Android: build/runtime fixes for modern Clang + ETDump path override (pytorch#19293)

## Summary

Five small fixes needed to build and run ExecuTorch Android on hosts
with Apple Clang 21, on devices where `/data/local/tmp` isn't
app-writable, and to make the Android build script behave correctly on
hosts with multiple Python interpreters:

1. **flatcc `-Werror` on modern Clang.** flatcc's vendored
`CMakeLists.txt` hits `-Wimplicit-int-conversion-on-negation` and
`-Wunterminated-string-initialization` on new Clang. Pass
`FLATCC_ALLOW_WERROR=OFF` into the `flatcc_ep` ExternalProject so the
third-party build doesn't fail.

2. **`EXECUTORCH_ANDROID_PROFILING` was never forwarded.**
`scripts/build_android_library.sh` set `EXECUTORCH_ENABLE_EVENT_TRACER`
from `$EXECUTORCH_ANDROID_PROFILING` but never forwarded the separate
`-DEXECUTORCH_ANDROID_PROFILING` define that gates `ETDumpGen`
allocation in `extension/android/jni/jni_layer.cpp`. As a result the
event tracer was compiled in but the JNI never attached it, and
`Module.etdump()` always returned `false`.

3. **`Module.etdump(String outputPath)` overload.** Add a JNI method
(`etdumpToNative`) and Java wrapper so apps can write the ETDump into
their own `filesDir`. The existing `etdump()` hardcodes
`/data/local/tmp/result.etdump`, which is not app-writable on modern
Android (Pixel 9 / API 36 returns `EACCES`). The new overload factors
the write path through a shared `etdump_to_path()` helper; `etdump()`
now delegates to it with the legacy path.

4. **`define_overridable_option` was expanding `${DESCRIPTION}`
unquoted.** The `set(... CACHE TYPE DOCSTRING [FORCE])` form expects the
docstring as a single argument, but `${DESCRIPTION}` was unquoted, so
any `STRING` option with a multi-word description that the user
overrides on the cmake command line had its description text spill into
subsequent `set()` arguments. One-character fix: quote
`"${DESCRIPTION}"` in both `set()` calls.

5. **`scripts/build_android_library.sh` never forwarded
`PYTHON_EXECUTABLE` to cmake.** The script defaults
`PYTHON_EXECUTABLE=python3` and honors a caller override, but only used
it for `which`. cmake's `find_package(Python3)` then picked up whatever
interpreter came first on PATH (e.g. Homebrew's `python3` on macOS)
instead of the project's torch-equipped venv, failing at
`find_package_torch_headers` with `'NoneType' object has no attribute
'submodule_search_locations'`. Pass
`-DPYTHON_EXECUTABLE="${PYTHON_EXECUTABLE}"` so the script's documented
behavior actually flows through.

## Test plan
- [x] Apple Clang 21 host build: `scripts/build_android_library.sh` now
succeeds (flatcc no longer fails `-Werror`).
- [x] With `EXECUTORCH_ANDROID_PROFILING=1`:
`-DEXECUTORCH_ANDROID_PROFILING` reaches the JNI compile, `ETDumpGen` is
attached, `Module.etdump()` returns `true` and writes a valid `.etdump`.
- [x] On Pixel 9 / API 36: `Module.etdump(context.getFilesDir() +
"/result.etdump")` succeeds; legacy `Module.etdump()` still works on
devices where `/data/local/tmp` is writable.
- [x] Default build (`EXECUTORCH_ANDROID_PROFILING` unset): unchanged —
`etdump()` returns `false` as before.
- [x] `cmake -DEXECUTORCH_VULKAN_FP16_PRECISION=mediump …` (multi-word
description STRING option) now configures cleanly.
- [x] Hosts with both Homebrew `python3` and a torch-equipped venv:
`PYTHON_EXECUTABLE=$VENV/bin/python scripts/build_android_library.sh`
configures correctly.

