gh-150467: Fix linking python3.dll and python3t.dll with ASan on Windows

The stable ABI forwarder DLLs are linked with /NOENTRY, so nothing pulls
in the CRT. With AddressSanitizer enabled, the linker adds the ASan
runtime thunk library, which needs the CRT, and the link fails with
unresolved symbols. Keep the default CRT entry point for these DLLs
when building with ASan.

Author: ck0i

Misc/NEWS.d/next/Build/2026-10-30-23-16-35.gh-issue-150467.qMGY7M.rst

@@ -0,0 +1,2 @@
+Fix linking ``python3.dll`` and ``python3t.dll`` on Windows when building with
+AddressSanitizer.

PCbuild/pyproject.props

@@ -25,6 +25,14 @@
     <SupportSigning Condition="'$(Configuration)' == 'Debug'">false</SupportSigning>
     <SupportSigning Condition="'$(ConfigurationType)' == 'StaticLibrary'">false</SupportSigning>
     <LinkIncremental Condition="$(Configuration) != 'Debug'">false</LinkIncremental>
+
+    <!--
+    AddressSanitizer may be enabled with the EnableASAN property or through the
+    _CL_ and _LINK_ environment variables, so check all of them.
+    -->
+    <PyUsingAsan Condition="'$(PyUsingAsan)' == '' and '$(EnableASAN)' == 'true'">true</PyUsingAsan>
+    <PyUsingAsan Condition="'$(PyUsingAsan)' == '' and $(_CL_.ToLowerInvariant().Contains('fsanitize=address'))">true</PyUsingAsan>
+    <PyUsingAsan Condition="'$(PyUsingAsan)' == '' and $(_LINK_.ToLowerInvariant().Contains('inferasanlibs'))">true</PyUsingAsan>
   </PropertyGroup>
 
   <PropertyGroup Condition="$(TargetExt) != ''">

PCbuild/python3dll.vcxproj

@@ -95,7 +95,11 @@
       <BufferSecurityCheck>false</BufferSecurityCheck>
     </ClCompile>
     <Link>
-      <NoEntryPoint>true</NoEntryPoint>
+      <!--
+      The ASan runtime thunks need the CRT to be initialized, so keep the
+      default entry point when building with AddressSanitizer.
+      -->
+      <NoEntryPoint Condition="$(PyUsingAsan) != 'true'">true</NoEntryPoint>
     </Link>
   </ItemDefinitionGroup>
   <ItemGroup>

PCbuild/python3tdll.vcxproj

@@ -95,7 +95,11 @@
       <BufferSecurityCheck>false</BufferSecurityCheck>
     </ClCompile>
     <Link>
-      <NoEntryPoint>true</NoEntryPoint>
+      <!--
+      The ASan runtime thunks need the CRT to be initialized, so keep the
+      default entry point when building with AddressSanitizer.
+      -->
+      <NoEntryPoint Condition="$(PyUsingAsan) != 'true'">true</NoEntryPoint>
     </Link>
   </ItemDefinitionGroup>
   <ItemGroup>