[BridgingHeader] Provide a deterministic unique pch file path

Teach swift driver to use a deterministic path for compiled PCH output.
Also when dependency scanner is updated to return a module scanning or
PCH hash from the scanning result, it will add that to the code path to
have an unique PCH file path for the specific compilation.

Author: cachemeifyoucan

Sources/SwiftDriver/Driver/Driver.swift

@@ -336,7 +336,13 @@ public struct Driver {
   let importedObjCHeader: VirtualPath.Handle?
 
   /// The path to the pch for the imported Objective-C header.
-  let bridgingPrecompiledHeader: VirtualPath.Handle?
+  lazy var bridgingPrecompiledHeader: VirtualPath.Handle? = {
+    Self.computeBridgingPrecompiledHeader(&parsedOptions,
+                                          compilerMode: compilerMode,
+                                          importedObjCHeader: importedObjCHeader,
+                                          outputFileMap: outputFileMap,
+                                          planner: explicitDependencyBuildPlanner)
+  }()
 
   /// Path to the dependencies file.
   let dependenciesFilePath: VirtualPath.Handle?
@@ -801,10 +807,6 @@ public struct Driver {
       recordedInputModificationDates: recordedInputModificationDates)
 
     self.importedObjCHeader = try Self.computeImportedObjCHeader(&parsedOptions, compilerMode: compilerMode, diagnosticEngine: diagnosticEngine)
-    self.bridgingPrecompiledHeader = try Self.computeBridgingPrecompiledHeader(&parsedOptions,
-                                                                               compilerMode: compilerMode,
-                                                                               importedObjCHeader: importedObjCHeader,
-                                                                               outputFileMap: outputFileMap)
 
     self.supportedFrontendFlags =
       try Self.computeSupportedCompilerArgs(of: self.toolchain,
@@ -829,7 +831,7 @@ public struct Driver {
         diagnosticsEngine.emit(.warning("-cache-compile-job cannot be used without explicit module build, turn off caching"),
                                location: nil)
         self.enableCaching = false
-      } else if importedObjCHeader != nil && bridgingPrecompiledHeader == nil {
+      } else if importedObjCHeader != nil, !parsedOptions.hasFlag(positive: .enableBridgingPch, negative: .disableBridgingPch, default: true) {
         diagnosticsEngine.emit(.warning("-cache-compile-job cannot be used with -disable-bridging-pch, turn off caching"),
                                location: nil)
         self.enableCaching = false
@@ -1779,7 +1781,7 @@ extension Driver {
   /// The swift-driver doesn't have actions, so the logic here takes the jobs and tries
   /// to mimic the actions that would be created by the C++ driver and
   /// prints them in *hopefully* the same order.
-  private func printActions(_ jobs: [Job]) {
+  private mutating func printActions(_ jobs: [Job]) {
     defer {
       stdoutStream.flush()
     }
@@ -2872,23 +2874,29 @@ extension Driver {
   static func computeBridgingPrecompiledHeader(_ parsedOptions: inout ParsedOptions,
                                                compilerMode: CompilerMode,
                                                importedObjCHeader: VirtualPath.Handle?,
-                                               outputFileMap: OutputFileMap?) throws -> VirtualPath.Handle? {
+                                               outputFileMap: OutputFileMap?,
+                                               planner: ExplicitDependencyBuildPlanner?) -> VirtualPath.Handle? {
     guard compilerMode.supportsBridgingPCH,
       let input = importedObjCHeader,
       parsedOptions.hasFlag(positive: .enableBridgingPch, negative: .disableBridgingPch, default: true) else {
         return nil
     }
 
-    if let outputPath = try outputFileMap?.existingOutput(inputFile: input, outputType: .pch) {
+    if let outputPath = try? outputFileMap?.existingOutput(inputFile: input, outputType: .pch) {
       return outputPath
     }
 
-    let inputFile = VirtualPath.lookup(input)
-    let pchFileName = inputFile.basenameWithoutExt.appendingFileTypeExtension(.pch)
+    let pchFile : String
+    var baseName = VirtualPath.lookup(input).basenameWithoutExt
+    if let contextHash = try? planner?.getMainModuleContextHash() {
+      pchFile = baseName + "-" + contextHash + ".pch"
+    } else {
+      pchFile = baseName.appendingFileTypeExtension(.pch)
+    }
     if let outputDirectory = parsedOptions.getLastArgument(.pchOutputDir)?.asSingle {
-      return try VirtualPath(path: outputDirectory).appending(component: pchFileName).intern()
+      return try? VirtualPath(path: outputDirectory).appending(component: pchFile).intern()
     } else {
-      return try VirtualPath.createUniqueTemporaryFile(RelativePath(validating: pchFileName)).intern()
+      return try? VirtualPath.temporary(RelativePath(validating: pchFile)).intern()
     }
   }
 }

Sources/SwiftDriver/ExplicitModuleBuilds/ExplicitDependencyBuildPlanner.swift

@@ -425,6 +425,13 @@ public typealias ExternalTargetModuleDetailsMap = [ModuleDependencyId: ExternalT
     commandLine.append(contentsOf: mainModuleDependenciesArgs.commandLine)
   }
 
+  /// Get the context hash for the main module.
+  public func getMainModuleContextHash() throws -> String? {
+    let mainModuleId: ModuleDependencyId = .swift(dependencyGraph.mainModuleName)
+    let mainModuleDetails = try dependencyGraph.swiftModuleDetails(of: mainModuleId)
+    return mainModuleDetails.contextHash
+  }
+
   /// Resolve all module dependencies of the main module and add them to the lists of
   /// inputs and command line flags.
   public mutating func resolveBridgingHeaderDependencies(inputs: inout [TypedVirtualPath],

Tests/SwiftDriverTests/CachingBuildTests.swift

@@ -324,7 +324,7 @@ final class CachingBuildTests: XCTestCase {
               let baseName = "testCachingBuildJobs"
               XCTAssertTrue(matchTemporary(outputFilePath, basename: baseName, fileExtension: "o") ||
                             matchTemporary(outputFilePath, basename: baseName, fileExtension: "autolink") ||
-                            matchTemporary(outputFilePath, basename: "Bridging-", fileExtension: "pch"))
+                            matchTemporary(outputFilePath, basename: "Bridging", fileExtension: "pch"))
             default:
               XCTFail("Unexpected module dependency build job output: \(outputFilePath)")
           }