AK+Kernel: Support snprintf

In contrast to sprintf, which might overflow the given buffer.

I feel bad about the code duplication, but that is a pre-existing issue.

AK/kstdio.h

@@ -37,6 +37,7 @@ int vdbgprintf(const char* fmt, va_list);
 int dbgprintf(const char* fmt, ...);
 ssize_t dbgputstr(const char*, ssize_t);
 int sprintf(char* buf, const char* fmt, ...);
+int snprintf(char* buffer, size_t, const char* fmt, ...);
 }
 #    endif
 #else

Kernel/kprintf.cpp

@@ -144,6 +144,34 @@ int sprintf(char* buffer, const char* fmt, ...)
     return ret;
 }
 
+static size_t __vsnprintf_space_remaining;
+ALWAYS_INLINE void sized_buffer_putch(char*& bufptr, char ch)
+{
+    if (__vsnprintf_space_remaining) {
+        *bufptr++ = ch;
+        --__vsnprintf_space_remaining;
+    }
+}
+
+int snprintf(char* buffer, size_t size, const char* fmt, ...)
+{
+    va_list ap;
+    va_start(ap, fmt);
+    if (size) {
+        __vsnprintf_space_remaining = size - 1;
+    } else {
+        __vsnprintf_space_remaining = 0;
+    }
+    int ret = printf_internal(sized_buffer_putch, buffer, fmt, ap);
+    if (__vsnprintf_space_remaining) {
+        buffer[ret] = '\0';
+    } else if (size > 0) {
+        buffer[size - 1] = '\0';
+    }
+    va_end(ap);
+    return ret;
+}
+
 static void debugger_out(char ch)
 {
     if (serial_debug)

Kernel/kstdio.h

@@ -34,6 +34,7 @@ int dbgputstr(const char*, int);
 int kernelputstr(const char*, int);
 int kprintf(const char* fmt, ...);
 int sprintf(char* buf, const char* fmt, ...);
+int snprintf(char* buf, size_t, const char* fmt, ...);
 void set_serial_debug(bool on_or_off);
 int get_serial_debug();
 }