[linker] fix #51340

[radekdoulik]

• preserve the GetInstance method whenever the
  Mono.Unix.Native.FileNameMarshaler type is used

• it avoids errors like:

    ReadlinkTest
    	ReadLink [FAIL] : System.ApplicationException : Custom marshaler 'FileNameMarshaler' does not implement a static GetInstance method that takes a single string parameter and returns an ICustomMarshaler.
    		  at (wrapper managed-to-native) Mono.Unix.Native.Syscall:readlinkat (int,string,byte[],ulong)
    		  at Mono.Unix.Native.Syscall.readlinkat (System.Int32 dirfd, System.String pathname, System.Byte[] buf) [0x0000a] in <75558c6e0e1f444f8a03d1c694a92a08>:0
    		  at MonoTests.Mono.Unix.ReadlinkTest.SetUp () [0x00007] in <9df8009b87984e1cb863e771d21f9f39>:0
    		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
    		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <9f55149ed93f4bad9226ce1d7634fe5b>:0
    	readlink_byte [FAIL] : System.ApplicationException : Custom marshaler 'FileNameMarshaler' does not implement a static GetInstance method that takes a single string parameter and returns an ICustomMarshaler.
    TearDown : System.ArgumentException : ** Unknown error code: 9**
      ----> Mono.Unix.UnixIOException : ** Unknown error code: 9** [EBADF].
    		  at (wrapper managed-to-native) Mono.Unix.Native.Syscall:readlinkat (int,string,byte[],ulong)
    		  at Mono.Unix.Native.Syscall.readlinkat (System.Int32 dirfd, System.String pathname, System.Byte[] buf) [0x0000a] in <75558c6e0e1f444f8a03d1c694a92a08>:0
    		  at MonoTests.Mono.Unix.ReadlinkTest.SetUp () [0x00007] in <9df8009b87984e1cb863e771d21f9f39>:0
    		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
    		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <9f55149ed93f4bad9226ce1d7634fe5b>:0
    		--TearDown
    		  at Mono.Unix.UnixMarshal.ThrowExceptionForLastError () [0x00006] in <75558c6e0e1f444f8a03d1c694a92a08>:0
    		  at MonoTests.Mono.Unix.ReadlinkTest.TearDown () [0x00011] in <9df8009b87984e1cb863e771d21f9f39>:0
    		  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
    		  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in <9f55149ed93f4bad9226ce1d7634fe5b>:0
    		--UnixIOException
  

[jonpryor]

Instead of special-casing Mono.Unix.Native.FileNameMarshaler, I think you should instead preserve the static ICustomMarshaler GetInstance() method on any type that implements ICustomMarshaler. That would automatically handle FileNameMarshaler, and any other custom marshaler.

[radekdoulik]

Indeed (https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.icustommarshaler(v=vs.110).aspx).

Updated the patch.

[jonpryor]

build

[jonpryor]

build

[jonpryor]

Do we need to check the remaining methods once this if has executed? Perhaps we can break after the MarkMethod(method)?

[jonpryor]

Please use braces around the foreach.

[radekdoulik]

Updated. It would probably still not build as it is on cycle9 branch.

[jonpryor]

I don't understand why, I don't know what's going on, but the PR builder doesn't like this commit.

Pr Builds 451, 455, and 456 have all taken "unreasonable" amounts of time -- taking over 5 hours, where a normal PR build is less than 20 minutes -- because they can't connect to the emulator:

03:32:54 		Executing: "/Users/builder/android-toolchain/sdk/platform-tools/adb"   uninstall Mono.Android_Tests
03:32:54 		Environment variables being passed to the tool:
03:32:55 		- waiting for device -

I don't understand why this only appears to impact this PR, and none of the others.

[jonpryor]

Updated. It would probably still not build as it is on cycle9 branch.

Oh! That's because PR builds don't work on the cycle 9 branch.

Doh!

Why is this going to the cycle9 branch and not master? Surely master needs this fix, too, no? Or is the fix in external/linker?

[radekdoulik]

It is cycle9 bug, so it needs to be fixed in cycle9 I guess.

For master I would like to create patch for mono/linker repo.