Linux: Check renameat2() is available in the header files and kernel.

- Add a dummy _CF_renameat2() if SYS_renameat2 is not defined.

- Add a check that the system call is defined and the kernel version
  is high enough (>= 3.15) to call SYS_renameat2().

Author: spevans

CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h

@@ -59,6 +59,7 @@
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #elif TARGET_OS_LINUX
+#include <errno.h>
 #include <features.h>
 
 #if __GLIBC_PREREQ(2, 28) == 0
@@ -594,10 +595,21 @@ _stat_with_btime(const char *filename, struct stat *buffer, struct timespec *bti
 #endif // __NR_statx
 
 static unsigned int const _CF_renameat2_RENAME_EXCHANGE = 1 << 1;
-static int _CF_renameat2(int olddirfd, const char *_Nonnull oldpath,
-                            int newdirfd, const char *_Nonnull newpath, unsigned int flags) {
+#ifdef SYS_renameat2
+static _Bool const _CFHasRenameat2 = 1;
+static inline int _CF_renameat2(int olddirfd, const char *_Nonnull oldpath,
+                                int newdirfd, const char *_Nonnull newpath, unsigned int flags) {
     return syscall(SYS_renameat2, olddirfd, oldpath, newdirfd, newpath, flags);
 }
+#else
+static _Bool const _CFHasRenameat2 = 0;
+static inline int _CF_renameat2(int olddirfd, const char *_Nonnull oldpath,
+                                int newdirfd, const char *_Nonnull newpath, unsigned int flags) {
+    return ENOSYS;
+}
+#endif // __SYS_renameat2
+
+
 #endif // TARGET_OS_LINUX
 
 #if __HAS_STATX

Foundation/FileManager+POSIX.swift

@@ -1185,7 +1185,7 @@ internal func _contentsEqual(atPath path1: String, andPath path2: String) -> Boo
             throw _NSErrorWithErrno(error, reading: false, path: path)
         }
     }
-    
+
     internal func _replaceItem(at originalItemURL: URL, withItemAt newItemURL: URL, backupItemName: String?, options: ItemReplacementOptions = [], allowPlatformSpecificSyscalls: Bool = true) throws -> URL? {
 
         // 1. Make a backup, if asked to.
@@ -1245,11 +1245,21 @@ internal func _contentsEqual(atPath path1: String, andPath path2: String) -> Boo
                     return newItemURL.withUnsafeFileSystemRepresentation { (newItemFS) -> Int32? in
                         if let originalFS = originalFS,
                            let newItemFS = newItemFS {
-                            if _CF_renameat2(AT_FDCWD, originalFS, AT_FDCWD, newItemFS, _CF_renameat2_RENAME_EXCHANGE) == 0 {
-                                return nil
-                            } else {
-                                return errno
-                            }
+
+                                #if os(Linux)
+                                if _CFHasRenameat2 && kernelSupportsRenameat2 {
+                                    if _CF_renameat2(AT_FDCWD, originalFS, AT_FDCWD, newItemFS, _CF_renameat2_RENAME_EXCHANGE) == 0 {
+                                        return nil
+                                    } else {
+                                        return errno
+                                    }
+                                }
+                                #endif
+                                if renameat(AT_FDCWD, originalFS, AT_FDCWD, newItemFS) == 0 {
+                                    return nil
+                                } else {
+                                    return errno
+                                }
                         } else {
                             return Int32(EINVAL)
                         }

Foundation/FileManager.swift

@@ -841,6 +841,12 @@ open class FileManager : NSObject {
         let requiredVersion = OperatingSystemVersion(majorVersion: 4, minorVersion: 11, patchVersion: 0)
         return ProcessInfo.processInfo.isOperatingSystemAtLeast(requiredVersion)
     }()
+
+    // renameat2() is only supported by Linux kernels >= 3.15
+    internal lazy var kernelSupportsRenameat2: Bool = {
+        let requiredVersion = OperatingSystemVersion(majorVersion: 3, minorVersion: 15, patchVersion: 0)
+        return ProcessInfo.processInfo.isOperatingSystemAtLeast(requiredVersion)
+    }()
 #endif
 
     /* -contentsEqualAtPath:andPath: does not take into account data stored in the resource fork or filesystem extended attributes.