log: Use malloc for long messages.

For short messages, use a stack buffer that is
significantly smaller than SDL_MAX_LOG_MESSAGE.

The rationale for this is that we don't want to risk
blowing the stack, while at the same time we would
like to not put pressure on the memory allocator unless
absolutely necessary.

src/SDL_log.c

@@ -39,6 +39,9 @@
 #include <android/log.h>
 #endif
 
+/* The size of the stack buffer to use for rendering log messages. */
+#define SDL_MAX_LOG_MESSAGE_STACK 256
+
 #define DEFAULT_PRIORITY                SDL_LOG_PRIORITY_CRITICAL
 #define DEFAULT_ASSERT_PRIORITY         SDL_LOG_PRIORITY_WARN
 #define DEFAULT_APPLICATION_PRIORITY    SDL_LOG_PRIORITY_INFO
@@ -285,8 +288,10 @@ GetCategoryPrefix(int category)
 void
 SDL_LogMessageV(int category, SDL_LogPriority priority, const char *fmt, va_list ap)
 {
-    static char message[SDL_MAX_LOG_MESSAGE];
+    char *message = NULL;
+    char stack_buf[SDL_MAX_LOG_MESSAGE_STACK];
     size_t len;
+    va_list aq;
 
     /* Nothing to do if we don't have an output function */
     if (!SDL_log_function) {
@@ -308,26 +313,45 @@ SDL_LogMessageV(int category, SDL_LogPriority priority, const char *fmt, va_list
         log_function_mutex = SDL_CreateMutex();
     }
 
-    if (log_function_mutex) {
-        SDL_LockMutex(log_function_mutex);
-    }
+    /* Render into stack buffer */
+    va_copy(aq, ap);
+    len = SDL_vsnprintf(stack_buf, sizeof(stack_buf), fmt, aq);
+    va_end(aq);
 
-    SDL_vsnprintf(message, SDL_MAX_LOG_MESSAGE, fmt, ap);
+    /* If message truncated, allocate and re-render */
+    if (len >= sizeof(stack_buf)) {
+        message = (char *)SDL_malloc(SDL_MAX_LOG_MESSAGE);
+        if (!message)
+            return;
+        va_copy(aq, ap);
+        len = SDL_vsnprintf(message, SDL_MAX_LOG_MESSAGE, fmt, aq);
+        va_end(aq);
+    } else {
+        message = stack_buf;
+    }
 
     /* Chop off final endline. */
-    len = SDL_strlen(message);
     if ((len > 0) && (message[len-1] == '\n')) {
         message[--len] = '\0';
         if ((len > 0) && (message[len-1] == '\r')) {  /* catch "\r\n", too. */
             message[--len] = '\0';
         }
     }
 
+    if (log_function_mutex) {
+        SDL_LockMutex(log_function_mutex);
+    }
+
     SDL_log_function(SDL_log_userdata, category, priority, message);
 
     if (log_function_mutex) {
         SDL_UnlockMutex(log_function_mutex);
     }
+
+    /* Free only if dynamically allocated */
+    if (message != stack_buf) {
+        SDL_free(message);
+    }
 }
 
 #if defined(__WIN32__) && !defined(HAVE_STDIO_H) && !defined(__WINRT__)