[WIP] Expose a SwiftPM BSP interface based on Swift Build

Fixtures/Miscellaneous/VersionSpecificManifest/Foo.swift

@@ -0,0 +1,3 @@
+public func foo() {
+	{}()
+}

Fixtures/Miscellaneous/VersionSpecificManifest/Package.swift

@@ -0,0 +1,12 @@
+// swift-tools-version:6.0
+import PackageDescription
+
+let package = Package(
+    name: "Foo",
+    products: [
+        .library(name: "Foo", targets: ["Foo"]),
+    ],
+    targets: [
+        .target(name: "Foo", path: "./"),
+    ]
+)

Fixtures/Miscellaneous/VersionSpecificManifest/Package@swift-5.0.swift

@@ -0,0 +1,12 @@
+// swift-tools-version:5.2
+import PackageDescription
+
+let package = Package(
+    name: "Foo",
+    products: [
+        .library(name: "Foo", targets: ["Foo"]),
+    ],
+    targets: [
+        .target(name: "Foo", path: "./"),
+    ]
+)

Package.swift

@@ -575,6 +575,23 @@ let package = Package(
             ]
         ),
 
+        // MARK: BSP
+        .target(
+            name: "SwiftPMBuildServer",
+            dependencies: [
+                "Basics",
+                "Build",
+                "PackageGraph",
+                "PackageLoading",
+                "PackageModel",
+                "SPMBuildCore",
+                "SourceControl",
+                "SourceKitLSPAPI",
+                "SwiftBuildSupport",
+                .product(name: "SWBBuildServerProtocol", package: "swift-build"),
+            ] + swiftTSCBasicsDeps
+        ),
+
         // MARK: Commands
 
         .target(
@@ -613,6 +630,7 @@ let package = Package(
                 "XCBuildSupport",
                 "SwiftBuildSupport",
                 "SwiftFixIt",
+                "SwiftPMBuildServer",
             ] + swiftSyntaxDependencies(["SwiftIDEUtils", "SwiftRefactor"]),
             exclude: ["CMakeLists.txt", "README.md"],
             swiftSettings: swift6CompatibleExperimentalFeatures + [
@@ -1053,6 +1071,14 @@ if ProcessInfo.processInfo.environment["SWIFTCI_DISABLE_SDK_DEPENDENT_TESTS"] ==
                 "dummy-swiftc",
             ]
         ),
+        .testTarget(
+            name: "SwiftPMBuildServerTests",
+            dependencies: [
+                "SwiftPMBuildServer",
+                "_InternalTestSupport",
+                .product(name: "SWBBuildServerProtocol", package: "swift-build"),
+            ]
+        ),
     ])
 }
 

Sources/Commands/PackageCommands/BuildServer.swift

@@ -0,0 +1,76 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2025 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 ArgumentParser
+import TSCBasic
+import SWBBuildServerProtocol
+import CoreCommands
+import Foundation
+import PackageGraph
+import SwiftPMBuildServer
+import SPMBuildCore
+import SwiftBuildSupport
+
+struct BuildServer: AsyncSwiftCommand {
+    static let configuration = CommandConfiguration(
+        commandName: "build-server",
+        abstract: "Launch a build server for Swift Packages"
+    )
+
+    @OptionGroup(visibility: .hidden)
+    var globalOptions: GlobalOptions
+
+    func run(_ swiftCommandState: SwiftCommandState) async throws {
+        // Dup stdout and redirect the fd to stderr so that a careless print()
+        // will not break our connection stream.
+        let realStdout = dup(STDOUT_FILENO)
+        if realStdout == -1 {
+            fatalError("failed to dup stdout: \(strerror(errno)!)")
+        }
+        if dup2(STDERR_FILENO, STDOUT_FILENO) == -1 {
+            fatalError("failed to redirect stdout -> stderr: \(strerror(errno)!)")
+        }
+
+        let realStdoutHandle = FileHandle(fileDescriptor: realStdout, closeOnDealloc: false)
+
+        let clientConnection = JSONRPCConnection(
+            name: "client",
+            protocol: bspRegistry,
+            inFD: FileHandle.standardInput,
+            outFD: realStdoutHandle,
+            inputMirrorFile: nil,
+            outputMirrorFile: nil
+        )
+
+        guard let buildSystem = try await swiftCommandState.createBuildSystem() as? SwiftBuildSystem else {
+            print("Build server requires --build-system swiftbuild")
+            Self.exit()
+        }
+
+        guard let packagePath = try swiftCommandState.getWorkspaceRoot().packages.first else {
+            throw StringError("unknown package")
+        }
+
+        let server = try await SwiftPMBuildServer(packageRoot: packagePath, buildSystem: buildSystem, workspace: swiftCommandState.getActiveWorkspace(), connectionToClient: clientConnection, exitHandler: {_ in Self.exit() })
+        clientConnection.start(
+            receiveHandler: server,
+            closeHandler: {
+                Self.exit()
+            }
+        )
+
+        // Park the main function by sleeping for 10 years.
+        while true {
+            try? await Task.sleep(for: .seconds(60 * 60 * 24 * 365 * 10))
+        }
+    }
+}

Sources/Commands/PackageCommands/SwiftPackageCommand.swift

@@ -39,6 +39,7 @@ public struct SwiftPackageCommand: AsyncParsableCommand {
             AddTargetDependency.self,
             AddSetting.self,
             AuditBinaryArtifact.self,
+            BuildServer.self,
             Clean.self,
             PurgeCache.self,
             Reset.self,

