LibCore: Let IODevice::can_read_line() buffer until \n or EOF

If a line was larger than 1024 bytes or the file ended without a
newline character, can_read_line would return false.

IODevice::can_read_line() now reads until a newline is found or
EOF is reached.

fixes #5907

2 files changed, 21 insertions, 6 deletions

diff --git a/AK/Vector.h b/AK/Vector.h
index fd25487ce6..e75e6a58c9 100644
--- a/AK/Vector.h
+++ b/AK/Vector.h
@@ -161,8 +161,6 @@ public:
         return false;
     }
 
-    // NOTE: Vector::is_null() exists for the benefit of String::copy().
-    bool is_null() const { return false; }
     bool is_empty() const { return size() == 0; }
     ALWAYS_INLINE size_t size() const { return m_size; }
     size_t capacity() const { return m_capacity; }
diff --git a/Userland/Libraries/LibCore/IODevice.cpp b/Userland/Libraries/LibCore/IODevice.cpp
index 6dd7e3dc9c..fb45152999 100644
--- a/Userland/Libraries/LibCore/IODevice.cpp
+++ b/Userland/Libraries/LibCore/IODevice.cpp
@@ -105,14 +105,31 @@ bool IODevice::can_read_line() const
 {
     if (m_eof && !m_buffered_data.is_empty())
         return true;
+
     if (m_buffered_data.contains_slow('\n'))
         return true;
+
     if (!can_read_from_fd())
         return false;
-    populate_read_buffer();
-    if (m_eof && !m_buffered_data.is_empty())
-        return true;
-    return m_buffered_data.contains_slow('\n');
+
+    while (true) {
+        // Populate buffer until a newline is found or we reach EOF.
+
+        auto previous_buffer_size = m_buffered_data.size();
+        populate_read_buffer();
+        auto new_buffer_size = m_buffered_data.size();
+
+        if (m_error)
+            return false;
+
+        if (m_eof)
+            return !m_buffered_data.is_empty();
+
+        if (m_buffered_data.contains_in_range('\n', previous_buffer_size, new_buffer_size - 1))
+            return true;
+    }
+
+    return true;
 }
 
 bool IODevice::can_read() const