[monodroid] Allow the SGEN Bridge version to be specified (#219)

[Mono has support for improved SGEN GC Bridge behavior][0] which bumps
the value of `SGEN_BRIDGE_VERSION` from `4` to `5`, and will be
present within Mono 4.8.

Xamarin.Android, meanwhile, is currently targeting Mono 4.6, which
won't be getting support for this new SGEN bridge version.

What is desired is a way to use xamarin-android with Mono 4.7/4.8 to
test out the new GC bridge behavior, without breaking existing
Mono 4.6 support.

The solution is a new MSBuild property, `$(MonoSgenBridgeVersion)`,
which allows overriding the `SGEN_BRIDGE_VERSION` value used when
building the `libmono-android` native library.

By default this MSBuild prpoperty is `4`, for compatibility with
Mono 4.6.

[0]: https://github.com/mono/mono/pull/3422/files

Author: jonpryor

Configuration.props

@@ -25,6 +25,7 @@
     <NeedMxe Condition=" '$(HostOS)' == 'Darwin' ">true</NeedMxe>
     <MakeConcurrency Condition=" '$(MakeConcurrency)' == '' And '$(HostCpuCount)' != '' ">-j$(HostCpuCount)</MakeConcurrency>
     <ManagedRuntime Condition=" '$(ManagedRuntime)' == '' And '$(OS)' != 'Windows_NT' ">mono</ManagedRuntime>
+    <MonoSgenBridgeVersion Condition=" '$(MonoSgenBridgeVersion)' == '' ">4</MonoSgenBridgeVersion>
     <TargetFrameworkRootPath>$(MSBuildThisFileDirectory)bin\$(Configuration)\lib\xbuild-frameworks</TargetFrameworkRootPath>
     <HOME Condition=" '$(HOME)' == '' ">$(HOMEDRIVE)$(HOMEPATH)</HOME>
     <AndroidApiLevel Condition=" '$(AndroidApiLevel)' == '' ">24</AndroidApiLevel>

README.md

@@ -93,7 +93,12 @@ Overridable MSBuild properties include:
     `external/Java.Interop` directory, maintained by `git submodule update`.
 * `$(MakeConcurrency)`: **make**(1) parameters to use intended to influence
     the number of CPU cores used when **make**(1) executes. By default this uses
-		`-jCOUNT`, where `COUNT` is obtained from `sysctl hw.ncpu`.
+    `-jCOUNT`, where `COUNT` is obtained from `sysctl hw.ncpu`.
+* `$(MonoSgenBridgeVersion)`: The Mono SGEN Bridge version to support.
+    Valid values include:
+
+    * `4`: Mono 4.6 support. This is the default.
+    * `5`: Mono 4.8 support.
 
 # Build Requirements
 

src/monodroid/jni/Android.mk

@@ -11,6 +11,7 @@ endif
 LOCAL_CFLAGS =	$(COMMON_CFLAGS) \
 	-std=c99 \
 	-DHAVE_LINUX_NETLINK_H=1 -DHAVE_LINUX_RTNETLINK_H=1 \
+	-DSGEN_BRIDGE_VERSION=$(SGEN_BRIDGE_VERSION) \
 	-D_REENTRANT -DPLATFORM_ANDROID -DANDROID -DLINUX -Dlinux -D__linux_ \
 	-DHAVE_CONFIG_H -DJI_DLL_EXPORT -DMONO_DLL_EXPORT \
 	-I$(topdir)/libmonodroid/zip -I$(BUILDDIR)/include -I$(BUILDDIR)/include/eglib \

src/monodroid/jni/dylib-mono.h

@@ -120,9 +120,13 @@ typedef uint8_t  mono_byte;
 #endif
 #endif
 
-enum {
-	SGEN_BRIDGE_VERSION = 4
-};
+#ifndef SGEN_BRIDGE_VERSION
+#error  SGEN_BRIDGE_VERSION must be defined! (Use the `$(MonoSgenBridgeVersion)` MSBuild property)
+#endif  /* ndef SGEN_BRIDGE_VERSION */
+
+#if (SGEN_BRIDGE_VERSION < 4) || (SGEN_BRIDGE_VERSION >= 6)
+#error  Only SGEN_BRIDGE_VERSION/$(MonoSgenBridgeVersion) values of 4 or 5 are supported.
+#endif  /* SGEN_BRIDGE_VERSION check */
 
