osbuild: add a mutex to the _jsonseq() writer

This commit fixes a race/threading issue with the way the monitor
works. The osbuild monitor can be called from multiple threads,
e.g. in buildroot.py:run() monitor.log() is called but also
in host.py:_stdout_ready(). This can lead to out-of-order writes
when many messages need to be processed.

We did not notice this so far because we were lucky and also
log was just used for information. But now it is used to transmit
the jsonseq data which means out-of-order communication results
in broken json.

Closes: https://github.com/osbuild/image-builder-cli/issues/110

osbuild/monitor.py

@@ -16,6 +16,7 @@ import json
 import os
 import sys
 import time
+from threading import Lock
 from typing import Dict, Optional, Set, Union
 
 import osbuild
@@ -234,6 +235,7 @@ class BaseMonitor(abc.ABC):
     def result(self, result: Union[BuildResult, DownloadResult]) -> None:
         """Called when a module (stage/assembler) is done with its result"""
 
+    # note that this should be re-entrant
     def log(self, message: str, origin: Optional[str] = None):
         """Called for all module log outputs"""
 
@@ -316,6 +318,7 @@ class JSONSeqMonitor(BaseMonitor):
         self._ctx_ids: Set[str] = set()
         self._progress = Progress("pipelines/sources", total_steps)
         self._context = Context(origin="org.osbuild")
+        self._jsonseq_mu = Lock()
 
     def begin(self, pipeline: osbuild.Pipeline):
         self._context.set_pipeline(pipeline)
@@ -362,10 +365,11 @@ class JSONSeqMonitor(BaseMonitor):
         ))
 
     def _jsonseq(self, entry: dict) -> None:
-        # follow rfc7464 (application/json-seq)
-        self.out.write("\x1e")
-        json.dump(entry, self.out)
-        self.out.write("\n")
+        with self._jsonseq_mu:
+            # follow rfc7464 (application/json-seq)
+            self.out.write("\x1e")
+            json.dump(entry, self.out)
+            self.out.write("\n")
 
 
 def make(name: str, fd: int, total_steps: int) -> BaseMonitor: