Kernel: Implement peek() function for DoubleBuffer

This allows us to "peek" into a DoubleBuffer without incrementing
the m_read_buffer_index, which is needed to implement MSG_PEEK.

2 files changed, 25 insertions, 2 deletions

diff --git a/Kernel/DoubleBuffer.cpp b/Kernel/DoubleBuffer.cpp
index ef5fd497e1..2d3b39adac 100644
--- a/Kernel/DoubleBuffer.cpp
+++ b/Kernel/DoubleBuffer.cpp
@@ -44,7 +44,6 @@ ssize_t DoubleBuffer::write(const UserOrKernelBuffer& data, size_t size)
 {
     if (!size || m_storage.is_null())
         return 0;
-    VERIFY(size > 0);
     Locker locker(m_lock);
     size_t bytes_to_write = min(size, m_space_for_writing);
     u8* write_ptr = m_write_buffer->data + m_write_buffer->size;
@@ -61,7 +60,6 @@ ssize_t DoubleBuffer::read(UserOrKernelBuffer& data, size_t size)
 {
     if (!size || m_storage.is_null())
         return 0;
-    VERIFY(size > 0);
     Locker locker(m_lock);
     if (m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size != 0)
         flip();
@@ -77,4 +75,23 @@ ssize_t DoubleBuffer::read(UserOrKernelBuffer& data, size_t size)
     return (ssize_t)nread;
 }
 
+ssize_t DoubleBuffer::peek(UserOrKernelBuffer& data, size_t size)
+{
+    if (!size || m_storage.is_null())
+        return 0;
+    Locker locker(m_lock);
+    if (m_read_buffer_index >= m_read_buffer->size && m_write_buffer->size != 0) {
+        flip();
+    }
+    if (m_read_buffer_index >= m_read_buffer->size)
+        return 0;
+    size_t nread = min(m_read_buffer->size - m_read_buffer_index, size);
+    if (!data.write(m_read_buffer->data + m_read_buffer_index, nread))
+        return -EFAULT;
+    compute_lockfree_metadata();
+    if (m_unblock_callback && m_space_for_writing > 0)
+        m_unblock_callback();
+    return (ssize_t)nread;
+}
+
 }
diff --git a/Kernel/DoubleBuffer.h b/Kernel/DoubleBuffer.h
index fc3b3472db..8bf58f7532 100644
--- a/Kernel/DoubleBuffer.h
+++ b/Kernel/DoubleBuffer.h
@@ -29,6 +29,12 @@ public:
         auto buffer = UserOrKernelBuffer::for_kernel_buffer(data);
         return read(buffer, size);
     }
+    [[nodiscard]] ssize_t peek(UserOrKernelBuffer&, size_t);
+    [[nodiscard]] ssize_t peek(u8* data, size_t size)
+    {
+        auto buffer = UserOrKernelBuffer::for_kernel_buffer(data);
+        return peek(buffer, size);
+    }
 
     bool is_empty() const { return m_empty; }