compat.expat 改为静态:host tool 链接它才能真的跑起来 (#291)

compat.expat was shared by analogy with compat.libdrm. The analogy does not
hold, and the shared form is actively broken for its only consumer.

WHY LIBDRM IS SHARED AND EXPAT NEED NOT BE. libdrm must be one mapping because
Mesa's payload has DT_NEEDED on libdrm.so.2 and the library keeps mutable
file-static state — drmHashTable, nr_fds, connection — over a shared set of
fds, so a second copy is a split ledger. Expat has no equivalent: parser state
hangs off the XML_Parser the caller owns.

WHY SHARED IS BROKEN HERE. The only consumer is freedesktop.wayland-scanner, a
 HOST TOOL that mcpp builds in a sub-build and then runs during
another package's build.mcpp. A host tool linking a shared dependency comes out
with a DT_NEEDED nothing satisfies — mcpp does not stage the .so beside the
tool, and the sub-build's bin/ holds the executable alone:

    wayland-scanner: error while loading shared libraries:
    libexpat.so.1: cannot open shared object file

Reported as mcpp-community/mcpp#535.

The failure only reproduces without a graphics stack, which is why it reached
CI rather than being caught here: with Mesa installed the tool's RPATH reaches
<registry>/subos/default/lib and binds to xim:expat's copy instead — a
DIFFERENT library, silently succeeding. Static removes the question.

The test's dladdr check keeps its meaning: with objects merged it reports the
executable, and what it asserts is unchanged — that the code which just parsed
did not come from the ecosystem's copy.

Co-authored-by: Sunrisepeak <x.d2learn.org@gmail.com>

Author: Sunrisepeak

pkgs/c/compat.expat.lua

@@ -86,7 +86,33 @@ package = {
             "lib/xmlrole.c",
         },
 
-        targets = { ["expat"] = { kind = "shared", soname = "libexpat.so.1" } },
+        -- STATIC, unlike compat.libdrm's shared build, and the difference is
+        -- not an oversight.
+        --
+        -- libdrm is shared because Mesa's payload has DT_NEEDED on
+        -- `libdrm.so.2` and the two must be ONE mapping — libdrm keeps mutable
+        -- file-static state (`drmHashTable`, `nr_fds`, `connection`) over a
+        -- shared set of fds, so a second copy is a split ledger. Expat has no
+        -- equivalent: every bit of parser state hangs off the XML_Parser the
+        -- caller owns, so a consumer that merges these objects while Mesa loads
+        -- the payload's libexpat.so.1 is not sharing anything to corrupt.
+        --
+        -- And there is a concrete reason to prefer static here. Expat's only
+        -- consumer in this index is `freedesktop.wayland-scanner`, a
+        -- `kind = "bin"` HOST TOOL that mcpp builds in a sub-build and then
+        -- RUNS during another package's build.mcpp. A host tool linking a
+        -- shared dependency comes out with a DT_NEEDED nothing satisfies —
+        -- mcpp does not stage the .so beside the tool, and the sub-build's
+        -- bin/ holds the executable alone:
+        --
+        --     wayland-scanner: error while loading shared libraries:
+        --     libexpat.so.1: cannot open shared object file
+        --
+        -- That reproduces only on a machine without a graphics stack: with
+        -- Mesa installed the tool's RPATH reaches `<registry>/subos/default/lib`
+        -- and silently binds to `xim:expat`'s copy instead — the WRONG library,
+        -- succeeding. Static removes the question.
+        targets = { ["expat"] = { kind = "lib" } },
         deps    = {},
 
         generated_files = {

tests/examples/expat/tests/xml.cpp

@@ -97,17 +97,22 @@ int main()
     }
 
     // ── 4. It is THIS build that ran ─────────────────────────────────────
+    // The package is `kind = "lib"`, so these objects are merged into the
+    // consumer and dladdr reports the EXECUTABLE. The check that matters is
+    // the same either way: the code that just parsed must not have come from
+    // the ecosystem's `xim:expat`, which is present whenever Mesa is and would
+    // otherwise answer silently.
     {
         Dl_info info{};
         const bool located =
             ::dladdr(reinterpret_cast<void *>(&XML_ParserCreate), &info) != 0
             && info.dli_fname != nullptr;
-        check(located, "dladdr locates the loaded libexpat");
+        check(located, "dladdr locates the code that parsed");
         if (located) {
             const std::string from = info.dli_fname;
-            std::printf("   loaded from: %s\n", from.c_str());
+            std::printf("   resolved from: %s\n", from.c_str());
             check(from.find("xim-x-expat") == std::string::npos,
-                  "the loaded libexpat is not the ecosystem payload's copy");
+                  "it is not the ecosystem payload's libexpat");
         }
     }