 typedef enum {
 	/* Instances of this class should be scanned when computing the transitive dependency among bridges. E.g. List<object>*/

src/monodroid/monodroid.targets

@@ -13,14 +13,15 @@
       Outputs="@(AndroidSupportedTargetJitAbi->'$(OutputPath)\%(Identity)\libmono-android.debug.so');@(AndroidSupportedTargetJitAbi->'$(OutputPath)\%(Identity)\libmono-android.release.so')">
     <PropertyGroup>
       <_AppAbi>@(AndroidSupportedTargetJitAbi->'%(Identity)', ' ')</_AppAbi>
+      <_NdkBuildArgs>CONFIGURATION=$(Configuration) SGEN_BRIDGE_VERSION=$(MonoSgenBridgeVersion)</_NdkBuildArgs>
     </PropertyGroup>
     <WriteLinesToFile
         File="jni\Application.mk"
         Lines="APP_ABI := $(_AppAbi)"
         Overwrite="True"
     />
     <Exec
-        Command="$(AndroidToolchainDirectory)\ndk\ndk-build CONFIGURATION=$(Configuration) NDK_LIBS_OUT=./libs/Debug NDK_OUT=./obj/Debug V=1"
+        Command="$(AndroidToolchainDirectory)\ndk\ndk-build $(_NdkBuildArgs) NDK_LIBS_OUT=./libs/Debug NDK_OUT=./obj/Debug V=1"
     />
     <Copy
         SourceFiles="@(AndroidSupportedTargetJitAbi->'obj\Debug\local\%(Identity)\libmonodroid.so')"
@@ -31,7 +32,7 @@
         DestinationFiles="@(AndroidSupportedTargetJitAbi->'$(OutputPath)\%(Identity)\libmono-android.debug.so')"
     />
     <Exec
-        Command="$(AndroidToolchainDirectory)\ndk\ndk-build CONFIGURATION=$(Configuration) NDK_LIBS_OUT=./libs/Release NDK_OUT=./obj/Release  V=1"
+        Command="$(AndroidToolchainDirectory)\ndk\ndk-build $(_NdkBuildArgs) NDK_LIBS_OUT=./libs/Release NDK_OUT=./obj/Release  V=1"
     />
     <Copy
         SourceFiles="@(AndroidSupportedTargetJitAbi->'obj\Release\local\%(Identity)\libmonodroid.so')"
@@ -45,10 +46,10 @@
   <Target Name="_CleanRuntimes"
       AfterTargets="Clean">
     <Exec
-        Command="$(AndroidToolchainDirectory)\ndk\ndk-build CONFIGURATION=$(Configuration) NDK_LIBS_OUT=./libs/Debug NDK_OUT=./obj/Debug  V=1 clean"
+        Command="$(AndroidToolchainDirectory)\ndk\ndk-build $(_NdkBuildArgs) NDK_LIBS_OUT=./libs/Debug NDK_OUT=./obj/Debug  V=1 clean"
     />
     <Exec
-        Command="$(AndroidToolchainDirectory)\ndk\ndk-build CONFIGURATION=$(Configuration) NDK_LIBS_OUT=./libs/Release NDK_OUT=./obj/Release  V=1 clean"
+        Command="$(AndroidToolchainDirectory)\ndk\ndk-build $(_NdkBuildArgs) NDK_LIBS_OUT=./libs/Release NDK_OUT=./obj/Release  V=1 clean"
     />
     <RemoveDir Directories="obj\local;libs" />
     <Delete Files="jni\config.include;jni\machine.config.include;jni\Application.mk" />