Sources/SwiftBuildSupport/SwiftBuildSystem.swift

@@ -109,7 +109,9 @@ func withSession(
     }
 }
 
-private final class PlanningOperationDelegate: SWBPlanningOperationDelegate, Sendable {
+package final class SwiftBuildSystemPlanningOperationDelegate: SWBPlanningOperationDelegate, SWBIndexingDelegate, Sendable {
+    package init() {}
+
     public func provisioningTaskInputs(
         targetGUID: String,
         provisioningSourceData: SWBProvisioningTaskInputsSourceData
@@ -194,7 +196,7 @@ public struct PluginConfiguration {
 }
 
 public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
-    private let buildParameters: BuildParameters
+    package let buildParameters: BuildParameters
     private let packageGraphLoader: () async throws -> ModulesGraph
     private let packageManagerResourcesDirectory: Basics.AbsolutePath?
     private let logLevel: Basics.Diagnostic.Severity
@@ -349,7 +351,7 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
             )
         }
 
-        try await writePIF(buildParameters: buildParameters)
+        try await writePIF()
 
         return try await startSWBuildOperation(
             pifTargetName: subset.pifTargetName,
@@ -545,7 +547,7 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
 
                     let operation = try await session.createBuildOperation(
                         request: request,
-                        delegate: PlanningOperationDelegate()
+                        delegate: SwiftBuildSystemPlanningOperationDelegate()
                     )
 
                     var buildState = BuildState()
@@ -631,7 +633,7 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         )
     }
 
-    private func makeBuildParameters(session: SWBBuildServiceSession, genSymbolGraph: Bool) async throws -> SwiftBuild.SWBBuildParameters {
+    package func makeBuildParameters(session: SWBBuildServiceSession, genSymbolGraph: Bool) async throws -> SwiftBuild.SWBBuildParameters {
         // Generate the run destination parameters.
         let runDestination = makeRunDestination()
 
@@ -767,8 +769,8 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
             buildProductsPath: ddPathPrefix + "/Products",
             buildIntermediatesPath: ddPathPrefix + "/Intermediates.noindex",
             pchPath: ddPathPrefix + "/PCH",
-            indexRegularBuildProductsPath: nil,
-            indexRegularBuildIntermediatesPath: nil,
+            indexRegularBuildProductsPath: ddPathPrefix + "/Index/Products",
+            indexRegularBuildIntermediatesPath: ddPathPrefix + "/Index/Intermediates.noindex",
             indexPCHPath: ddPathPrefix,
             indexDataStoreFolderPath: ddPathPrefix,
             indexEnableDataStore: request.parameters.arenaInfo?.indexEnableDataStore ?? false
@@ -902,7 +904,9 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         }
     }
 
-    public func writePIF(buildParameters: BuildParameters) async throws {
+    public func writePIF() async throws {
+        pifBuilder = .init()
+        packageGraph = .init()
         let pifBuilder = try await getPIFBuilder()
         let pif = try await pifBuilder.generatePIF(
             printPIFManifestGraphviz: buildParameters.printPIFManifestGraphviz,
@@ -912,6 +916,27 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         try self.fileSystem.writeIfChanged(path: buildParameters.pifManifest, string: pif)
     }
 
+    package struct LongLivedBuildServiceSession {
+        package var session: SWBBuildServiceSession
+        package var diagnostics: [SwiftBuildMessage.DiagnosticInfo]
+        package var teardownHandler: () async throws -> Void
+    }
+
+    package func createLongLivedSession(name: String) async throws -> LongLivedBuildServiceSession {
+        let service = try await SWBBuildService(connectionMode: .inProcessStatic(swiftbuildServiceEntryPoint))
+        do {
+            let (session, diagnostics) = try await createSession(service: service, name: name, toolchainPath: buildParameters.toolchain.toolchainDir, packageManagerResourcesDirectory: packageManagerResourcesDirectory)
+            let teardownHandler = {
+                try await session.close()
+                await service.close()
+            }
+            return LongLivedBuildServiceSession(session: session, diagnostics: diagnostics, teardownHandler: teardownHandler)
+        } catch {
+            await service.close()
+            throw error
+        }
+    }
+
     public func cancel(deadline: DispatchTime) throws {}
 
     /// Returns the package graph using the graph loader closure.

Sources/SwiftPMBuildServer/DisableSigpipe.swift

@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if canImport(Glibc)
+import Glibc
+#elseif canImport(Musl)
+import Musl
+#elseif canImport(Android)
+import Android
+#endif
+
+#if canImport(Glibc) || canImport(Musl) || canImport(Android)
+// This is a lazily initialised global variable that when read for the first time, will ignore SIGPIPE.
+private let globallyIgnoredSIGPIPE: Bool = {
+  /* no F_SETNOSIGPIPE on Linux :( */
+  _ = signal(SIGPIPE, SIG_IGN)
+  return true
+}()
+#endif
+
+/// We receive a `SIGPIPE` if we write to a pipe that points to a crashed process. This in particular happens if the
+/// target of a `JSONRPCConnection` has crashed and we try to send it a message or if swift-format crashes and we try
+/// to send the source file to it.
+///
+/// On Darwin, `DispatchIO` ignores `SIGPIPE` for the pipes handled by it and swift-tools-support-core offers
+/// `LocalFileOutputByteStream.disableSigpipe`, but that features is not available on Linux.
+///
+/// Instead, globally ignore `SIGPIPE` on Linux to prevent us from crashing if the `JSONRPCConnection`'s target crashes.
+///
+/// On Darwin platforms and on Windows this is a no-op.
+package func globallyDisableSigpipeIfNeeded() {
+  #if !canImport(Darwin) && !os(Windows)
+  let haveWeIgnoredSIGPIEThisIsHereToTriggerIgnoringIt = globallyIgnoredSIGPIPE
+  guard haveWeIgnoredSIGPIEThisIsHereToTriggerIgnoringIt else {
+    fatalError("globallyIgnoredSIGPIPE should always be true")
+  }
+  #endif
+}