describe: also describe "configure" targets in more detail

Most built-in rules are just described by their name, assuming they
are known to the user anyway. One exception, however, are "export"
targets, as those serve as interfaces to (logical) repositories and
hence is supposed to have a documentation of the target itself. Over
time, however, "configure" targets have evolved to become internal
interfaces (maybe even for an external target), e.g., to provide
default values and hence make better use of the actual export target.
Therefore it is desirable to provide a bit more information when
asked to describe a "configure" target.
- As "configure" targets have a fixed set of keys, adding a "doc"
  field is a conservative extension; this can be useful to give an
  overview what the target is about.
- An important information of a "configure" target is the target
  that is configured. While this, in general, is an expression, in
  the typical cases, the description is very short (a literal target
  name, or a variable). So we can afford to show the definition.

5 files changed, 123 insertions, 0 deletions

diff --git a/doc/concepts/doc-strings.md b/doc/concepts/doc-strings.md
index ed4ee84b..22115cee 100644
--- a/doc/concepts/doc-strings.md
+++ b/doc/concepts/doc-strings.md
@@ -131,6 +131,16 @@ strings describing the targeted in general and `"config_doc"` is a map
 from (some of) the variables of the `"flexible_config"` to an array of
 strings describing this parameter.
 
+Configure targets
+-----------------
+
+As configure targets often serve as internal interface to external
+export targets (e.g., in order to set a needed configuration), we
+support documentation here as well. As configure targets, being
+built-in, have a fixed set of fields, a `"doc"` field can be used
+for this purpose without conflicts. Again, the `"doc"` field is an
+array of strings describing the target in general.
+
 Presentation of the documentation
 ---------------------------------
 
diff --git a/src/buildtool/build_engine/target_map/built_in_rules.cpp b/src/buildtool/build_engine/target_map/built_in_rules.cpp
index 7d858840..81f679e5 100644
--- a/src/buildtool/build_engine/target_map/built_in_rules.cpp
+++ b/src/buildtool/build_engine/target_map/built_in_rules.cpp
@@ -76,6 +76,7 @@ auto const kInstallRuleFields =
 auto const kConfigureRuleFields =
     std::unordered_set<std::string>{"arguments_config",
                                     "config",
+                                    "doc",
                                     "tainted",
                                     "target",
                                     "type"};
diff --git a/src/buildtool/main/describe.cpp b/src/buildtool/main/describe.cpp
index 4739ba88..e3ab1223 100644
--- a/src/buildtool/main/describe.cpp
+++ b/src/buildtool/main/describe.cpp
@@ -431,6 +431,17 @@ auto DescribeTarget(BuildMaps::Target::ConfiguredTarget const& id,
                 PrintFields(*flexible_config, config_doc, " - ", "   | ");
             }
         }
+        else if (*rule_it == "configure") {
+            auto target = desc.find("target");
+            auto doc = desc.find("doc");
+            if (doc != desc.end()) {
+                PrintDoc(*doc, " | ");
+            }
+            if (target != desc.end()) {
+                std::cout << "The target to be configured is defined as "
+                          << target->dump() << "." << std::endl;
+            }
+        }
         return kExitSuccess;
     }
     auto rule_name = BuildMaps::Base::ParseEntityNameFromJson(
diff --git a/test/end-to-end/cli/TARGETS b/test/end-to-end/cli/TARGETS
index 03f1db49..f7237f28 100644
--- a/test/end-to-end/cli/TARGETS
+++ b/test/end-to-end/cli/TARGETS
@@ -60,6 +60,12 @@
   , "test": ["log-limit.sh"]
   , "deps": [["", "tool-under-test"], ["", "mr-tool-under-test"]]
   }
+, "describe":
+  { "type": ["@", "rules", "shell/test", "script"]
+  , "name": ["describe"]
+  , "test": ["describe.sh"]
+  , "deps": [["", "tool-under-test"]]
+  }
 , "TESTS":
   { "type": "install"
   , "tainted": ["test"]
@@ -75,6 +81,7 @@
         , "git cas -P"
         , "install --archive"
         , "conflict report"
+        , "describe"
         ]
       , { "type": "if"
         , "cond": {"type": "var", "name": "TEST_BOOTSTRAP_JUST_MR"}
diff --git a/test/end-to-end/cli/describe.sh b/test/end-to-end/cli/describe.sh
new file mode 100644
index 00000000..378ee4d2
--- /dev/null
+++ b/test/end-to-end/cli/describe.sh
@@ -0,0 +1,94 @@
+#!/bin/sh
+# Copyright 2024 Huawei Cloud Computing Technology Co., Ltd.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -eu
+
+ROOT="$(pwd)"
+JUST="${ROOT}/bin/tool-under-test"
+LBR="${TEST_TMPDIR}/build-root"
+OUT="${TEST_TMPDIR}/output"
+
+mkdir work
+cd work
+touch ROOT
+
+cat > TARGETS <<'EOF'
+{ "payload":
+  { "type": "file_gen"
+  , "arguments_config": ["NAME"]
+  , "name": "greeting.txt"
+  , "data":
+    {"type": "join", "$1": ["Hello ", {"type": "var", "name": "NAME"}, "!"]}
+  }
+, "exported-target":
+  { "type": "export"
+  , "doc": ["Here we export the canonical GREETING text"]
+  , "target": "payload"
+  , "flexible_config": ["NAME"]
+  , "config_doc": {"NAME": ["The RECIPIENT of the greeting"]}
+  }
+, "":
+  { "type": "configure"
+  , "doc": ["Configure the canonical greeting to set a DEFAULT name"]
+  , "arguments_config": ["NAME"]
+  , "target": "exported-target"
+  , "config":
+    { "type": "singleton_map"
+    , "key": "NAME"
+    , "value": {"type": "var", "name": "NAME", "default": "world"}
+    }
+  }
+}
+EOF
+
+# Sanity check: the target description actually works
+"${JUST}" install --local-build-root "${LBR}" -o "${OUT}" 2>&1
+grep 'Hello world' "${OUT}/greeting.txt"
+
+# Verify the description of the export target
+"${JUST}" describe --local-build-root "${LBR}" exported-target \
+          > "${OUT}/export.doc" 2> "${OUT}/log"
+echo
+cat "${OUT}/log"
+echo
+cat "${OUT}/export.doc"
+echo
+echo
+# - rule name
+grep '"export"' "${OUT}/export.doc"
+# - top-level description
+grep GREETING "${OUT}/export.doc"
+# - flexible config
+grep NAME "${OUT}/export.doc"
+# - documentation of variable
+grep RECIPIENT "${OUT}/export.doc"
+
+# Verify the description of the configure target
+"${JUST}" describe --local-build-root "${LBR}" \
+          > "${OUT}/configure.doc" 2> "${OUT}/log"
+echo
+cat "${OUT}/log"
+echo
+cat "${OUT}/configure.doc"
+echo
+echo
+# - rule name
+grep '"configure"' "${OUT}/configure.doc"
+# - top-level description
+grep DEFAULT "${OUT}/configure.doc"
+# - the expression defining what to configure
+grep '"exported-target"' "${OUT}/configure.doc"
+
+echo OK