fix(android): use --wrap=pthread_create instead of raw symbol override

Build #10 failed with:
  ld.lld: error: duplicate symbol: pthread_create
    >>> defined at pthread_shim.c:30
    >>> ... in archive libpthread_shim.a
  (the other definition coming from libstd's bundled libc.a stub)

The raw-symbol-override approach was naive: when two static archives
both define the same symbol the linker refuses instead of picking one.

Switch to GNU-ld's `--wrap=pthread_create` mechanism:
  - All `pthread_create` references get rewritten to `__wrap_pthread_create`
  - Our shim now defines `__wrap_pthread_create` (no symbol clash)
  - Inside the shim we `dlopen("libc.so")` + `dlsym("pthread_create")` to
    get the real runtime symbol directly, bypassing BOTH the broken static
    stub (libstd's libc.a copy) AND libstd's own pthread_create path
  - `--real_pthread_create` is deliberately NOT used — it would alias the
    same broken stub the wrap exists to avoid

The wrap flag is emitted via `cargo:rustc-link-arg` in build.rs so it
only affects the Android target (the Android-branch of build.rs is the
only place that emits it).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/build.rs

@@ -112,6 +112,12 @@ fn build_oboe_android(target: &str) {
     // Oboe requires Android log + OpenSLES backends
     println!("cargo:rustc-link-lib=log");
     println!("cargo:rustc-link-lib=OpenSLES");
+
+    // Wrap pthread_create: redirect every `pthread_create` reference to our
+    // `__wrap_pthread_create` in pthread_shim.c, which forwards to the real
+    // libc.so symbol via dlsym. Without this the linker binds to libstd's
+    // bundled broken static pthread_create stub (see pthread_shim.c).
+    println!("cargo:rustc-link-arg=-Wl,--wrap=pthread_create");
 }
 
 /// Recursively add all .cpp files from a directory to a cc::Build.

desktop/src-tauri/cpp/pthread_shim.c

@@ -1,19 +1,19 @@
 /* pthread_shim.c
  *
- * Interpose pthread_create to bypass the broken static stub that Rust's
+ * Interpose pthread_create via linker --wrap to bypass Rust libstd's broken
- * pre-compiled libstd for aarch64-linux-android drags in.
+ * static pthread_create stub (pulled in from an old NDK libc.a), which
+ * transitively calls __init_tcb+4 and SIGSEGVs in any .so loaded via dlopen.
  *
- * The stub (from an old NDK libc.a) calls __init_tcb(bionic_tcb*, ...)+4,
+ * Link flags (see build.rs):
- * which SIGSEGVs in .so libraries because __init_tcb expects TCB state that
+ *   -Wl,--wrap=pthread_create
- * only the static-libc init path sets up. In a dlopen-loaded shared lib
- * nothing ever initialises that state.
  *
- * By providing our own pthread_create at link time (which takes priority
+ * The wrap flag makes the linker redirect every unresolved reference to
- * over the one dragged in by libstd) and forwarding it via dlsym(RTLD_NEXT)
+ * `pthread_create` → `__wrap_pthread_create` (below). Inside the shim we
- * to the REAL pthread_create in libc.so, we completely sidestep the static
+ * explicitly open libc.so and look up the real, fully working runtime
- * stub — libc.so's pthread_create is the fully working runtime version.
+ * pthread_create, bypassing libstd's bundled archive entirely.
  *
- * The same trick handles `getauxval` via getauxval_fix.c.
+ * We deliberately do NOT call `__real_pthread_create` — that alias is the
+ * SAME broken stub the wrap is designed to get around.
  */
 
 #ifdef __ANDROID__
@@ -26,20 +26,29 @@
 typedef int (*pthread_create_fn)(pthread_t *, const pthread_attr_t *,
                                  void *(*)(void *), void *);
 
-int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
+int __wrap_pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                           void *(*start_routine)(void *), void *arg) {
-    static pthread_create_fn real_pthread_create = NULL;
+    static pthread_create_fn real = NULL;
-    if (real_pthread_create == NULL) {
+    if (real == NULL) {
-        /* RTLD_NEXT: skip the symbol we're currently defining and return
+        /* Explicitly open libc.so and fetch the runtime pthread_create.
-         * the next one in the search order — which is the real pthread_create
+         * RTLD_NOLOAD would be wrong here — we want the mapping even if
-         * exported from libc.so. */
+         * bionic already loaded libc. The standard filename "libc.so" is
-        real_pthread_create =
+         * safe on Android; bionic's dynamic linker resolves it to
-            (pthread_create_fn)dlsym(RTLD_NEXT, "pthread_create");
+         * /apex/com.android.runtime/lib64/bionic/libc.so at runtime. */
-        if (real_pthread_create == NULL) {
+        void *libc = dlopen("libc.so", RTLD_LAZY | RTLD_GLOBAL);
+        if (libc != NULL) {
+            real = (pthread_create_fn)dlsym(libc, "pthread_create");
+        }
+        if (real == NULL) {
+            /* Fallback: try RTLD_DEFAULT — may or may not work depending on
+             * link order but it's better than segfaulting. */
+            real = (pthread_create_fn)dlsym(RTLD_DEFAULT, "pthread_create");
+        }
+        if (real == NULL) {
             return -1;
         }
     }
-    return real_pthread_create(thread, attr, start_routine, arg);
+    return real(thread, attr, start_routine, arg);
 }
 
 #endif /* __ANDROID__ */