🤖 Authored with [Claude Code](https://claude.com/claude-code)

cc @kirklandsign @cbilgin

Author: jgibson2

extension/android/executorch_android/src/main/java/org/pytorch/executorch/Module.java

@@ -227,6 +227,28 @@ public String[] readLogBuffer() {
   @DoNotStrip
   public native boolean etdump();
 
+  /**
+   * Dump the ExecuTorch ETDump file to {@code outputPath}.
+   *
+   * @param outputPath absolute path to write the etdump file to.
+   * @return true if the etdump was successfully written, false otherwise.
+   */
+  @Experimental
+  public boolean etdump(String outputPath) {
+    mLock.lock();
+    try {
+      if (!mHybridData.isValid()) {
+        throw new IllegalStateException("Module has been destroyed");
+      }
+      return etdumpToNative(outputPath);
+    } finally {
+      mLock.unlock();
+    }
+  }
+
+  @DoNotStrip
+  private native boolean etdumpToNative(String outputPath);
+
   /**
    * Explicitly destroys the native Module object. Calling this method is not required, as the
    * native object will be destroyed when this object is garbage-collected. However, the timing of

extension/android/jni/jni_layer.cpp

@@ -466,34 +466,15 @@ class ExecuTorchJni : public facebook::jni::HybridClass<ExecuTorchJni> {
   }
 
   jboolean etdump() {
-#ifdef EXECUTORCH_ANDROID_PROFILING
-    executorch::etdump::ETDumpGen* etdumpgen =
-        (executorch::etdump::ETDumpGen*)module_->event_tracer();
-    auto etdump_data = etdumpgen->get_etdump_data();
+    return etdump_to_path("/data/local/tmp/result.etdump");
+  }
 
-    if (etdump_data.buf != nullptr && etdump_data.size > 0) {
-      int etdump_file =
-          open("/data/local/tmp/result.etdump", O_WRONLY | O_CREAT, 0644);
-      if (etdump_file == -1) {
-        ET_LOG(Error, "Cannot create result.etdump error: %d", errno);
-        return false;
-      }
-      ssize_t bytes_written =
-          write(etdump_file, (uint8_t*)etdump_data.buf, etdump_data.size);
-      if (bytes_written == -1) {
-        ET_LOG(Error, "Cannot write result.etdump error: %d", errno);
-        return false;
-      } else {
-        ET_LOG(Info, "ETDump written %d bytes to file.", bytes_written);
-      }
-      close(etdump_file);
-      free(etdump_data.buf);
-      return true;
-    } else {
-      ET_LOG(Error, "No ETDump data available!");
+  jboolean etdumpTo(facebook::jni::alias_ref<jstring> outputPath) {
+    if (!outputPath) {
+      ET_LOG(Error, "etdumpTo called with null outputPath");
+      return false;
     }
-#endif
-    return false;
+    return etdump_to_path(outputPath->toStdString().c_str());
   }
 
   facebook::jni::local_ref<facebook::jni::JArrayClass<jstring>> getMethods() {
@@ -565,10 +546,51 @@ class ExecuTorchJni : public facebook::jni::HybridClass<ExecuTorchJni> {
         makeNativeMethod(
             "readLogBufferStaticNative", ExecuTorchJni::readLogBufferStatic),
         makeNativeMethod("etdump", ExecuTorchJni::etdump),
+        makeNativeMethod("etdumpToNative", ExecuTorchJni::etdumpTo),
         makeNativeMethod("getMethods", ExecuTorchJni::getMethods),
         makeNativeMethod("getUsedBackends", ExecuTorchJni::getUsedBackends),
     });
   }
+
+ private:
+  jboolean etdump_to_path(const char* path) {
+#ifdef EXECUTORCH_ANDROID_PROFILING
+    auto* tracer = module_->event_tracer();
+    if (!tracer) {
+      ET_LOG(Error, "ETDump not available: no event tracer attached");
+      return false;
+    }
+    auto* etdumpgen = static_cast<executorch::etdump::ETDumpGen*>(tracer);
+    auto etdump_data = etdumpgen->get_etdump_data();
+
+    if (etdump_data.buf != nullptr && etdump_data.size > 0) {
+      int etdump_file = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0644);
+      if (etdump_file == -1) {
+        ET_LOG(Error, "Cannot create %s error: %d", path, errno);
+        free(etdump_data.buf);
+        return false;
+      }
+      ssize_t bytes_written =
+          write(etdump_file, (uint8_t*)etdump_data.buf, etdump_data.size);
+      if (bytes_written == -1) {
+        ET_LOG(Error, "Cannot write %s error: %d", path, errno);
+        close(etdump_file);
+        free(etdump_data.buf);
+        return false;
+      } else {
+        ET_LOG(Info, "ETDump written %zd bytes to %s.", bytes_written, path);
+      }
+      close(etdump_file);
+      free(etdump_data.buf);
+      return true;
+    } else {
+      ET_LOG(Error, "No ETDump data available!");
+    }
+#else
+    (void)path;
+#endif
+    return false;
+  }
 };
 } // namespace executorch::extension
 

