Fix overlapping plugins/products build database access

Sources/Build/BuildOperation.swift

@@ -455,9 +455,16 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         let prebuildCommandResults: [ResolvedTarget.ID: [PrebuildCommandResult]]
         // Invoke any build tool plugins in the graph to generate prebuild commands and build commands.
         if let pluginConfiguration, !self.productsBuildParameters.shouldSkipBuilding {
+            // Hacky workaround for rdar://120560817, but it replicates precisely enough the original behavior before
+            // products/tools build parameters were split. Ideally we want to have specify the correct path at the time
+            // when `toolsBuildParameters` is initialized, but we have too many places in the codebase where that's
+            // done, which makes it hard to realign them all at once.
+            var pluginsBuildParameters = self.toolsBuildParameters
+            pluginsBuildParameters.dataPath = pluginsBuildParameters.dataPath.parentDirectory.appending(components: ["plugins", "tools"])
             let buildOperationForPluginDependencies = BuildOperation(
-                productsBuildParameters: self.productsBuildParameters,
-                toolsBuildParameters: self.toolsBuildParameters,
+                // FIXME: this doesn't maintain the products/tools split cleanly
+                productsBuildParameters: pluginsBuildParameters,
+                toolsBuildParameters: pluginsBuildParameters,
                 cacheBuildManifest: false,
                 packageGraphLoader: { return graph },
                 additionalFileRules: self.additionalFileRules,
@@ -469,7 +476,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
             )
             buildToolPluginInvocationResults = try graph.invokeBuildToolPlugins(
                 outputDir: pluginConfiguration.workDirectory.appending("outputs"),
-                buildParameters: self.toolsBuildParameters,
+                buildParameters: pluginsBuildParameters,
                 additionalFileRules: self.additionalFileRules,
                 toolSearchDirectories: [self.toolsBuildParameters.toolchain.swiftCompilerPath.parentDirectory],
                 pkgConfigDirectories: self.pkgConfigDirectories,

Tests/BuildTests/PluginsBuildPlanTests.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2014-2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+import SPMTestSupport
+import XCTest
+
+final class PluginsBuildPlanTests: XCTestCase {
+    func testBuildToolsDatabasePath() throws {
+        try fixture(name: "Miscellaneous/Plugins/MySourceGenPlugin") { fixturePath in
+            let (stdout, stderr) = try executeSwiftBuild(fixturePath)
+            XCTAssertMatch(stdout, .contains("Build complete!"))
+            XCTAssertTrue(localFileSystem.exists(fixturePath.appending(RelativePath(".build/plugins/tools/build.db"))))
+        }
+    }
+}