pipeline: return logs in --json mode

A pipeline run only returned logs in the `StageFailed` and
`AssemblerFailed` exceptions. Remove those and always return structured
data instead.

It only returns data for stages that actually ran (i.e., didn't come
from the cache). This is similar to the output in interactive mode.

Also change osbuildtest to be able to deal with output that is larger
than the pipe buffer by using subprocess.communicate().

osbuild/__main__.py

@@ -38,29 +38,24 @@ def main():
         pipeline.prepend_build_env(build_pipeline, runner)
 
     try:
-        pipeline.run(args.store, interactive=not args.json, libdir=args.libdir)
+        r = pipeline.run(args.store, interactive=not args.json, libdir=args.libdir)
     except KeyboardInterrupt:
         print()
         print(f"{RESET}{BOLD}{RED}Aborted{RESET}")
         return 130
-    except (osbuild.StageFailed, osbuild.AssemblerFailed) as error:
-        print()
-        print(f"{RESET}{BOLD}{RED}{error.name} failed with code {error.returncode}{RESET}")
-        if args.json:
-            print(error.output)
-        return 1
 
     if args.json:
-        json.dump({
-            "tree_id": pipeline.tree_id,
-            "output_id": pipeline.output_id,
-        }, sys.stdout)
+        json.dump(r, sys.stdout)
         sys.stdout.write("\n")
     else:
-        print("tree id:", pipeline.tree_id)
-        print("output id:", pipeline.output_id)
+        if r["success"]:
+            print("tree id:", pipeline.tree_id)
+            print("output id:", pipeline.output_id)
+        else:
+            print()
+            print(f"{RESET}{BOLD}{RED}Failed{RESET}")
 
-    return 0
+    return 0 if r["success"] else 1
 
 
 if __name__ == "__main__":