Added description of debugging gstreamer in magicleap

support/magicleap/README.md

@@ -29,3 +29,104 @@ This builds the application `.out/Servo2D/Servo2D.mpk`.
 Alternatively, in Visual Studio code, open the `support/magicleap/Servo2D` directory,
 and use the `Terminal/Run Build Task...` menu option to build the
 Servo2D application.
+
+## Debugging gstreamer
+
+By default, Servo links against release builds of gstreamer, even for debug builds,
+so if you want to use gdb on gstreamer, you've got some work to do...
+
+First off, you'll need to build the gstreamer libraries with debug symbols.
+To do this, edit `support/magicleap/gstreamer/mlsdk.txt.in` and add `'-g', '-O0`,` to
+`c_args` and `cpp_args`:
+
+```
+[properties]
+c_args = [
+  '-g', '-O0',
+  '--sysroot=@MAGICLEAP_SDK@/lumin/usr',
+  '-I@MAGICLEAP_SDK@/include',
+  '-I@MAGICLEAP_SDK@/staging/include',
+  '-I@INSTALL_DIR@/system/include',
+  ]
+cpp_args = [
+  '-g', '-O0',
+  '--sysroot=@MAGICLEAP_SDK@/lumin/usr',
+  '-I@MAGICLEAP_SDK@/include',
+  '-I@MAGICLEAP_SDK@/staging/include',
+  '-I@INSTALL_DIR@/system/include',
+  ]
+```
+then build the libraries with `./gstreamer.sh` from inside `support/magicleap/gstreamer`.
+
+The libraries will be built in `_install/system`, and should be moved over to
+where `mach` expects them to be:
+
+```
+rm -r target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system
+cp -r support/magicleap/gstreamer/_install/system target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/
+```
+You can now build, package and install as normal:
+```
+./mach build -d --magicleap
+./mach package -d --magicleap
+$MAGICLEAP_SDK/tools/mldb/mldb install -u target/magicleap/aarch64-linux-android/debug/Servo.mpk
+```
+to launch gdb on the application:
+```
+$MAGICLEAP_SDK/debug --package com.mozilla.servo support/magicleap/Servo2D/.out/debug_lumin_clang-3.8_aarch64/Servo2D
+```
+Using the debug libraries in gdb is slightly tricky because you need to set everything up
+in the right order, setting `solib-search-path` should happen before doing any dynamic loading,
+but setting `sysroot` after dynamic loading has started. The easiest thing to do is to place a
+breakpoint somewhere in Servo after gstreamer has started loading, for example:
+
+
+```
+(gdb) set solib-search-path /Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib:/Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/
+warning: Unable to find dynamic linker breakpoint function.
+GDB will be unable to debug shared library initializers
+and track explicitly loaded dynamic code.
+
+(gdb) rbreak RenderAndroid::new
+Breakpoint 1 at 0xaaaac59953a4
+struct Option<servo_media_gstreamer_render_android::RenderAndroid> servo_media_gstreamer_render_android::RenderAndroid::new::he5ddafe679ae0dff(struct Box<PlayerGLContext>);
+Breakpoint 2 at 0xaaaac423f184: file /Users/ajeffrey/github/asajeffrey/media/backends/gstreamer/render-android/lib.rs, line 81.
+static struct Option<gstreamer_gl::auto::gl_display::GLDisplay> servo_media_gstreamer_render_android::RenderAndroid::new::_$u7b$$u7b$closure$u7d$$u7d$::h53b9e99990e38d92(struct closure, struct GLDisplayEGL);
+
+(gdb) c
+
+...
+Thread 2 "ScriptThread Pi" hit Breakpoint 1, 0x0000aaaac59953a4 in servo_media_gstreamer_render_android::RenderAndroid::new::he5ddafe679ae0dff
+    (app_gl_context=...)
+(gdb) set sysroot /Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libc.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/liblog.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libz.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libEGL.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libGLESv2.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libGLESv3.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libm.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libvulkan.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libstdc++.so" is not at the expected address (wrong library or version mismatch?)
+warning: .dynamic section for "/Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libGLESv1_CM.so" is not at the expected address (wrong library or version mismatch?)
+warning: Could not load shared library symbols for 191 libraries, e.g. /system/bin/linker64.
+Use the "info sharedlibrary" command to see the complete listing.
+Do you need "set solib-search-path" or "set sysroot"?
+Reading symbols from /Users/ajeffrey/MagicLeap/mlsdk/v0.22.0/lumin/usr/lib/libc.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libglib-2.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgobject-2.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstreamer-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstapp-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstaudio-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstbase-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstgl-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstplayer-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstsdp-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstvideo-1.0.so...done.
+Reading symbols from /Users/ajeffrey/github/asajeffrey/servo/target/magicleap/aarch64-linux-android/native/gstreamer-1.16.0/system/lib64/libgstwebrtc-1.0.so...done.
+...
+
+(gdb) break gst_gl_context_activate
+Breakpoint 3 at 0x40003ba0bb98: file ../gst-build/subprojects/gst-plugins-base/gst-libs/gst/gl/gstglcontext.c, line 746.
+```
+At this point, setting breakpoints and step-debugging should work as expected.