[DependencyScan] Prevent command-line flags added again on re-scan

Add a flag `finalized` to indicate that a module entry in the dependency
cache is finalized and no longer needs to be updated. This prevents the
command-line flags from dependency inputs get added multiple times on
re-scan with the same service.

While during normal compilation, adding the same command-line flags
multiple times are fine, it is bad for caching builds as a new
compilation cache key needs to be computed every time.

Author: cachemeifyoucan

include/swift/AST/ModuleDependencies.h

@@ -116,7 +116,7 @@ class ModuleDependencyInfoStorageBase {
                                   const std::vector<std::string> &moduleImports,
                                   StringRef moduleCacheKey = "")
       : dependencyKind(dependencyKind), moduleImports(moduleImports),
-        moduleCacheKey(moduleCacheKey.str()), resolved(false)  {}
+        moduleCacheKey(moduleCacheKey.str()), resolved(false), finalized(false)  {}
 
   virtual ModuleDependencyInfoStorageBase *clone() const = 0;
 
@@ -138,6 +138,7 @@ class ModuleDependencyInfoStorageBase {
   std::string moduleCacheKey;
 
   bool resolved;
+  bool finalized;
 };
 
 struct CommonSwiftTextualModuleDependencyDetails {
@@ -604,6 +605,13 @@ class ModuleDependencyInfo {
     storage->resolved = isResolved;
   }
 
+  bool isFinalized() const {
+    return storage->finalized;
+  }
+  void setIsFinalized(bool isFinalized) {
+    storage->finalized = isFinalized;
+  }
+
   /// For a Source dependency, register a `Testable` import
   void addTestableImport(ImportPath::Module module);
 

lib/DependencyScan/ScanDependencies.cpp

@@ -302,6 +302,10 @@ static llvm::Error resolveExplicitModuleInputs(
   if (moduleID.second == ModuleDependencyKind::SwiftPlaceholder)
     return llvm::Error::success();
 
+  // If the dependency is already finalized, nothing needs to be done.
+  if (resolvingDepInfo.isFinalized())
+    return llvm::Error::success();
+
   std::vector<std::string> rootIDs;
   if (auto ID = resolvingDepInfo.getCASFSRootID())
     rootIDs.push_back(*ID);
@@ -505,6 +509,7 @@ static llvm::Error resolveExplicitModuleInputs(
         return E;
     }
   }
+  dependencyInfoCopy.setIsFinalized(true);
   cache.updateDependency(moduleID, dependencyInfoCopy);
 
   return llvm::Error::success();