_frontend/widget: Allow showing the CAS digest

This adds the cas-digest format string to the show command.

man/bst-show.1

@@ -45,6 +45,7 @@ Show elements in the pipeline
         %{deps}           A list of all dependencies
         %{build-deps}     A list of build dependencies
         %{runtime-deps}   A list of runtime dependencies
+        %{cas-digest}     The CAS digest
 .PP
     The value of the %{symbol} without the leading '%' character is understood
     as a pythonic formatting string, so python formatting features apply,

src/buildstream/_frontend/cli.py

@@ -613,6 +613,7 @@ def show(app, elements, deps, except_, order, format_):
         %{deps}           A list of all dependencies
         %{build-deps}     A list of build dependencies
         %{runtime-deps}   A list of runtime dependencies
+        %{cas-digest}     The CAS digest
 
     The value of the %{symbol} without the leading '%' character is understood
     as a pythonic formatting string, so python formatting features apply,
@@ -638,7 +639,8 @@ def show(app, elements, deps, except_, order, format_):
         state_match = re.search(r"%(\{(state)[^%]*?\})", format_)
         key_match = re.search(r"%(\{(key)[^%]*?\})", format_)
         full_key_match = re.search(r"%(\{(full-key)[^%]*?\})", format_)
-        need_state = bool(state_match or key_match or full_key_match)
+        cas_digest_match = re.search(r"%(\{(cas-digest)[^%]*?\})", format_)
+        need_state = bool(state_match or key_match or full_key_match or cas_digest_match)
 
         if not elements:
             elements = app.project.get_default_targets()

src/buildstream/_frontend/widget.py

@@ -437,6 +437,14 @@ def show_pipeline(self, dependencies, format_):
                 runtime_deps = [e._get_full_name() for e in element._dependencies(_Scope.RUN, recurse=False)]
                 line = p.fmt_subst(line, "runtime-deps", _yaml.roundtrip_dump_string(runtime_deps).rstrip("\n"))
 
+            # CAS Digest
+            if "%{cas-digest" in format_:
+                cas_digest = element._get_cas_digest()
+                if cas_digest is not None:
+                    line = p.fmt_subst(line, "cas-digest", cas_digest)
+                else:
+                    line = p.fmt_subst(line, "cas-digest", "(no CAS digest)", fg="yellow")
+
             report += line + "\n"
 
         return report.rstrip("\n")

src/buildstream/element.py

@@ -1272,6 +1272,22 @@ def _get_cache_key(self, strength=_KeyStrength.STRONG):
         else:
             return self.__weak_cache_key
 
+    # _get_cas_digest():
+    #
+    # Returns the CAS digest
+    #
+    # Returns:
+    #    (str|None): The CAS digest for this Element, or None
+    #
+    # None is returned if no CAS artifact is available for this element.
+    #
+    def _get_cas_digest(self):
+        if self.__artifact is None:
+            return None
+        if not self.__artifact.query_cache():
+            return None
+        return self.__artifact.get_files().get_cas_digest()
+
     # _can_query_cache():
     #
     # Returns whether the cache key required for cache queries is available.

src/buildstream/storage/_casbaseddirectory.py

@@ -529,6 +529,10 @@ def is_dir_in(entry: _IndexEntry, directory: CasBasedDirectory) -> bool:
 
         self.__invalidate_digest()
 
+    def get_cas_digest(self):
+        digest = self._get_digest()
+        return "{}/{}".format(digest.hash, digest.size_bytes)
+
     #############################################################
     #                      Private methods                      #
     #############################################################