scripts/build_android_library.sh

@@ -37,9 +37,11 @@ build_android_native_library() {
 
   cmake . -DCMAKE_INSTALL_PREFIX="${CMAKE_OUT}" \
     -DCMAKE_TOOLCHAIN_FILE="${ANDROID_NDK}/build/cmake/android.toolchain.cmake" \
+    -DPYTHON_EXECUTABLE="${PYTHON_EXECUTABLE}" \
     --preset "android-${ANDROID_ABI}" \
     -DANDROID_PLATFORM=android-26 \
     -DEXECUTORCH_ENABLE_EVENT_TRACER="${EXECUTORCH_ANDROID_PROFILING:-OFF}" \
+    -DEXECUTORCH_ANDROID_PROFILING="${EXECUTORCH_ANDROID_PROFILING:-OFF}" \
     -DEXECUTORCH_BUILD_EXTENSION_LLM="${EXECUTORCH_BUILD_EXTENSION_LLM:-ON}" \
     -DEXECUTORCH_BUILD_EXTENSION_LLM_RUNNER="${EXECUTORCH_BUILD_EXTENSION_LLM:-ON}" \
     -DEXECUTORCH_BUILD_EXTENSION_ASR_RUNNER="${EXECUTORCH_BUILD_EXTENSION_LLM:-ON}" \
@@ -51,6 +53,7 @@ build_android_native_library() {
     -DQNN_SDK_ROOT="${QNN_SDK_ROOT}" \
     -DEXECUTORCH_BUILD_VULKAN="${EXECUTORCH_BUILD_VULKAN}" \
     -DXNNPACK_ENABLE_ARM_SME2="${XNNPACK_ENABLE_ARM_SME2}" \
+    -DFLATCC_ALLOW_WERROR=OFF \
     -DSUPPORT_REGEX_LOOKAHEAD=ON \
     -DCMAKE_BUILD_TYPE="${EXECUTORCH_CMAKE_BUILD_TYPE}" \
     -B"${CMAKE_OUT}"

third-party/CMakeLists.txt

@@ -100,6 +100,7 @@ ExternalProject_Add(
     -DFLATCC_TEST=OFF
     -DFLATCC_REFLECTION=OFF
     -DFLATCC_DEBUG_CLANG_SANITIZE=OFF
+    -DFLATCC_ALLOW_WERROR=OFF
     -DFLATCC_INSTALL=ON
     -DCMAKE_POLICY_VERSION_MINIMUM=3.5
     -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>

tools/cmake/common/preset.cmake

@@ -82,12 +82,12 @@ macro(define_overridable_option NAME DESCRIPTION VALUE_TYPE DEFAULT_VALUE)
   if(DEFINED ${NAME} AND NOT DEFINED CACHE{${NAME}})
     set(${NAME}
         ${${NAME}}
-        CACHE ${VALUE_TYPE} ${DESCRIPTION} FORCE
+        CACHE ${VALUE_TYPE} "${DESCRIPTION}" FORCE
     )
   else()
     set(${NAME}
         ${DEFAULT_VALUE}
-        CACHE ${VALUE_TYPE} ${DESCRIPTION}
+        CACHE ${VALUE_TYPE} "${DESCRIPTION}"
     )
   endif()