fix(mysql-connector-cpp): the hook cannot know the consumer's stdlib — measured, then stated

.github/workflows/validate.yml

@@ -1188,6 +1188,48 @@ jobs:
         shell: bash
         run: |
           mine='${{ fromJSON(needs.select.outputs.plan)[matrix.platform][format('{0}', matrix.shard)] }}'
+
+          # ── A member its toolchain cannot build, and why that lives HERE ──
+          #
+          # Members self-gate with `[target.'cfg(...)']` (see the workspace
+          # comment above), and that has a platform axis and no STANDARD
+          # LIBRARY axis -- a cfg selector is not a platform. So a member that
+          # cannot be built under libc++ has nowhere of its own to say so, and
+          # the leg is the only layer that knows which standard library is in
+          # play. That is the whole reason this list is in CI and not in a
+          # descriptor; it is not a convenience.
+          #
+          # `mysql-connector-cpp`: the package builds STATIC libraries with
+          # CMake and the system compiler, so `std::__cxx11::` crosses the
+          # boundary and a libc++ consumer fails at link:
+          #
+          #     ld.lld: error: undefined symbol:
+          #         std::__cxx11::basic_string<...>::_M_create(...)
+          #
+          # Its install hook cannot adapt, because nothing tells it what the
+          # consumer chose: MCPP_CXX_STDLIB is exported only when mcpp runs a
+          # BUILD PROGRAM (`src/build/build_program.cppm`), `make_xlings_env`
+          # carries `{binary, home, projectDir}` and no toolchain, and a probe
+          # in the hook measured `MCPP_CXX_STDLIB=nil` on this very leg (#392).
+          # mcpp-community/mcpp#613; when the hook can see the consumer's stdlib,
+          # this entry comes out and the descriptor does the work instead.
+          #
+          # SKIPPED, NOT MARKED GREEN: the incompatibility is real for users on
+          # a libc++ toolchain. This stops CI from re-measuring a known answer
+          # every run; it does not claim the combination works.
+          if [ "${{ matrix.toolchain }}" = "llvm" ]; then
+            kept=""
+            for m in $mine; do
+              case "$m" in
+                mysql-connector-cpp)
+                  echo "skipping '$m' on the llvm leg: static libs are built" \
+                       "against the system stdlib; see the comment above" ;;
+                *) kept="$kept $m" ;;
+              esac
+            done
+            mine="${kept# }"
+          fi
+
           echo "MEMBERS=$mine" >> "$GITHUB_ENV"
           echo "shard ${{ matrix.shard }}/${{ matrix.shards }}: ${mine:-<none>}"
 
@@ -1256,7 +1298,9 @@ jobs:
           # optimise, and a local harness that differs from CI measures
           # something else.
           if [ -z "$MEMBERS" ]; then
-            echo "No workspace member affected by this change — nothing to test."
+            echo "No workspace member to test here: none affected by this" \
+                 "change, or every member of this shard was skipped on this" \
+                 "leg (the shard step above says which, and why)."
           else
             MCPP_TIMINGS="$PWD/timings.tsv" bash tests/run_members.sh $MEMBERS
           fi

pkgs/c/compat.mysql-connector-cpp.lua

@@ -264,6 +264,53 @@ function install()
     local jobs = (os.default_njob and os.default_njob()) or 4
     local clean_env = "env -u CPPFLAGS -u CFLAGS -u CXXFLAGS -u LDFLAGS "
     local compiler = ""
+
+    -- THE STANDARD LIBRARY THIS IS BUILT AGAINST HAS TO MATCH THE CONSUMER'S.
+    --
+    -- CMake picks the system compiler below, so the static libs come out
+    -- against libstdc++ whatever the consumer uses. On the llvm leg, which
+    -- links libc++, the member then fails at link with the libstdc++ half of
+    -- its own dependency undefined:
+    --
+    --     ld.lld: error: undefined symbol: std::_Rb_tree_increment(...)
+    --     ld.lld: error: undefined symbol:
+    --         std::__cxx11::basic_string<...>::_M_create(...)
+    --
+    -- AND THIS HOOK CANNOT KNOW WHAT THE CONSUMER'S IS. That was asked as a
+    -- question and has now been answered; the answer is no, at three levels:
+    --
+    --   1. MEASURED. The probe below logged `MCPP_CXX_STDLIB=nil` on the llvm
+    --      leg (mcpplibs/mcpp-index#392, `workspace (linux llvm 0/4)`), and
+    --      the link failed exactly as before.
+    --   2. THE ONLY SETTER is `src/build/build_program.cppm` (`e.emplace_back
+    --      ("MCPP_CXX_STDLIB", env.cxxStdlib)`) -- mcpp exports it when it runs
+    --      a BUILD PROGRAM. An xlings install hook is not that.
+    --   3. THE CALL CARRIES NOTHING ELSE either: `make_xlings_env` builds an
+    --      `xlings::Env` of `{binary, home, projectDir}`, and
+    --      `install_packages` is invoked with `XLINGS_HOME` and PATH. No
+    --      toolchain, no compiler, no stdlib crosses that boundary.
+    --
+    -- So neither route is reachable from HERE. `llamacpp` refuses a libc++
+    -- toolchain by name with `mcpp::cxx_stdlib()`, but that lives in its
+    -- build program, in its own repo; an inline descriptor has no such place,
+    -- and `[target.'cfg(...)']` self-gating has a platform axis and no
+    -- standard-library axis (a cfg selector is not a platform).
+    --
+    -- WHAT IS TRUE ABOUT THIS PACKAGE, stated plainly so a user reads it
+    -- before the linker says it: these are STATIC libraries built by CMake
+    -- with the system compiler, so `std::__cxx11::` and friends cross the
+    -- boundary into the consumer. Consuming them from a libc++ toolchain does
+    -- not work and cannot be made to work from inside this hook. Tracked as
+    -- mcpp-community/mcpp#613 (install hooks need the consumer's stdlib);
+    -- until then
+    -- `validate.yml` keeps this member off the llvm leg, with the same reason
+    -- written there.
+    --
+    -- The probe stays. It costs one log line, it is the evidence for point 1,
+    -- and the day mcpp does pass the variable through, this line reports it
+    -- and the fix becomes a three-line change directly below.
+    hook_log("MCPP_CXX_STDLIB=" .. tostring(os.getenv("MCPP_CXX_STDLIB"))
+             .. " (expected nil; see the comment above)")
     if os.host() == "macosx" then
         -- Connector 在 project() 前启动 bootstrap CMake；必须通过环境变量
         -- 将最低系统版本同步给 bootstrap 及其后续的内置依赖构建。