When possible, clone files on Linux and FreeBSD. (swiftlang#1736)

* When possible, clone files on Linux and FreeBSD.

This change adds support for file cloning on Linux and FreeBSD where supported
by the underlying OS and filesystem.

I don't have a great story for unit testing here as if either call fails, we'll
fall back to block-based copying which, in the general case, will appear to the
caller like a successful copy. Furthermore, our CI systems aren't configured
with file systems that support file cloning anyway.

Resolves swiftlang#1727.

* That's not what those are called...

* FICLONE is a complex macro

* Include fs.h in the right place

* Move shims

* Remove duplicate include

Author: grynspan

Sources/FoundationEssentials/FileManager/FileOperations.swift

@@ -872,6 +872,10 @@ enum _FileOperations {
     }
 #else
     #if !canImport(Darwin)
+    #if os(FreeBSD)
+    private static let _freeBSDRelease = getosreldate()
+    #endif
+
     private static func _copyRegularFile(_ srcPtr: UnsafePointer<CChar>, _ dstPtr: UnsafePointer<CChar>, delegate: some LinkOrCopyDelegate) throws {
         let srcfd = open(srcPtr, O_RDONLY)
         guard srcfd >= 0 else {
@@ -906,8 +910,25 @@ enum _FileOperations {
             // no copying required
             return
         }
-        
+
         let total: Int = Int(fileInfo.st_size)
+
+        // Attempt to clone the file using platform-specific API. If this operation fails, don't throw
+        // an error and just fall back to chunked writes.
+        #if os(Linux)
+        if ioctl(dstfd, _filemanager_shims_FICLONE(), srcfd) != -1 {
+            return
+        }
+        #elseif os(FreeBSD)
+        if _freeBSDRelease >= 1500000 {
+            // `COPY_FILE_RANGE_CLONE` was introduced in FreeBSD 15.0.
+            let flags = _filemanager_shims_COPY_FILE_RANGE_CLONE()
+            if copy_file_range(srcfd, nil, dstfd, nil, total, flags) != -1 {
+                return
+            }
+        }
+        #endif
+
         // Respect the optimal block size for the file system if available
         // Some platforms including WASI don't provide this information, so we
         // fall back to the default chunk size 4KB, which is a common page size.

Sources/_FoundationCShims/include/filemanager_shims.h

@@ -47,6 +47,24 @@
 extern int _mkpath_np(const char *path, mode_t omode, const char **firstdir);
 #endif
 
+#if TARGET_OS_LINUX
+#include <linux/fs.h>
+
+static inline unsigned long _filemanager_shims_FICLONE(void) { return FICLONE; }
+#endif
+
+#if TARGET_OS_BSD
+static inline unsigned int _filemanager_shims_COPY_FILE_RANGE_CLONE(void) {
+#if defined(COPY_FILE_RANGE_CLONE)
+  return COPY_FILE_RANGE_CLONE;
+#else
+  // Compiled against an older unistd.h, but presumably running on FreeBSD 15.0
+  // or newer. SEE: https://github.com/freebsd/freebsd-src/blob/main/sys/sys/unistd.h
+  return 0x00800000;
+#endif
+}
+#endif
+
 #if TARGET_OS_ANDROID && __ANDROID_API__ <= 23
 #include <grp.h>
 #include <sys/types.h>