From 6e29619dcbe6af0344f983fbf62897433baf2d36 Mon Sep 17 00:00:00 2001
From: Tim Schumacher <timschumi@gmx.de>
Date: Mon, 28 Nov 2022 18:54:14 +0100
Subject: LibCore: Add `Stream::discard()`

---
 Userland/Libraries/LibCore/Stream.cpp | 30 ++++++++++++++++++++++++++++++
 Userland/Libraries/LibCore/Stream.h   |  8 ++++++++
 2 files changed, 38 insertions(+)

diff --git a/Userland/Libraries/LibCore/Stream.cpp b/Userland/Libraries/LibCore/Stream.cpp
index cf19566079..b70c9f1518 100644
--- a/Userland/Libraries/LibCore/Stream.cpp
+++ b/Userland/Libraries/LibCore/Stream.cpp
@@ -73,6 +73,22 @@ ErrorOr<ByteBuffer> Stream::read_all_impl(size_t block_size, size_t expected_fil
     return data;
 }
 
+ErrorOr<void> Stream::discard(size_t discarded_bytes)
+{
+    // Note: This was chosen arbitrarily.
+    // Note: This can't be PAGE_SIZE because it is defined to sysconf() on Lagom.
+    constexpr size_t continuous_read_size = 4096;
+
+    Array<u8, continuous_read_size> buffer;
+
+    while (discarded_bytes > 0) {
+        auto slice = TRY(read(buffer.span().slice(0, min(discarded_bytes, continuous_read_size))));
+        discarded_bytes -= slice.size();
+    }
+
+    return {};
+}
+
 bool Stream::write_or_error(ReadonlyBytes buffer)
 {
     VERIFY(buffer.size());
@@ -120,6 +136,12 @@ ErrorOr<off_t> SeekableStream::size()
     return seek_result.value();
 }
 
+ErrorOr<void> SeekableStream::discard(size_t discarded_bytes)
+{
+    TRY(seek(discarded_bytes, SeekMode::FromCurrentPosition));
+    return {};
+}
+
 ErrorOr<NonnullOwnPtr<File>> File::open(StringView filename, OpenMode mode, mode_t permissions)
 {
     auto file = TRY(adopt_nonnull_own_or_enomem(new (nothrow) File(mode)));
@@ -729,6 +751,14 @@ ErrorOr<Bytes> WrappedAKInputStream::read(Bytes bytes)
     return bytes.slice(0, bytes_read);
 }
 
+ErrorOr<void> WrappedAKInputStream::discard(size_t discarded_bytes)
+{
+    if (!m_stream->discard_or_error(discarded_bytes))
+        return Error::from_string_literal("Underlying InputStream indicated an error");
+
+    return {};
+}
+
 ErrorOr<size_t> WrappedAKInputStream::write(ReadonlyBytes)
 {
     VERIFY_NOT_REACHED();
diff --git a/Userland/Libraries/LibCore/Stream.h b/Userland/Libraries/LibCore/Stream.h
index 3fc95f1cc4..2a28bcae75 100644
--- a/Userland/Libraries/LibCore/Stream.h
+++ b/Userland/Libraries/LibCore/Stream.h
@@ -41,6 +41,10 @@ public:
     /// is returned once EOF is encountered. The block size determines the size
     /// of newly allocated chunks while reading.
     virtual ErrorOr<ByteBuffer> read_all(size_t block_size = 4096);
+    /// Discards the given number of bytes from the stream.
+    /// Unless specifically overwritten, this just uses read() to read into an
+    /// internal stack-based buffer.
+    virtual ErrorOr<void> discard(size_t discarded_bytes);
 
     virtual bool is_writable() const { return false; }
     /// Tries to write the entire contents of the buffer. It is possible for
@@ -97,6 +101,9 @@ public:
     /// Shrinks or extends the stream to the given size. Returns an errno in
     /// the case of an error.
     virtual ErrorOr<void> truncate(off_t length) = 0;
+    /// Seeks until after the given amount of bytes to be discarded instead of
+    /// reading and discarding everything manually;
+    virtual ErrorOr<void> discard(size_t discarded_bytes) override;
 };
 
 /// The Socket class is the base class for all concrete BSD-style socket
@@ -973,6 +980,7 @@ class WrappedAKInputStream final : public Stream {
 public:
     WrappedAKInputStream(NonnullOwnPtr<InputStream> stream);
     virtual ErrorOr<Bytes> read(Bytes) override;
+    virtual ErrorOr<void> discard(size_t discarded_bytes) override;
     virtual ErrorOr<size_t> write(ReadonlyBytes) override;
     virtual bool is_eof() const override;
     virtual bool is_open() const override;
-- 
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