Adding _DisplayKey type

Instead of passing around untyped tuples for cache keys, lets have
a clearly typed object for this.

This makes for more readable code, and additionally corrects the
data model statement of intent that some cache keys should be displayed
as "dim", instead informing the frontend about whether the cache key
is "strict" or not, allowing the frontend to decide how to display
a strict or non-strict key.

This patch does the following:

  * types.py: Add _DisplayKey

  * element.py: Return a _DisplayKey from Element._get_display_key()

  * Other sources: Updated to use the display key object

6 files changed, 39 insertions, 22 deletions

diff --git a/src/buildstream/_frontend/app.py b/src/buildstream/_frontend/app.py
index 59bc3513b..2f4235733 100644
--- a/src/buildstream/_frontend/app.py
+++ b/src/buildstream/_frontend/app.py
@@ -454,13 +454,13 @@ class App:
     def shell_prompt(self, element):
 
         element_name = element._get_full_name()
-
-        _, key, dim = element._get_display_key()
+        display_key = element._get_display_key()
 
         if self.colors:
+            dim_key = not display_key.strict
             prompt = (
                 self._format_profile.fmt("[")
-                + self._content_profile.fmt(key, dim=dim)
+                + self._content_profile.fmt(display_key.brief, dim=dim_key)
                 + self._format_profile.fmt("@")
                 + self._content_profile.fmt(element_name)
                 + self._format_profile.fmt(":")
@@ -469,7 +469,7 @@ class App:
                 + " "
             )
         else:
-            prompt = "[{}@{}:${{PWD}}]$ ".format(key, element_name)
+            prompt = "[{}@{}:${{PWD}}]$ ".format(display_key.brief, element_name)
 
         return prompt
 
diff --git a/src/buildstream/_frontend/widget.py b/src/buildstream/_frontend/widget.py
index 65f74d475..7b2ff2688 100644
--- a/src/buildstream/_frontend/widget.py
+++ b/src/buildstream/_frontend/widget.py
@@ -209,15 +209,16 @@ class CacheKey(Widget):
         if message.element_name is None:
             return " " * self._key_length
 
-        missing = False
+        dim = False
         key = " " * self._key_length
         if message.element_key:
-            _, key, missing = message.element_key
+            key = message.element_key.brief
+            dim = not message.element_key.strict
 
         if message.message_type in ERROR_MESSAGES:
             text = self._err_profile.fmt(key)
         else:
-            text = self.content_profile.fmt(key, dim=missing)
+            text = self.content_profile.fmt(key, dim=dim)
 
         return text
 
@@ -340,11 +341,12 @@ class LogLine(Widget):
         for element in dependencies:
             line = format_
 
-            full_key, cache_key, dim_keys = element._get_display_key()
+            key = element._get_display_key()
+            dim_keys = not key.strict
 
             line = p.fmt_subst(line, "name", element._get_full_name(), fg="blue", bold=True)
-            line = p.fmt_subst(line, "key", cache_key, fg="yellow", dim=dim_keys)
-            line = p.fmt_subst(line, "full-key", full_key, fg="yellow", dim=dim_keys)
+            line = p.fmt_subst(line, "key", key.brief, fg="yellow", dim=dim_keys)
+            line = p.fmt_subst(line, "full-key", key.full, fg="yellow", dim=dim_keys)
 
             try:
                 if not element._has_all_sources_resolved():
diff --git a/src/buildstream/_scheduler/jobs/job.py b/src/buildstream/_scheduler/jobs/job.py
index c8ff853ed..388fe0f5d 100644
--- a/src/buildstream/_scheduler/jobs/job.py
+++ b/src/buildstream/_scheduler/jobs/job.py
@@ -289,7 +289,7 @@ class Job:
     # key for for the issuing message (if an element is related to the Job).
     #
     # Args:
-    #     element_key (tuple): The element_key tuple to be supplied to the Message() constructor
+    #     element_key (_DisplayKey): The element_key tuple to be supplied to the Message() constructor
     #
     def set_message_element_key(self, element_key):
         self._message_element_key = element_key
diff --git a/src/buildstream/_scheduler/queues/queue.py b/src/buildstream/_scheduler/queues/queue.py
index 9e444b393..f5aa2ca4c 100644
--- a/src/buildstream/_scheduler/queues/queue.py
+++ b/src/buildstream/_scheduler/queues/queue.py
@@ -356,9 +356,9 @@ class Queue:
 
     def _element_log_path(self, element):
         project = element._get_project()
-        key = element._get_display_key()[1]
+        key = element._get_display_key()
         action = self.action_name.lower()
-        logfile = "{key}-{action}".format(key=key, action=action)
+        logfile = "{key}-{action}".format(key=key.brief, action=action)
 
         return os.path.join(project.name, element.normal_name, logfile)
 
diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index c9480d4fc..9d108c878 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -102,7 +102,7 @@ from .plugin import Plugin
 from .sandbox import SandboxFlags, SandboxCommandError
 from .sandbox._config import SandboxConfig
 from .sandbox._sandboxremote import SandboxRemote
-from .types import _Scope, _CacheBuildTrees, _KeyStrength, OverlapAction
+from .types import _Scope, _CacheBuildTrees, _KeyStrength, OverlapAction, _DisplayKey
 from ._artifact import Artifact
 from ._elementproxy import ElementProxy
 from ._elementsources import ElementSources
@@ -1398,15 +1398,13 @@ class Element(Plugin):
     # Returns cache keys for display purposes
     #
     # Returns:
-    #    (str): A full hex digest cache key for this Element
-    #    (str): An abbreviated hex digest cache key for this Element
-    #    (bool): True if key should be shown as dim, False otherwise
+    #    (_DisplayKey): The display key
     #
     # Question marks are returned if information for the cache key is missing.
     #
     def _get_display_key(self):
         context = self._get_context()
-        dim_key = True
+        strict = False
 
         cache_key = self._get_cache_key()
 
@@ -1415,10 +1413,10 @@ class Element(Plugin):
         elif cache_key == self.__strict_cache_key:
             # Strong cache key used in this session matches cache key
             # that would be used in strict build mode
-            dim_key = False
+            strict = True
 
         length = min(len(cache_key), context.log_key_length)
-        return (cache_key, cache_key[0:length], dim_key)
+        return _DisplayKey(cache_key, cache_key[0:length], strict)
 
     # _get_brief_display_key()
     #
@@ -1430,8 +1428,7 @@ class Element(Plugin):
     # Question marks are returned if information for the cache key is missing.
     #
     def _get_brief_display_key(self):
-        _, display_key, _ = self._get_display_key()
-        return display_key
+        return self._get_display_key().brief
 
     # _tracking_done():
     #
diff --git a/src/buildstream/types.py b/src/buildstream/types.py
index a5cf7afb8..67383f86a 100644
--- a/src/buildstream/types.py
+++ b/src/buildstream/types.py
@@ -208,6 +208,24 @@ class _KeyStrength(FastEnum):
     WEAK = 2
 
 
+# _DisplayKey():
+#
+# The components of a cache key which need to be displayed
+#
+# This is a part of Message() so it needs to be a simple serializable object.
+#
+# Args:
+#    full: A full hex digest cache key for an Element
+#    brief: An abbreviated hex digest cache key for an Element
+#    strict: Whether the key matches the key which would be used in strict mode
+#
+class _DisplayKey:
+    def __init__(self, full: str, brief: str, strict: bool):
+        self.full = full  # type: str
+        self.brief = brief  # type: str
+        self.strict = strict  # type: bool
+
+
 # _SchedulerErrorAction()
 #
 # Actions the scheduler can take on error