From 0e62ad88e8de985b52d3686b69eec0c85e903c87 Mon Sep 17 00:00:00 2001
From: Adam Sharp <adam@sharplet.me>
Date: Sat, 29 May 2021 11:34:00 -0400
Subject: [PATCH] Add non-throwing FileDescriptor.currentOffset helper

Provided the file descriptor is still valid, accessing the current
offset shouldn't fail.
---
 Sources/System/FileOperations.swift        |  6 ++++++
 Tests/SystemTests/FileOperationsTest.swift | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/Sources/System/FileOperations.swift b/Sources/System/FileOperations.swift
index e2332eee..f493b52e 100644
--- a/Sources/System/FileOperations.swift
+++ b/Sources/System/FileOperations.swift
@@ -116,6 +116,12 @@ extension FileDescriptor {
     try _seek(offset: offset, from: whence).get()
   }
 
+  /// Return the current offset for the given file descriptor.
+  @_alwaysEmitIntoClient
+  public var currentOffset: Int64 {
+    try! _seek(offset: 0, from: .current).get()
+  }
+
   @usableFromInline
   internal func _seek(
     offset: Int64, from whence: FileDescriptor.SeekOrigin
diff --git a/Tests/SystemTests/FileOperationsTest.swift b/Tests/SystemTests/FileOperationsTest.swift
index f39a052a..211f889d 100644
--- a/Tests/SystemTests/FileOperationsTest.swift
+++ b/Tests/SystemTests/FileOperationsTest.swift
@@ -90,6 +90,17 @@ final class FileOperationsTest: XCTestCase {
     // TODO: Test writeAll, writeAll(toAbsoluteOffset), closeAfter
   }
 
+  func testCurrentOffset() throws {
+    let fd = try FileDescriptor.open("/tmp/a.txt", .readWrite, options: [.create, .truncate], permissions: .ownerReadWrite)
+    XCTAssertEqual(fd.currentOffset, 0)
+
+    try fd.writeAll("abc".utf8)
+    XCTAssertEqual(fd.currentOffset, 3)
+
+    try fd.seek(offset: -1, from: .current)
+    XCTAssertEqual(fd.currentOffset, 2)
+  }
+
   func testAdHocOpen() {
     // Ad-hoc test touching a file system.
     do {