From dbdc036858b04a888cd855e270187bdd03c31353 Mon Sep 17 00:00:00 2001
From: Ben Langmuir <blangmuir@apple.com>
Date: Tue, 17 Jun 2014 22:35:27 +0000
Subject: [PATCH] Retry building modules that were compiled by other instances
 and are out-of-date

When another clang instance builds a module, it may still be considered
"out of date" for the current instance in a couple of cases*.  This
patch prevents us from giving spurious errors when compilers race to
build a module by allowing the module load to fail when the pcm was
built by a different compiler instance.

* Cases where a module can be out of date despite just having been
built:

1) There are different -I paths between invocations that result in
finding a different module map file for some dependent module. This is
not an error, and should never be diagnosed.

<rdar://problem/16843887>

2) There are file system races where the headers making up a module are
touched or moved. Although this can sometimes mean trouble, diagnosing
it only during a build-race is worse than useless and we cannot detect
this in general.  It is more robust to just rebuild.  This was causing
spurious issues in some setups where only the modtime of headers was
bumped during a build.

<rdar://problem/16157638>

llvm-svn: 211129
---
 clang/lib/Frontend/CompilerInstance.cpp | 60 +++++++++++++++++++--------------
 1 file changed, 34 insertions(+), 26 deletions(-)

diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index bca4a59..e8ca080 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -896,23 +896,28 @@ static void compileModuleImpl(CompilerInstance &ImportingInstance,
   }
 }
 
-static void compileModule(CompilerInstance &ImportingInstance,
-                          SourceLocation ImportLoc,
-                          Module *Module,
-                          StringRef ModuleFileName) {
+static bool compileAndLoadModule(CompilerInstance &ImportingInstance,
+                                 SourceLocation ImportLoc,
+                                 SourceLocation ModuleNameLoc,
+                                 Module *Module,
+                                 StringRef ModuleFileName) {
   // FIXME: have LockFileManager return an error_code so that we can
   // avoid the mkdir when the directory already exists.
   StringRef Dir = llvm::sys::path::parent_path(ModuleFileName);
   llvm::sys::fs::create_directories(Dir);
 
   while (1) {
+    unsigned ModuleLoadCapabilities = ASTReader::ARR_Missing;
     llvm::LockFileManager Locked(ModuleFileName);
     switch (Locked) {
     case llvm::LockFileManager::LFS_Error:
-      return;
+      return false;
 
     case llvm::LockFileManager::LFS_Owned:
-      // We're responsible for building the module ourselves. Do so below.
+      // We're responsible for building the module ourselves.
+      // FIXME: if there are errors, don't attempt to load the module.
+      compileModuleImpl(ImportingInstance, ModuleNameLoc, Module,
+                        ModuleFileName);
       break;
 
     case llvm::LockFileManager::LFS_Shared:
@@ -920,11 +925,28 @@ static void compileModule(CompilerInstance &ImportingInstance,
       // finish.
       if (Locked.waitForUnlock() == llvm::LockFileManager::Res_OwnerDied)
         continue; // try again to get the lock.
-      return;
+      ModuleLoadCapabilities |= ASTReader::ARR_OutOfDate;
+      break;
     }
 
-    return compileModuleImpl(ImportingInstance, ImportLoc, Module,
-                             ModuleFileName);
+    // Try to read the module file, now that we've compiled it.
+    ASTReader::ASTReadResult ReadResult =
+        ImportingInstance.getModuleManager()->ReadAST(
+            ModuleFileName, serialization::MK_Module, ImportLoc,
+            ModuleLoadCapabilities);
+
+    if (ReadResult == ASTReader::OutOfDate &&
+        Locked == llvm::LockFileManager::LFS_Shared) {
+      // The module may be out of date in the presence of file system races,
+      // or if one of its imports depends on header search paths that are not
+      // consistent with this ImportingInstance.  Try again...
+      continue;
+    } else if (ReadResult == ASTReader::Missing) {
+      ImportingInstance.getDiagnostics().Report(ModuleNameLoc,
+                                                diag::err_module_not_built)
+          << Module->Name << SourceRange(ImportLoc, ModuleNameLoc);
+    }
+    return ReadResult == ASTReader::Success;
   }
 }
 
@@ -1235,23 +1257,9 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
         return ModuleLoadResult();
       }
 
-      // Try to compile the module.
-      compileModule(*this, ModuleNameLoc, Module, ModuleFileName);
-
-      // Try to read the module file, now that we've compiled it.
-      ASTReader::ASTReadResult ReadResult
-        = ModuleManager->ReadAST(ModuleFileName,
-                                 serialization::MK_Module, ImportLoc,
-                                 ASTReader::ARR_Missing);
-      if (ReadResult != ASTReader::Success) {
-        if (ReadResult == ASTReader::Missing) {
-          getDiagnostics().Report(ModuleNameLoc,
-                                  Module? diag::err_module_not_built
-                                        : diag::err_module_not_found)
-            << ModuleName
-            << SourceRange(ImportLoc, ModuleNameLoc);
-        }
-
+      // Try to compile and then load the module.
+      if (!compileAndLoadModule(*this, ImportLoc, ModuleNameLoc, Module,
+                                ModuleFileName)) {
         if (getPreprocessorOpts().FailedModules)
           getPreprocessorOpts().FailedModules->addFailed(ModuleName);
         KnownModules[Path[0].first] = nullptr;
-- 
2.7.4