[APIDiff] Control paralellism of api-digester invocations with -j

Author: owenv

Sources/Commands/APIDigester.swift

@@ -138,8 +138,10 @@ struct APIDigesterBaselineDumper {
         // Dump the SDK JSON.
         try localFileSystem.createDirectory(baselineDir, recursive: true)
         let group = DispatchGroup()
+        let semaphore = DispatchSemaphore(value: Int(buildParameters.jobs))
         let errors = ThreadSafeArrayStore<Swift.Error>()
         for module in modulesToDiff {
+            semaphore.wait()
             DispatchQueue.sharedConcurrent.async(group: group) {
                 do {
                     try apiDigesterTool.emitAPIBaseline(
@@ -150,6 +152,7 @@ struct APIDigesterBaselineDumper {
                 } catch {
                     errors.append(error)
                 }
+                semaphore.signal()
             }
         }
         group.wait()

Sources/Commands/SwiftPackageTool.swift

@@ -344,6 +344,7 @@ extension SwiftPackageTool {
 
             let results = ThreadSafeArrayStore<SwiftAPIDigester.ComparisonResult>()
             let group = DispatchGroup()
+            let semaphore = DispatchSemaphore(value: Int(buildOp.buildParameters.jobs))
             var skippedModules: Set<String> = []
 
             for module in modulesToDiff {
@@ -353,6 +354,7 @@ extension SwiftPackageTool {
                     skippedModules.insert(module)
                     continue
                 }
+                semaphore.wait()
                 DispatchQueue.sharedConcurrent.async(group: group) {
                     if let comparisonResult = apiDigesterTool.compareAPIToBaseline(
                         at: moduleBaselinePath,
@@ -361,6 +363,7 @@ extension SwiftPackageTool {
                     ) {
                         results.append(comparisonResult)
                     }
+                    semaphore.signal()
                 }
             }
 

Tests/CommandsTests/APIDiffTests.swift

@@ -65,7 +65,7 @@ final class APIDiffTests: XCTestCase {
             try localFileSystem.writeFileContents(packageRoot.appending(components: "Sources", "Qux", "Qux.swift")) {
                 $0 <<< "public class Qux<T, U> { private let x = 1 }"
             }
-            XCTAssertThrowsError(try execute(["experimental-api-diff", "1.2.3"], packagePath: packageRoot)) { error in
+            XCTAssertThrowsError(try execute(["experimental-api-diff", "1.2.3", "-j", "2"], packagePath: packageRoot)) { error in
                 guard case SwiftPMProductError.executionFailure(error: _, output: let output, stderr: _) = error else {
                     XCTFail("Unexpected error")
                     return