself.  For downstream elements, most notably decoders, that require the use of a specific GstAllocator this means some pipelines fail.

For example, assuming myaudiodecoder has a GstAllocator and cannot receive GstBuffers allocated by the default allocator:

 gst-launch-1.0 filesrc location=/usr/test.mp3 ! myaudiodecoder ! myaudiosink

works, i.e. filesrc allocates buffers using an allocator assigned by myaudiodecoder, but


 gst-launch-1.0 filesrc location=/usr/test.mp3 ! identity ! myaudiodecoder ! myaudiosink

fails because the 'identity' element (base class GstBaseTransform) fails to forward the GST_QUERY_ALLOCATION from filesrc (base class GstBaseSrc) to myaudiodecoder because filesrc never emitted a GST_EVENT_CAPS.