lib/std/posix: test ftruncate via std.fs.File.setEndPos()

Add a test for std.fs.File's `setEndPos` (which is a simple wrapper around
`std.posix.ftruncate`) to exercise some success and failure paths.

Explicitly check that the `ftruncate` length isn't negative when
interpreted as a signed value.  This avoids having to decode overloaded
`EINVAL` errors.

Add errno handling to Windows path to map INVALID_PARAMETER to FileTooBig.

Fixes #22960

lib/std/fs/test.zig

@@ -1391,6 +1391,60 @@ test "pwritev, preadv" {
     try testing.expectEqualStrings(&buf2, "line1\n");
 }
 
+test "setEndPos" {
+    // https://github.com/ziglang/zig/issues/20747 (open fd does not have write permission)
+    if (native_os == .wasi and builtin.link_libc) return error.SkipZigTest;
+
+    var tmp = tmpDir(.{});
+    defer tmp.cleanup();
+
+    const file_name = "afile.txt";
+    try tmp.dir.writeFile(.{ .sub_path = file_name, .data = "ninebytes" });
+    const f = try tmp.dir.openFile(file_name, .{ .mode = .read_write });
+    defer f.close();
+
+    const initial_size = try f.getEndPos();
+    var buffer: [32]u8 = undefined;
+
+    {
+        try f.setEndPos(initial_size);
+        try testing.expectEqual(initial_size, try f.getEndPos());
+        try testing.expectEqual(initial_size, try f.preadAll(&buffer, 0));
+        try testing.expectEqualStrings("ninebytes", buffer[0..@intCast(initial_size)]);
+    }
+
+    {
+        const larger = initial_size + 4;
+        try f.setEndPos(larger);
+        try testing.expectEqual(larger, try f.getEndPos());
+        try testing.expectEqual(larger, try f.preadAll(&buffer, 0));
+        try testing.expectEqualStrings("ninebytes\x00\x00\x00\x00", buffer[0..@intCast(larger)]);
+    }
+
+    {
+        const smaller = initial_size - 5;
+        try f.setEndPos(smaller);
+        try testing.expectEqual(smaller, try f.getEndPos());
+        try testing.expectEqual(smaller, try f.preadAll(&buffer, 0));
+        try testing.expectEqualStrings("nine", buffer[0..@intCast(smaller)]);
+    }
+
+    try f.setEndPos(0);
+    try testing.expectEqual(0, try f.getEndPos());
+    try testing.expectEqual(0, try f.preadAll(&buffer, 0));
+
+    // Invalid file length should error gracefully. Actual limit is host
+    // and file-system dependent, but 1PB should fail most everywhere.
+    // Except MacOS APFS limit is 8 exabytes.
+    f.setEndPos(0x4_0000_0000_0000) catch |err| if (err != error.FileTooBig) {
+        return err;
+    };
+
+    try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u63))); // Maximum signed value
+
+    try testing.expectError(error.FileTooBig, f.setEndPos(std.math.maxInt(u64)));
+}
+
 test "access file" {
     try testWithAllSupportedPathTypes(struct {
         fn impl(ctx: *TestContext) !void {

lib/std/posix.zig

@@ -1037,11 +1037,15 @@ pub const TruncateError = error{
     PermissionDenied,
 } || UnexpectedError;
 
+/// Length must be positive when treated as an i64.
 pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
+    const signed_len: i64 = @bitCast(length);
+    if (signed_len < 0) return error.FileTooBig; // avoid ambiguous EINVAL errors
+
     if (native_os == .windows) {
         var io_status_block: windows.IO_STATUS_BLOCK = undefined;
         var eof_info = windows.FILE_END_OF_FILE_INFORMATION{
-            .EndOfFile = @bitCast(length),
+            .EndOfFile = signed_len,
         };
 
         const rc = windows.ntdll.NtSetInformationFile(
@@ -1057,6 +1061,7 @@ pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
             .INVALID_HANDLE => unreachable, // Handle not open for writing
             .ACCESS_DENIED => return error.AccessDenied,
             .USER_MAPPED_FILE => return error.AccessDenied,
+            .INVALID_PARAMETER => return error.FileTooBig,
             else => return windows.unexpectedStatus(rc),
         }
     }
@@ -1069,7 +1074,7 @@ pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
             .PERM => return error.PermissionDenied,
             .TXTBSY => return error.FileBusy,
             .BADF => unreachable, // Handle not open for writing
-            .INVAL => unreachable, // Handle not open for writing
+            .INVAL => unreachable, // Handle not open for writing, negative length, or non-resizable handle
             .NOTCAPABLE => return error.AccessDenied,
             else => |err| return unexpectedErrno(err),
         }
@@ -1077,7 +1082,7 @@ pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
 
     const ftruncate_sym = if (lfs64_abi) system.ftruncate64 else system.ftruncate;
     while (true) {
-        switch (errno(ftruncate_sym(fd, @bitCast(length)))) {
+        switch (errno(ftruncate_sym(fd, signed_len))) {
             .SUCCESS => return,
             .INTR => continue,
             .FBIG => return error.FileTooBig,
@@ -1085,7 +1090,7 @@ pub fn ftruncate(fd: fd_t, length: u64) TruncateError!void {
             .PERM => return error.PermissionDenied,
             .TXTBSY => return error.FileBusy,
             .BADF => unreachable, // Handle not open for writing
-            .INVAL => unreachable, // Handle not open for writing
+            .INVAL => unreachable, // Handle not open for writing, negative length, or non-resizable handle
             else => |err| return unexpectedErrno(err),
         }
     }