Merge pull request #24464 from ziglang/fixes

std.Io fixes, docs, and tests

lib/std/Io/Reader.zig

@@ -1303,6 +1303,13 @@ fn takeMultipleOf7Leb128(r: *Reader, comptime Result: type) TakeLeb128Error!Resu
 }
 
 /// Left-aligns data such that `r.seek` becomes zero.
+///
+/// If `r.seek` is not already zero then `buffer` is mutated, making it illegal
+/// to call this function with a const-casted `buffer`, such as in the case of
+/// `fixed`. This issue can be avoided:
+/// * in implementations, by attempting a read before a rebase, in which
+///   case the read will return `error.EndOfStream`, preventing the rebase.
+/// * in usage, by copying into a mutable buffer before initializing `fixed`.
 pub fn rebase(r: *Reader) void {
     if (r.seek == 0) return;
     const data = r.buffer[r.seek..r.end];
@@ -1315,6 +1322,13 @@ pub fn rebase(r: *Reader) void {
 /// if necessary.
 ///
 /// Asserts `capacity` is within the buffer capacity.
+///
+/// If the rebase occurs then `buffer` is mutated, making it illegal to call
+/// this function with a const-casted `buffer`, such as in the case of `fixed`.
+/// This issue can be avoided:
+/// * in implementations, by attempting a read before a rebase, in which
+///   case the read will return `error.EndOfStream`, preventing the rebase.
+/// * in usage, by copying into a mutable buffer before initializing `fixed`.
 pub fn rebaseCapacity(r: *Reader, capacity: usize) void {
     if (r.end > r.buffer.len - capacity) rebase(r);
 }

lib/std/Io/Writer.zig

@@ -2194,8 +2194,10 @@ pub const Discarding = struct {
         const d: *Discarding = @alignCast(@fieldParentPtr("writer", w));
         d.count += w.end;
         w.end = 0;
+        if (limit == .nothing) return 0;
         if (file_reader.getSize()) |size| {
             const n = limit.minInt64(size - file_reader.pos);
+            if (n == 0) return error.EndOfStream;
             file_reader.seekBy(@intCast(n)) catch return error.Unimplemented;
             w.end = 0;
             d.count += n;
@@ -2489,18 +2491,17 @@ pub const Allocating = struct {
 
     fn sendFile(w: *Writer, file_reader: *File.Reader, limit: std.io.Limit) FileError!usize {
         if (File.Handle == void) return error.Unimplemented;
+        if (limit == .nothing) return 0;
         const a: *Allocating = @fieldParentPtr("writer", w);
         const gpa = a.allocator;
         var list = a.toArrayList();
         defer setArrayList(a, list);
         const pos = file_reader.pos;
         const additional = if (file_reader.getSize()) |size| size - pos else |_| std.atomic.cache_line;
+        if (additional == 0) return error.EndOfStream;
         list.ensureUnusedCapacity(gpa, limit.minInt64(additional)) catch return error.WriteFailed;
         const dest = limit.slice(list.unusedCapacitySlice());
-        const n = file_reader.read(dest) catch |err| switch (err) {
-            error.ReadFailed => return error.ReadFailed,
-            error.EndOfStream => 0,
-        };
+        const n = try file_reader.read(dest);
         list.items.len += n;
         return n;
     }
@@ -2522,3 +2523,43 @@ pub const Allocating = struct {
         try testing.expectEqualSlices(u8, "x: 42\ny: 1234\n", a.getWritten());
     }
 };
+
+test "discarding sendFile" {
+    var tmp_dir = testing.tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    const file = try tmp_dir.dir.createFile("input.txt", .{ .read = true });
+    defer file.close();
+    var r_buffer: [256]u8 = undefined;
+    var file_writer: std.fs.File.Writer = .init(file, &r_buffer);
+    try file_writer.interface.writeByte('h');
+    try file_writer.interface.flush();
+
+    var file_reader = file_writer.moveToReader();
+    try file_reader.seekTo(0);
+
+    var w_buffer: [256]u8 = undefined;
+    var discarding: std.io.Writer.Discarding = .init(&w_buffer);
+
+    _ = try file_reader.interface.streamRemaining(&discarding.writer);
+}
+
+test "allocating sendFile" {
+    var tmp_dir = testing.tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    const file = try tmp_dir.dir.createFile("input.txt", .{ .read = true });
+    defer file.close();
+    var r_buffer: [256]u8 = undefined;
+    var file_writer: std.fs.File.Writer = .init(file, &r_buffer);
+    try file_writer.interface.writeByte('h');
+    try file_writer.interface.flush();
+
+    var file_reader = file_writer.moveToReader();
+    try file_reader.seekTo(0);
+
+    var allocating: std.io.Writer.Allocating = .init(std.testing.allocator);
+    defer allocating.deinit();
+
+    _ = try file_reader.interface.streamRemaining(&allocating.writer);
+}