Element: Use cached buildtree in build shells and failure shells

This includes changes in app.py:
* Interactive failure shell no longer uses the failed build sysroot,
  defaulting to the cached build tree.

Changes in element.py are:
* Errors caused by building don't store the failed build sysroot,
  instead storing that a sandbox can be created to debug the error.
* When staging sources, will stage the element's cached build tree if it
  exists.

Changes in _exceptions.py:
* BstError.sandbox is now a flag of whether a sandbox can be opened up
  to debug the error.

Changes in widget.py:
* Don't try to print any information about the sandbox.

Changes in _message.py:
* Fix documentation so Message.sandbox is not a directory any more.

This is part of #539

5 files changed, 12 insertions, 21 deletions

diff --git a/buildstream/_exceptions.py b/buildstream/_exceptions.py
index 19606776e..a1c26d38c 100644
--- a/buildstream/_exceptions.py
+++ b/buildstream/_exceptions.py
@@ -111,10 +111,8 @@ class BstError(Exception):
         #
         self.detail = detail
 
-        # The build sandbox in which the error occurred, if the
-        # error occurred at element assembly time.
-        #
-        self.sandbox = None
+        # A sandbox can be created to debug this error
+        self.sandbox = False
 
         # When this exception occurred during the handling of a job, indicate
         # whether or not there is any point retrying the job.
diff --git a/buildstream/_frontend/app.py b/buildstream/_frontend/app.py
index eeb5f3eb2..87db8076a 100644
--- a/buildstream/_frontend/app.py
+++ b/buildstream/_frontend/app.py
@@ -597,7 +597,7 @@ class App():
                     click.echo("\nDropping into an interactive shell in the failed build sandbox\n", err=True)
                     try:
                         prompt = self.shell_prompt(element)
-                        self.stream.shell(element, Scope.BUILD, prompt, directory=failure.sandbox, isolate=True)
+                        self.stream.shell(element, Scope.BUILD, prompt, isolate=True)
                     except BstError as e:
                         click.echo("Error while attempting to create interactive shell: {}".format(e), err=True)
                 elif choice == 'log':
diff --git a/buildstream/_frontend/widget.py b/buildstream/_frontend/widget.py
index 478f0ff14..c0c45cec6 100644
--- a/buildstream/_frontend/widget.py
+++ b/buildstream/_frontend/widget.py
@@ -668,17 +668,6 @@ class LogLine(Widget):
 
             extra_nl = True
 
-        if message.sandbox is not None:
-            sandbox = self._indent + 'Sandbox directory: ' + message.sandbox
-
-            text += '\n'
-            if message.message_type == MessageType.FAIL:
-                text += self._err_profile.fmt(sandbox, bold=True)
-            else:
-                text += self._detail_profile.fmt(sandbox)
-            text += '\n'
-            extra_nl = True
-
         if message.scheduler and message.message_type == MessageType.FAIL:
             text += '\n'
 
diff --git a/buildstream/_message.py b/buildstream/_message.py
index 37630eb86..c2cdb8277 100644
--- a/buildstream/_message.py
+++ b/buildstream/_message.py
@@ -70,7 +70,7 @@ class Message():
         self.elapsed = elapsed            # The elapsed time, in timed messages
         self.depth = depth                # The depth of a timed message
         self.logfile = logfile            # The log file path where commands took place
-        self.sandbox = sandbox            # The sandbox directory where an error occurred (if any)
+        self.sandbox = sandbox            # The error that caused this message used a sandbox
         self.pid = os.getpid()            # The process pid
         self.unique_id = unique_id        # The plugin object ID issueing the message
         self.task_id = task_id            # The plugin object ID of the task
diff --git a/buildstream/element.py b/buildstream/element.py
index 848730529..83d7f9705 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -1385,6 +1385,7 @@ class Element(Plugin):
             # the same filing system as the rest of our cache.
             temp_staging_location = os.path.join(self._get_context().artifactdir, "staging_temp")
             temp_staging_directory = tempfile.mkdtemp(prefix=temp_staging_location)
+            import_dir = temp_staging_directory
 
             try:
                 workspace = self._get_workspace()
@@ -1395,12 +1396,16 @@ class Element(Plugin):
                         with self.timed_activity("Staging local files at {}"
                                                  .format(workspace.get_absolute_path())):
                             workspace.stage(temp_staging_directory)
+                elif self._cached():
+                    # We have a cached buildtree to use, instead
+                    artifact_base, _ = self.__extract()
+                    import_dir = os.path.join(artifact_base, 'buildtree')
                 else:
                     # No workspace, stage directly
                     for source in self.sources():
                         source._stage(temp_staging_directory)
 
-                vdirectory.import_files(temp_staging_directory)
+                vdirectory.import_files(import_dir)
 
             finally:
                 # Staging may produce directories with less than 'rwx' permissions
@@ -1566,9 +1571,8 @@ class Element(Plugin):
                     collect = self.assemble(sandbox)  # pylint: disable=assignment-from-no-return
                     self.__set_build_result(success=True, description="succeeded")
                 except BstError as e:
-                    # If an error occurred assembling an element in a sandbox,
-                    # then tack on the sandbox directory to the error
-                    e.sandbox = rootdir
+                    # Shelling into a sandbox is useful to debug this error
+                    e.sandbox = True
 
                     # If there is a workspace open on this element, it will have
                     # been mounted for sandbox invocations instead of being staged.