[release/7.0] Set AssemblyName.ProcessorArchitecture for compatibility. (#81101)

* Set AssemblyName.ProcessorArchitecture for compatibility.

* Simplified ProcArch computation.

* Make GetAssemblyName test more robust (#80878)

* undo AssemblyName.CoreCLR change

* Library servicing steps.

---------

Co-authored-by: vsadov <[email protected]>
Co-authored-by: Vladimir Sadov <[email protected]>

Author: github-actions[bot]

src/libraries/System.Reflection.Metadata/src/System.Reflection.Metadata.csproj

@@ -9,6 +9,8 @@
 
 The System.Reflection.Metadata library is built-in as part of the shared framework in .NET Runtime. The package can be installed when you need to use it in other target frameworks.</PackageDescription>
     <PackageReadmeFile>README.md</PackageReadmeFile>
+    <ServicingVersion>1</ServicingVersion>
+    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
   </PropertyGroup>
 
   <!-- DesignTimeBuild requires all the TargetFramework Derived Properties to not be present in the first property group. -->

src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/MetadataReader.netstandard.cs

@@ -85,6 +85,12 @@ public static unsafe AssemblyName GetAssemblyName(string assemblyFile)
                     peReader = new PEReader((byte*)safeBuffer.DangerousGetHandle(), (int)safeBuffer.ByteLength);
                     MetadataReader mdReader = peReader.GetMetadataReader(MetadataReaderOptions.None);
                     AssemblyName assemblyName = mdReader.GetAssemblyDefinition().GetAssemblyName();
+
+                    AssemblyFlags aFlags = mdReader.AssemblyTable.GetFlags();
+#pragma warning disable SYSLIB0037 // AssemblyName.ProcessorArchitecture is obsolete
+                    assemblyName.ProcessorArchitecture = CalculateProcArch(peReader, aFlags);
+#pragma warning restore SYSLIB0037
+
                     return assemblyName;
                 }
                 finally
@@ -101,6 +107,42 @@ public static unsafe AssemblyName GetAssemblyName(string assemblyFile)
             }
         }
 
+        private static ProcessorArchitecture CalculateProcArch(PEReader peReader, AssemblyFlags aFlags)
+        {
+            // 0x70 specifies "reference assembly".
+            // For these, CLR wants to return None as arch so they can be always loaded, regardless of process type.
+            if (((uint)aFlags & 0xF0) == 0x70)
+                return ProcessorArchitecture.None;
+
+            PEHeaders peHeaders = peReader.PEHeaders;
+            switch (peHeaders.CoffHeader.Machine)
+            {
+                case Machine.IA64:
+                    return ProcessorArchitecture.IA64;
+                case Machine.Arm:
+                    return ProcessorArchitecture.Arm;
+                case Machine.Amd64:
+                    return ProcessorArchitecture.Amd64;
+                case Machine.I386:
+                    {
+                        CorFlags flags = peHeaders.CorHeader!.Flags;
+                        if ((flags & CorFlags.ILOnly) != 0 &&
+                            (flags & CorFlags.Requires32Bit) == 0)
+                        {
+                            // platform neutral.
+                            return ProcessorArchitecture.MSIL;
+                        }
+
+                        // requires x86
+                        return ProcessorArchitecture.X86;
+                    }
+            }
+
+            // ProcessorArchitecture is a legacy API and does not cover other Machine kinds.
+            // For example ARM64 is not expressible
+            return ProcessorArchitecture.None;
+        }
+
         private static AssemblyNameFlags GetAssemblyNameFlags(AssemblyFlags flags)
         {
             AssemblyNameFlags assemblyNameFlags = AssemblyNameFlags.None;

src/libraries/System.Reflection.Metadata/tests/Metadata/MetadataReaderTests.cs

@@ -3090,8 +3090,13 @@ public void GetAssemblyName()
 
             if (PlatformDetection.HasAssemblyFiles)
             {
-                Assembly a = typeof(MetadataReaderTests).Assembly;
-                Assert.Equal(new AssemblyName(a.FullName).ToString(), MetadataReader.GetAssemblyName(AssemblyPathHelper.GetAssemblyLocation(a)).ToString());
+                Assembly a = typeof(MetadataReader).Assembly;
+                AssemblyName name = MetadataReader.GetAssemblyName(AssemblyPathHelper.GetAssemblyLocation(a));
+                Assert.Equal(new AssemblyName(a.FullName).ToString(), name.ToString());
+
+#pragma warning disable SYSLIB0037 // AssemblyName.ProcessorArchitecture is obsolete
+                Assert.Equal(ProcessorArchitecture.MSIL, name.ProcessorArchitecture);
+#pragma warning restore SYSLIB0037
             }
         }
     }