[SwiftScanCAS] Make sure that CAS size limitation can take effect

Previously the last CAS instance that was closing the database was not the one
that was getting the CAS size limitation setting, resulting in the limit not being in effect.

rdar://148836977
(cherry picked from commit 7134aa1)

Author: akyrtzi

Sources/SwiftDriver/SwiftScan/SwiftScanCAS.swift

@@ -170,9 +170,18 @@ public final class CacheReplayResult {
 
 public final class SwiftScanCAS {
   let cas: swiftscan_cas_t
-  private let scanner: SwiftScan
+  private var scanner: SwiftScan!
   deinit {
-    scanner.api.swiftscan_cas_dispose(cas)
+    // FIXME: `cas` needs to be disposed after `scanner`. This is because `scanner` contains a separate
+    // CAS instance contained in `clang::CASOptions` but `cas` is the one exposed to the build system
+    // and the one that a size limit is set on. When the `scanner` is disposed last then it's the last
+    // instance closing the database and it doesn't impose any size limit.
+    //
+    // This is extremely fragile, a proper fix would be to either eliminate the extra CAS instance
+    // from `scanner` or have the `scanner`'s CAS instance exposed to the build system.
+    let swiftscan_cas_dispose = scanner.api.swiftscan_cas_dispose!
+    scanner = nil
+    swiftscan_cas_dispose(cas)
   }
 
   init(cas: swiftscan_cas_t, scanner: SwiftScan) {

Tests/SwiftDriverTests/CachingBuildTests.swift

@@ -1124,4 +1124,92 @@ final class CachingBuildTests: XCTestCase {
       try cas.prune()
     }
   }
+
+  func testCASSizeLimiting() throws {
+    try withTemporaryDirectory { path in
+      let moduleCachePath = path.appending(component: "ModuleCache")
+      let casPath = path.appending(component: "cas")
+      try localFileSystem.createDirectory(moduleCachePath)
+
+      let main1 = path.appending(component: "testCachingBuild1.swift")
+      try localFileSystem.writeFileContents(main1) { $0.send("let x = 1") }
+      let main2 = path.appending(component: "testCachingBuild2.swift")
+      try localFileSystem.writeFileContents(main2) { $0.send("let x = 1") }
+
+      let cHeadersPath: AbsolutePath =
+          try testInputsPath.appending(component: "ExplicitModuleBuilds")
+                            .appending(component: "CHeaders")
+      let swiftModuleInterfacesPath: AbsolutePath =
+          try testInputsPath.appending(component: "ExplicitModuleBuilds")
+                            .appending(component: "Swift")
+      let sdkArgumentsForTesting = (try? Driver.sdkArgumentsForTesting()) ?? []
+
+      func createDriver(main: AbsolutePath) throws -> Driver {
+        return try Driver(args: ["swiftc",
+                                 "-I", cHeadersPath.nativePathString(escaped: true),
+                                 "-I", swiftModuleInterfacesPath.nativePathString(escaped: true),
+                                 "-explicit-module-build", "-Rcache-compile-job",
+                                 "-module-cache-path", moduleCachePath.nativePathString(escaped: true),
+                                 "-cache-compile-job", "-cas-path", casPath.nativePathString(escaped: true),
+                                 "-working-directory", path.nativePathString(escaped: true),
+                                 main.nativePathString(escaped: true)] + sdkArgumentsForTesting)
+      }
+
+      func buildAndGetSwiftCASKeys(main: AbsolutePath, forceCASLimit: Bool) throws -> [String] {
+        var driver = try createDriver(main: main)
+        let cas = try XCTUnwrap(driver.cas)
+        if forceCASLimit {
+          try cas.setSizeLimit(10)
+        }
+        let jobs = try driver.planBuild()
+        try driver.run(jobs: jobs)
+        XCTAssertFalse(driver.diagnosticEngine.hasErrors)
+
+        let dependencyOracle = driver.interModuleDependencyOracle
+
+        let scanLibPath = try XCTUnwrap(driver.getSwiftScanLibPath())
+        try dependencyOracle.verifyOrCreateScannerInstance(swiftScanLibPath: scanLibPath)
+
+        var keys: [String] = []
+        for job in jobs {
+          guard job.kind.supportCaching else { continue }
+          for (path, key) in job.outputCacheKeys {
+            if path.type == .swift {
+              keys.append(key)
+            }
+          }
+        }
+        return keys
+      }
+
+      func verifyKeys(exist: Bool, keys: [String], main: AbsolutePath, file: StaticString = #file, line: UInt = #line) throws {
+        let driver = try createDriver(main: main)
+        let cas = try XCTUnwrap(driver.cas)
+        for key in keys {
+          let comp = try cas.queryCacheKey(key, globally: false)
+          if exist {
+            XCTAssertNotNil(comp, file: file, line: line)
+          } else {
+            XCTAssertNil(comp, file: file, line: line)
+          }
+        }
+      }
+
+      do {
+        // Without CAS size limitation the keys will be preserved.
+        let keys = try buildAndGetSwiftCASKeys(main: main1, forceCASLimit: false)
+        _ = try buildAndGetSwiftCASKeys(main: main2, forceCASLimit: false)
+        try verifyKeys(exist: true, keys: keys, main: main1)
+      }
+
+      try localFileSystem.removeFileTree(casPath)
+
+      do {
+        // 2 separate builds with CAS size limiting, the keys of first build will not be preserved.
+        let keys = try buildAndGetSwiftCASKeys(main: main1, forceCASLimit: true)
+        _ = try buildAndGetSwiftCASKeys(main: main2, forceCASLimit: true)
+        try verifyKeys(exist: false, keys: keys, main: main1)
+      }
+    }
+  }
 }