kapi/system: reduce panic buffer size

Previously, we allocated a 512-byte buffer for every call of the
formatted panic function. This was heavily increasing the stack size for
a number of functions.

We now don't print to a statically allocated buffer, but rather use the
iterator-based formatting facility and a proxy iterator with a small,
self-flushing buffer. This reduces that static overhead of the formatted
panic function to by almost 500 bytes, while increasing possible message
fidelity, since we are not longer arbitrarily limited by the buffer.

1 files changed, 85 insertions, 5 deletions

diff --git a/kernel/kapi/system.cpp b/kernel/kapi/system.cpp
index 629e5990..34601b9d 100644
--- a/kernel/kapi/system.cpp
+++ b/kernel/kapi/system.cpp
@@ -11,19 +11,99 @@
 namespace kapi::system
 {
 
+  namespace
+  {
+
+    auto panic_write(kstd::error_code error) -> void
+    {
+      kstd::print(kstd::print_sink::stderr, " ({}) ", error);
+    }
+
+  }  // namespace
+
+  namespace detail
+  {
+
+    panic_buffer::~panic_buffer()
+    {
+      flush();
+    }
+
+    void panic_buffer::append(char character)
+    {
+      m_buffer[m_written++] = character;
+      if (m_written == m_buffer.size())
+      {
+        flush();
+      }
+    }
+
+    void panic_buffer::flush()
+    {
+      if (m_written > 0)
+      {
+        panic_write(std::string_view{m_buffer.data(), m_written});
+        m_written = 0;
+      }
+    }
+
+    panic_iterator::panic_iterator(panic_buffer & buffer)
+        : m_buffer(&buffer)
+    {}
+
+    auto panic_iterator::operator*() noexcept -> panic_iterator &
+    {
+      return *this;
+    }
+
+    auto panic_iterator::operator=(char character) -> panic_iterator &
+    {
+      m_buffer->append(character);
+      return *this;
+    }
+
+    auto panic_iterator::operator++() -> panic_iterator &
+    {
+      return *this;
+    }
+
+    auto panic_iterator::operator++(int) -> panic_iterator
+    {
+      return *this;
+    }
+
+  }  // namespace detail
+
   [[gnu::weak]]
   auto panic(std::string_view message, std::source_location location) -> void
   {
-    kstd::println(kstd::print_sink::stderr, "[PANIC] in {} : {} @ {}:{}", location.function_name(), message,
-                  location.file_name(), location.line());
-    cpu::halt();
+    panic_start(location);
+    panic_write(message);
+    panic_finish(location);
   }
 
   [[gnu::weak]]
   auto panic(std::string_view message, kstd::error_code error, std::source_location location) -> void
   {
-    kstd::println(kstd::print_sink::stderr, "[PANIC] in {} : {} ({}) @ {}:{}", location.function_name(), message, error,
-                  location.file_name(), location.line());
+    panic_start(location);
+    panic_write(message);
+    panic_write(error);
+    panic_finish(location);
+  }
+
+  auto panic_start(std::source_location location) -> void
+  {
+    kstd::print(kstd::print_sink::stderr, "[PANIC] in '{}': ", location.function_name());
+  }
+
+  auto panic_write(std::string_view message) -> void
+  {
+    kstd::print(kstd::print_sink::stderr, "{}", message);
+  }
+
+  auto panic_finish(std::source_location location) -> void
+  {
+    kstd::println(kstd::print_sink::stderr, " @ {}:{}", location.file_name(), location.line());
     cpu::halt();
   }