SR-4993: Read zero length files on Linux

- Some files (eg in /proc) report a filesize of zero and need to
  be read into a buffer instead of using mmap().

Author: spevans

Foundation/NSData.swift

@@ -146,6 +146,7 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
     public convenience init(bytesNoCopy bytes: UnsafeMutableRawPointer, length: Int, deallocator: ((UnsafeMutableRawPointer, Int) -> Void)? = nil) {
         self.init(bytes: bytes, length: length, copy: false, deallocator: deallocator)
     }
+
     public convenience init(contentsOfFile path: String, options readOptionsMask: ReadingOptions = []) throws {
         let readResult = try NSData.readBytesFromFileWithExtendedAttributes(path, options: readOptionsMask)
         self.init(bytes: readResult.bytes, length: readResult.length, copy: false, deallocator: readResult.deallocator)
@@ -380,7 +381,13 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
         }
 
         let length = Int(info.st_size)
-        
+
+#if os(Linux)
+        if length == 0 && (info.st_mode & S_IFMT == S_IFREG) {
+            return try readZeroSizeFile(fd)
+        }
+#endif
+
         if options.contains(.alwaysMapped) {
             let data = mmap(nil, length, PROT_READ, MAP_PRIVATE, fd, 0)
 
@@ -414,6 +421,39 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
             free(buffer)
         }
     }
+
+#if os(Linux)
+    internal static func readZeroSizeFile(_ fd: Int32) throws -> NSDataReadResult {
+        let blockSize = 1024 * 1024 // 1MB
+        var data: UnsafeMutableRawPointer? = nil
+        var bytesRead = 0
+        var amt = 0
+
+        repeat {
+            data = realloc(data, bytesRead + blockSize)
+            amt = read(fd, data!.advanced(by: bytesRead), blockSize)
+
+            // Dont continue on EINTR or EAGAIN as the file position may not
+            // have changed, see read(2).
+            if amt < 0 {
+                free(data!)
+                throw NSError(domain: NSPOSIXErrorDomain, code: Int(errno), userInfo: nil)
+            }
+            bytesRead += amt
+        } while amt > 0
+
+        if bytesRead == 0 {
+            free(data!)
+            data = malloc(0)
+        } else {
+            data = realloc(data, bytesRead) // shrink down the allocated block.
+        }
+
+        return NSDataReadResult(bytes: data!, length: bytesRead) { buffer, length in
+            free(buffer)
+        }
+    }
+#endif
 
     internal func makeTemporaryFile(inDirectory dirPath: String) throws -> (Int32, String) {
         let template = dirPath._nsObject.appendingPathComponent("tmp.XXXXXX")

TestFoundation/TestNSData.swift

@@ -37,6 +37,8 @@ class TestNSData: XCTestCase {
             ("test_base64Data_medium", test_base64Data_medium),
             ("test_base64Data_small", test_base64Data_small),
             ("test_openingNonExistentFile", test_openingNonExistentFile),
+            ("test_contentsOfFile", test_contentsOfFile),
+            ("test_contentsOfZeroFile", test_contentsOfZeroFile),
             ("test_basicReadWrite", test_basicReadWrite),
             ("test_bufferSizeCalculation", test_bufferSizeCalculation),
             // ("test_dataHash", test_dataHash),   Disabled due to lack of brdiging in swift runtime -- infinite loops
@@ -908,6 +910,45 @@ extension TestNSData {
         XCTAssertTrue(didCatchError)
     }
 
+    func test_contentsOfFile() {
+        let testDir = testBundle().resourcePath
+        let filename = testDir!.appending("/NSStringTestData.txt")
+
+        let contents = NSData(contentsOfFile: filename)
+        XCTAssertNotNil(contents)
+        if let contents = contents {
+            let ptr =  UnsafeMutableRawPointer(mutating: contents.bytes)
+            let str = String(bytesNoCopy: ptr, length: contents.length,
+                         encoding: .ascii, freeWhenDone: false)
+            XCTAssertEqual(str, "swift-corelibs-foundation")
+        }
+    }
+
+    func test_contentsOfZeroFile() {
+#if os(Linux)
+        let contents = NSData(contentsOfFile: "/proc/self/cmdline")
+        XCTAssertNotNil(contents)
+        if let contents = contents {
+            XCTAssertTrue(contents.length > 0)
+            let ptr = UnsafeMutableRawPointer(mutating: contents.bytes)
+            let str = String(bytesNoCopy: ptr, length: contents.length,
+                             encoding: .ascii, freeWhenDone: false)
+            XCTAssertNotNil(str)
+            if let str = str {
+                XCTAssertTrue(str.hasSuffix("TestFoundation"))
+            }
+        }
+
+        do {
+            let maps = try String(contentsOfFile: "/proc/self/maps", encoding: .utf8)
+            XCTAssertTrue(maps.characters.count > 0)
+            print(maps)
+        } catch {
+            XCTFail()
+        }
+#endif
+    }
+
     func test_basicReadWrite() {
         let url = URL(fileURLWithPath: NSTemporaryDirectory(), isDirectory: true).appendingPathComponent("testfile")
         let count = 1 << 24