Avoid error if missing xattr support (#690)

When building on a darwin host (e.g. macOS) using a file-system that
doesn't support xattrs (e.g. tmpfs) swift-build would fail trying to set
the "CreatedByBuildSystem" xattribute. This commit changes swift-build
to warn instead, allowing the build to complete.

Author: rauhul

Sources/SWBCore/WorkspaceContext.swift

@@ -401,13 +401,21 @@ extension FSProxy {
         return try getExtendedAttribute(path, key: Self.CreatedByBuildSystemAttribute) == Self.CreatedByBuildSystemAttributeOnValue
     }
 
+    /// Sets the "CreatedByBuildSystem" extended attribute on the specified
+    /// path.
+    ///
+    /// - Important: The caller is responsible for catching and handling
+    ///   failures if the filesystem does not support extended attributes. Many
+    ///   filesystems (e.g. various non-ext4 temporary filesystems on Linux)
+    ///   don't support xattrs and will return `ENOTSUP`. In particular, tmpfs
+    ///   doesn't support xattrs on Linux unless `CONFIG_TMPFS_XATTR` is enabled
+    ///   in the kernel config.
+    ///
+    /// - Parameter path: The path on which to set the extended attribute.
+    /// - Throws: An error if the operation fails, including when the filesystem
+    ///   doesn't support extended attributes.
     public func setCreatedByBuildSystemAttribute(_ path: Path) throws {
-        // Many filesystems on other platforms (e.g. various non-ext4 temporary filesystems on Linux) don't support xattrs and will return ENOTSUP.
-        // In particular, tmpfs doesn't support xattrs on Linux unless `CONFIG_TMPFS_XATTR` is enabled in the kernel config.
-        // FIXME: Detect whether the FS supports xattrs at runtime
-        #if canImport(Darwin)
         try setExtendedAttribute(path, key: Self.CreatedByBuildSystemAttribute, value: Self.CreatedByBuildSystemAttributeOnValue)
-        #endif
     }
 
     public func commandLineArgumentsToApplyCreatedByBuildSystemAttribute(to path: Path) -> [String] {

Sources/SWBTaskExecution/TaskActions/CreateBuildDirectoryTaskAction.swift

@@ -28,12 +28,22 @@ public final class CreateBuildDirectoryTaskAction: TaskAction {
 
         do {
             try fs.createDirectory(directoryPath, recursive: true)
-            try fs.setCreatedByBuildSystemAttribute(directoryPath)
-            return .succeeded
         } catch {
             outputDelegate?.emitError(error.localizedDescription)
             return .failed
         }
+
+        // Warn on Darwin if we are unable to set the "CreatedByBuildSystem"
+        // attribute
+        do {
+            try fs.setCreatedByBuildSystemAttribute(directoryPath)
+        } catch {
+            #if canImport(Darwin)
+            outputDelegate?.emitWarning("Failed to set build system attribute on \(directoryPath.str): \(error.localizedDescription)")
+            #endif
+        }
+
+        return .succeeded
     }
 
     public override func performTaskAction(