track SDK dependencies (rdar://115777026)

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -645,6 +645,12 @@ public final class SwiftTargetBuildDescription {
             }
         }
 
+        // rdar://115777026
+        // Compiler commands need to track SDK dependencies to trigger rebuilds when the SDK changes
+        if let sdkPath = self.buildParameters.toolchain.sdkRootPath, self.buildParameters.triple.isDarwin() {
+            args += ["-sdk", sdkPath.pathString]
+        }
+
         return args
     }
 

Sources/PackageModel/SwiftSDKs/SwiftSDK.swift

@@ -478,8 +478,8 @@ public struct SwiftSDK: Equatable {
         let sdkPath: AbsolutePath?
         #if os(macOS)
         // Get the SDK.
-        if let value = lookupExecutablePath(filename: ProcessEnv.vars["SDKROOT"]) {
-            sdkPath = value
+        if let value = ProcessEnv.vars["SDKROOT"] {
+            sdkPath = try AbsolutePath(validating: value)
         } else {
             // No value in env, so search for it.
             let sdkPathStr = try TSCBasic.Process.checkNonZeroExit(

Tests/BuildTests/IncrementalBuildTests.swift

@@ -14,6 +14,7 @@ import Basics
 import PackageModel
 import SPMTestSupport
 import XCTest
+import class TSCBasic.Process
 
 /// Functional tests of incremental builds.  These are fairly ad hoc at this
 /// point, and because of the time they take, they need to be kept minimal.
@@ -145,4 +146,28 @@ final class IncrementalBuildTests: XCTestCase {
             XCTAssertNoMatch(log2, .contains("Planning build"))
         }
     }
+    // testing the fix for tracking SDK dependencies to avoid triggering rebuilds when the SDK changes (rdar://115777026)
+    func testSDKTracking() throws {
+#if os(macOS)
+        try XCTSkipIf(!UserToolchain.default.supportsSDKDependentTests(), "skipping because test environment doesn't support this test")
+
+        try fixture(name: "ValidLayouts/SingleModule/Library") { fixturePath in
+            let dummySwiftcPath = SwiftPM.xctestBinaryPath(for: "dummy-swiftc")
+            let swiftCompilerPath = try UserToolchain.default.swiftCompilerPath
+            let environment = [
+                "SWIFT_EXEC": dummySwiftcPath.pathString,
+                "SWIFT_ORIGINAL_PATH": swiftCompilerPath.pathString
+            ]
+            let sdkPathStr = try TSCBasic.Process.checkNonZeroExit(
+                arguments: ["/usr/bin/xcrun", "--sdk", "macosx", "--show-sdk-path"],
+                environment: environment
+            ).spm_chomp()
+
+            let newSdkPathStr = "/tmp/../\(sdkPathStr)"
+            // Perform a full build again because SDK changed.
+            let log1 = try executeSwiftBuild(fixturePath, env: ["SDKROOT": newSdkPathStr]).stdout
+            XCTAssertMatch(log1, .contains("Compiling Library"))
+        }
+#endif
+    }
 }