Fix debug PCH builds with MSVC 2012 and later

With MSVC 2013 and newer, using pre-compiled headers with .pdb debugging
fails with the following error message:

fatal error C1041: cannot open program database '[...]\prog.pdb'; if multiple CL.EXE write to the same .PDB file, please use /FS

So we use /FS when PCH is enabled. When PCH is enabled and debugging is
disabled, this will have no effect since .pdb files will not be written.

mesonbuild/backend/ninjabackend.py

@@ -1694,9 +1694,9 @@ rule FORTRAN_DEP_HACK
 
         if target.has_pch():
             tfilename = self.get_target_filename_abs(target)
-            return compiler.get_compile_debugfile_args(tfilename)
+            return compiler.get_compile_debugfile_args(tfilename, pch=True)
         else:
-            return compiler.get_compile_debugfile_args(objfile)
+            return compiler.get_compile_debugfile_args(objfile, pch=False)
 
     def get_link_debugfile_args(self, linker, target, outname):
         return linker.get_link_debugfile_args(outname)

mesonbuild/compilers.py

@@ -435,7 +435,7 @@ class Compiler():
 
     # Some compilers (msvc) write debug info to a separate file.
     # These args specify where it should be written.
-    def get_compile_debugfile_args(self, rel_obj):
+    def get_compile_debugfile_args(self, rel_obj, **kwargs):
         return []
 
     def get_link_debugfile_args(self, rel_obj):
@@ -1883,10 +1883,19 @@ class VisualStudioCCompiler(CCompiler):
             raise MesonException('Compiling test app failed.')
         return not(warning_text in stde or warning_text in stdo)
 
-    def get_compile_debugfile_args(self, rel_obj):
+    def get_compile_debugfile_args(self, rel_obj, pch=False):
         pdbarr = rel_obj.split('.')[:-1]
         pdbarr += ['pdb']
-        return ['/Fd' + '.'.join(pdbarr)]
+        args = ['/Fd' + '.'.join(pdbarr)]
+        # When generating a PDB file with PCH, all compile commands write
+        # to the same PDB file. Hence, we need to serialize the PDB
+        # writes using /FS since we do parallel builds. This slows down the
+        # build obviously, which is why we only do this when PCH is on.
+        # This was added in Visual Studio 2013 (MSVC 18.0). Before that it was
+        # always on: https://msdn.microsoft.com/en-us/library/dn502518.aspx
+        if pch and mesonlib.version_compare(self.version, '>=18.0'):
+            args = ['/FS'] + args
+        return args
 
     def get_link_debugfile_args(self, targetfile):
         pdbarr = targetfile.split('.')[:-1]