Fix msbuild failing when '@' is present in path (2nd attempt) (grpc#3…

…1527)

* Fix msbuild failing when '@' is present in path

There was ambiguity in references to Source metadata on some Items. These
are now qualified to remove the ambiguity.

* Revert "Fix msbuild failing when '@' is present in path"

This reverts commit bb5d5dd.

* Unescape Indentity when creating Source

* Fix spelling in comment

src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets

@@ -149,8 +149,12 @@
       <!-- Remove files not for compile. -->
       <Protobuf_Compile Remove="@(Protobuf_Compile)" Condition=" '%(ProtoCompile)' != 'true' " />
       <!-- Ensure invariant Source=%(Identity). -->
+      <!-- Need to Unescape otherwise errors with characters such as '@' in paths 
+          See https://github.com/grpc/grpc/issues/30746
+          and https://github.com/dotnet/msbuild/issues/8104
+      -->
       <Protobuf_Compile>
-        <Source>%(Identity)</Source>
+        <Source>$([MSBuild]::Unescape(%(Identity)))</Source>
       </Protobuf_Compile>
     </ItemGroup>
   </Target>
@@ -190,17 +194,24 @@
     <ItemGroup>
       <_Protobuf_CodeCompile Include="@(Protobuf_ExpectedOutputs->Distinct())"
          Condition=" '%(Source)' != '' and '@(Protobuf_Compile->WithMetadataValue('CompileOutputs', 'true'))' != '' " />
-      <Compile Include="@(_Protobuf_CodeCompile)" />
+      <!-- Adding Distinct() is just a precaution as the CSC compiler complains if there are 
+           duplicate source files
+      -->
+      <Compile Include="@(_Protobuf_CodeCompile->Distinct())" />
     </ItemGroup>
   </Target>
 
   <!-- These invariants must be kept for compile up-to-date check to work. -->
   <Target Name="_Protobuf_EnforceInvariants">
     <!-- Enforce Source=Identity on proto files. The 'Source' metadata is used as a common
          key to match dependencies/expected outputs in the lists for up-to-date checks. -->
+    <!--  Need to Unescape otherwise errors with characters such as '@' in paths 
+          See https://github.com/grpc/grpc/issues/30746
+          and https://github.com/dotnet/msbuild/issues/8104
+      -->
     <ItemGroup>
       <Protobuf_Compile>
-        <Source>%(Identity)</Source>
+        <Source>$([MSBuild]::Unescape(%(Identity)))</Source>
       </Protobuf_Compile>
     </ItemGroup>