Use absolute form of export path when exporting pipelines (elyra-ai#690)

Because the notebook server's current working directory
doesn't always match the notebook directory, paths relative
to the notebook directory are not valid relative to the current
working directory causing things like pipeline export to break. 

This change leverages the change made in elyra-ai#673 to compute
an absolute path for the export's output.

Fixes elyra-ai#689

elyra/pipeline/processor.py

@@ -151,3 +151,12 @@ def process(self, pipeline) -> PipelineProcessorResponse:
     @abstractmethod
     def export(self, pipeline, pipeline_export_format, pipeline_export_path, overwrite):
         raise NotImplementedError()
+
+    def get_absolute_path(self, path):
+        """Checks if path is absolute or not.  If not absolute, `path` is appended to `root_dir`. """
+
+        absolute_path = path
+        if not os.path.isabs(path):
+            absolute_path = os.path.join(self.root_dir, path)
+
+        return absolute_path

elyra/pipeline/processor_kfp.py

@@ -91,20 +91,24 @@ def export(self, pipeline, pipeline_export_format, pipeline_export_path, overwri
 
         pipeline_name = pipeline.name
 
+        # Since pipeline_export_path may be relative to the notebook directory, ensure
+        # we're using its absolute form.
+        absolute_pipeline_export_path = self.get_absolute_path(pipeline_export_path)
+
         runtime_configuration = self._get_runtime_configuration(pipeline.runtime_config)
         api_endpoint = runtime_configuration.metadata['api_endpoint']
 
-        if os.path.exists(pipeline_export_path) and not overwrite:
-            raise ValueError("File " + pipeline_export_path + " already exists.")
+        if os.path.exists(absolute_pipeline_export_path) and not overwrite:
+            raise ValueError("File " + absolute_pipeline_export_path + " already exists.")
 
         self.log.info('Creating pipeline definition as a .' + pipeline_export_format + ' file')
         if pipeline_export_format != "py":
             try:
                 pipeline_function = lambda: self._cc_pipeline(pipeline, pipeline_name)  # nopep8
-                kfp.compiler.Compiler().compile(pipeline_function, pipeline_export_path)
+                kfp.compiler.Compiler().compile(pipeline_function, absolute_pipeline_export_path)
             except Exception as ex:
                 raise RuntimeError('Error compiling pipeline {} for export at {}'.
-                                   format(pipeline_name, pipeline_export_path), str(ex)) from ex
+                                   format(pipeline_name, absolute_pipeline_export_path), str(ex)) from ex
         else:
             # Load template from installed elyra package
             loader = PackageLoader('elyra', 'templates')
@@ -129,10 +133,10 @@ def export(self, pipeline, pipeline_export_format, pipeline_export_path, overwri
                                             pipeline_description="Elyra Pipeline")
 
             # Write to python file and fix formatting
-            with open(pipeline_export_path, "w") as fh:
+            with open(absolute_pipeline_export_path, "w") as fh:
                 fh.write(autopep8.fix_code(python_output))
 
-        return pipeline_export_path
+        return pipeline_export_path  # Return the input value, not its absolute form
 
     def _cc_pipeline(self, pipeline, pipeline_name):