test(e2e): 267 —— MCPP_TARGET_REQUESTED 的契约,以及它为什么不是 MCPP_TARGET

MCPP_TARGET 在没人指名目标时用宿主填上,对「这是给哪台机器的」是对的,而对平台包
必须问的另一个问题无用:**这次构建有没有被指向一个目标**。

⚠️ 这个问题的两种读法都已经在 openkal-macos 上被实测判错过:
· 用宿主读 → 交叉对,而 Mac 上 --target aarch64-macos 得到 library not found for -lSystem
· 用 MCPP_TARGET 读 → 交叉对,本机构建得到 undefined symbol: wcslen(它从不为空,
  于是包里三个名字的 stub 遮蔽了厂商完整的那份)

断言:本机构建为空;指名目标时携带被指名的三元组。

⚠️ 探针以**非零退出**汇报 —— mcpp 只打印失败的构建程序的输出,返回零的探针输出会被
丢弃,那样这个测试什么都不断言。
⚠️ 同一行上带阳性对照(MCPP_TARGET 必须被填上):没有它,mcpp 若停止设置全部变量,
上面那条断言同样会通过。

注入核验:把 MCPP_TARGET_REQUESTED 也改成「空则填宿主」⇒ 测试报
'should be empty for a native build' 并退非零。

Author: speak-agent

tests/e2e/267_target_requested_is_empty_for_a_native_build.sh

@@ -0,0 +1,99 @@
+#!/usr/bin/env bash
+# requires: unix-shell
+# `MCPP_TARGET_REQUESTED` — the value a build program needs and `MCPP_TARGET`
+# cannot give it.
+#
+# ⭐⭐ WHY THE TWO VARIABLES ARE NOT THE SAME QUESTION.
+#
+# `MCPP_TARGET` answers "which machine is this for", and it is filled in with
+# the host when nobody named a target — which is right for that question and
+# makes it useless for a different one a platform package has to ask: **was this
+# build POINTED at a target**, or is it an ordinary native build?
+#
+# The two differ even when the triples are equal. `mcpp build --target
+# aarch64-macos` on an arm64 Mac names the very machine the host is, and yet it
+# is the dependency graph that supplies the target side — so mcpp puts no system
+# SDK on the link, and the package that knows the system is the only thing that
+# can name one. A native build on the same machine gets the SDK and needs
+# nothing from the package.
+#
+# ⚠️ Both readings of that question have already been measured wrong in
+# `openkal-macos`:
+#
+#   from the host  → right for the cross, and `library not found for -lSystem`
+#                    for `--target aarch64-macos` ON a Mac
+#   from MCPP_TARGET → right for the cross, and `undefined symbol: wcslen` for
+#                    the native build, because it is never empty and the
+#                    package's three-name stub shadowed the vendor's complete one
+#
+# ⇒ The assertions below are the contract those two attempts needed: EMPTY for a
+# native build, and the requested triple otherwise.
+#
+# ⚠️ `"$MCPP"`, never a bare `mcpp`: the harness passes the binary under test,
+# and a bare name resolves through PATH to whichever engine is installed.
+set -e
+
+TMP=$(mktemp -d)
+trap "rm -rf $TMP" EXIT
+cd "$TMP"
+
+mkdir -p src
+cat > mcpp.toml <<'EOF'
+[package]
+name    = "target-requested-probe"
+version = "0.1.0"
+EOF
+printf 'int main() { return 0; }\n' > src/main.cpp
+
+# ⚠️ The build program reports through a NON-ZERO exit, because mcpp prints what
+# it captured only from a program that failed. A probe that returned zero would
+# have its output discarded and this test would assert nothing.
+cat > build.mcpp <<'EOF'
+import mcpp;
+import std;
+int main() {
+    const char* t = std::getenv("MCPP_TARGET");
+    const char* r = std::getenv("MCPP_TARGET_REQUESTED");
+    std::cerr << "PROBE target=[" << (t ? t : "") << "] "
+              << "requested=[" << (r ? r : "") << "]\n";
+    return 1;
+}
+EOF
+
+# ── A native build: nobody named a target ──────────────────────────────────
+native=$("$MCPP" build 2>&1 || true)
+echo "$native" | grep -q 'PROBE ' || {
+    echo "the build program did not report; mcpp prints a failing program's output" >&2
+    echo "$native" >&2
+    exit 1
+}
+echo "$native" | grep -qE 'requested=\[\]' || {
+    echo "MCPP_TARGET_REQUESTED should be empty for a native build" >&2
+    echo "$native" | grep 'PROBE ' >&2
+    exit 1
+}
+# ⚠️ And the positive control on the same line: `MCPP_TARGET` must be filled in.
+# Without this the assertion above would also pass if mcpp had stopped setting
+# any of them.
+echo "$native" | grep -qE 'target=\[[a-z0-9_]+-[a-z0-9-]+\]' || {
+    echo "MCPP_TARGET should carry the host triple for a native build" >&2
+    echo "$native" | grep 'PROBE ' >&2
+    exit 1
+}
+
+# ── A build pointed at a target ────────────────────────────────────────────
+# x86_64-linux-musl is chosen because every host in this suite can resolve it
+# without a payload download beyond what the other cross tests already use.
+cross=$("$MCPP" build --target x86_64-linux-musl 2>&1 || true)
+echo "$cross" | grep -q 'PROBE ' || {
+    echo "the build program did not report on the cross build" >&2
+    echo "$cross" >&2
+    exit 1
+}
+echo "$cross" | grep -qE 'requested=\[x86_64-linux-musl\]' || {
+    echo "MCPP_TARGET_REQUESTED should carry the triple that was named" >&2
+    echo "$cross" | grep 'PROBE ' >&2
+    exit 1
+}
+
+echo "MCPP_TARGET_REQUESTED distinguishes a native build from a named target"