Feature: Add support for generating json from bst show command

This adds a new flag called --json to the `bst show` command which when
specified will make bst output a machine readable JSON dump of the dependency
state within the current buildstream project. Having JSON support makes it
easier to write tools that need to inspect the configuration of a buildstream
enabled project.

Author: kevinschoon

src/buildstream/_frontend/cli.py

@@ -14,6 +14,7 @@
 import os
 import re
 import sys
+import json
 from functools import partial
 
 import shutil
@@ -434,7 +435,6 @@ def init(app, project_name, min_version, element_path, force, target_directory):
     """
     app.init_project(project_name, min_version, element_path, force, target_directory)
 
-
 ##################################################################
 #                          Build Command                         #
 ##################################################################
@@ -538,6 +538,7 @@ def build(
 @click.option(
     "--except", "except_", multiple=True, type=click.Path(readable=False), help="Except certain dependencies"
 )
+@click.option("--json", "as_json", is_flag=True, help="Output the information as machine readable JSON")
 @click.option(
     "--deps",
     "-d",
@@ -572,7 +573,7 @@ def build(
 )
 @click.argument("elements", nargs=-1, type=click.Path(readable=False))
 @click.pass_obj
-def show(app, elements, deps, except_, order, format_):
+def show(app, elements, deps, as_json, except_, order, format_):
     """Show elements in the pipeline
 
     Specifying no elements will result in showing the default targets
@@ -657,8 +658,12 @@ def show(app, elements, deps, except_, order, format_):
         if order == "alpha":
             dependencies = sorted(dependencies)
 
-        report = app.logger.show_pipeline(dependencies, format_)
-        click.echo(report)
+        if as_json:
+            serialized = app.logger.dump_pipeline(dependencies)
+            print(json.dumps(serialized))
+        else:
+            report = app.logger.show_pipeline(dependencies, format_)
+            click.echo(report)
 
 
 ##################################################################

src/buildstream/_frontend/widget.py

@@ -327,6 +327,55 @@ def __init__(
         logfile_tokens = self._parse_logfile_format(context.log_message_format, content_profile, format_profile)
         self._columns.extend(logfile_tokens)
 
+    # Dump the pipeline as a serializable object
+    def dump_pipeline(self, dependencies):
+
+        elements = []
+
+        for element in dependencies:
+
+            deps = [e._get_full_name() for e in element._dependencies(_Scope.ALL, recurse=False)]
+            build_deps = [e._get_full_name() for e in element._dependencies(_Scope.BUILD, recurse=False)]
+            runtime_deps = [e._get_full_name() for e in element._dependencies(_Scope.RUN, recurse=False)]
+
+            # Artifact CAS Digest
+            serialized_artifact = None
+            artifact = element._get_artifact()
+            if artifact.cached():
+                serialized_artifact = {"files": artifact.get_files(), "digest": artifact_files._get_digest()}
+            all_source_infos = []
+            for source in element.sources():
+                source_infos = source.collect_source_info()
+
+                if source_infos is not None:
+                    serialized_sources = []
+                    for s in source_infos:
+                        serialized = s.serialize()
+                        serialized_sources.append(serialized)
+
+                    all_source_infos += serialized_sources
+
+            # assert element._has_all_sources_resolved() == True
+            description = " ".join(element._description.splitlines())
+            serialized_element = {
+                "name": element._get_full_name(),
+                "description": description,
+                "key": element._get_display_key().brief,
+                "key_full": element._get_display_key().full,
+                "workspace": element._get_workspace(),
+                "dependencies": deps,
+                "build-dependencies": build_deps,
+                "runtime-dependencies": runtime_deps,
+                "variables": dict(element._Element__variables),
+                "environment": dict(element._Element__environment),
+                "sources": all_source_infos,
+                "artifact": serialized_artifact,
+            }
+
+            elements.append(serialized_element)
+
+        return elements
+
     # show_pipeline()
     #
     # Display a list of elements in the specified format.