[Glacier][Zion] Add a fix sized string builder to allow non-alloc debug.

lib/glacier/string/str_format.h

@@ -66,7 +66,8 @@ String StrFormat(StringView format, Args... args) {
 }
 
 template <typename... Args>
-void StrFormat(StringBuilder& builder, StringView format, Args... args) {
+void StrFormatIntoBuffer(StringBuilder& builder, StringView format,
+                         Args... args) {
   StrFormatInternal(builder, format, args...);
 }
 

lib/glacier/string/string_builder.cpp

@@ -27,4 +27,27 @@ VariableStringBuilder::operator StringView() const {
   return StringView(data_.RawPtr(), size());
 }
 
+void FixedStringBuilder::PushBack(const StringView& str) {
+  for (uint64_t i = 0; i < str.size(); i++) {
+    PushBack(str[i]);
+  }
+}
+
+void FixedStringBuilder::PushBack(const char str) {
+  if (size_ >= capacity_) {
+    // Somewhat random sequence of characters to show that we've overrun the
+    // buffer.
+    buffer_[capacity_ - 1] = '>';
+    buffer_[capacity_ - 2] = '!';
+  } else {
+    buffer_[size_++] = str;
+  }
+}
+
+String FixedStringBuilder::ToString() const { return String(buffer_, size_); }
+
+FixedStringBuilder::operator StringView() const {
+  return StringView(buffer_, size_);
+}
+
 }  // namespace glcr

lib/glacier/string/string_builder.h

@@ -41,4 +41,29 @@ class VariableStringBuilder : public StringBuilder {
   Vector<char> data_;
 };
 
+class FixedStringBuilder : public StringBuilder {
+ public:
+  FixedStringBuilder() = delete;
+  FixedStringBuilder(const FixedStringBuilder&) = delete;
+  FixedStringBuilder(FixedStringBuilder&&) = delete;
+  ~FixedStringBuilder() = default;
+
+  FixedStringBuilder(char* buffer, uint64_t size)
+      : buffer_(buffer), capacity_(size), size_(0) {}
+
+  virtual uint64_t size() const override { return size_; }
+
+  virtual void PushBack(const StringView& str) override;
+  virtual void PushBack(const char str) override;
+
+  virtual String ToString() const override;
+
+  virtual operator StringView() const override;
+
+ private:
+  char* buffer_;
+  uint64_t capacity_;
+  uint64_t size_;
+};
+
 }  // namespace glcr

zion/debug/debug.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <glacier/string/str_format.h>
+#include <glacier/string/string_builder.h>
 #include <stdarg.h>
 
 #include "include/ztypes.h"
@@ -21,13 +22,22 @@ void dbgln(const glcr::StringView& str);
 
 template <typename... Args>
 void dbgln(const char* str, Args... args) {
+  char buffer[256];
+  glcr::FixedStringBuilder builder(buffer, 256);
+  glcr::StrFormatIntoBuffer(builder, str, args...);
+  dbgln(builder);
+}
+
+template <typename... Args>
+void dbgln_large(const char* str, Args... args) {
   dbgln(glcr::StrFormat(str, args...));
 }
 
 template <typename... Args>
 void panic(const char* str, Args... args) {
-  dbgln(glcr::StrFormat(str, args...));
+  dbgln(str, args...);
   dbgln("PANIC");
+  asm volatile("hlt;");
 }
 
 #define UNREACHABLE                                \

zion/syscall/debug.cpp

@@ -5,6 +5,6 @@
 #include "debug/debug.h"
 
 z_err_t Debug(ZDebugReq* req) {
-  dbgln("[Debug] {}", req->message);
+  dbgln_large("[Debug] {}", req->message);
   return glcr::OK;
 }