AP_Common: add support for setting custom external buffer

libraries/AP_Common/ExpandingString.cpp

@@ -27,6 +27,10 @@ extern const AP_HAL::HAL& hal;
  */
 bool ExpandingString::expand(uint32_t min_extra_space_needed)
 {
+    if (external_buffer) {
+        // we can't expand an external buffer
+        return false;
+    }
     // expand a reasonable amount
     uint32_t newsize = (5*buflen/4) + EXPAND_INCREMENT;
     if (newsize - used < min_extra_space_needed) {
@@ -102,5 +106,22 @@ bool ExpandingString::append(const char *s, uint32_t len)
 
 ExpandingString::~ExpandingString()
 {
-    free(buf);
+    if (!external_buffer) {
+        free(buf);
+    }
+}
+
+
+void ExpandingString::set_buffer(char *s, uint32_t total_len, uint32_t used_len)
+{
+    if (buf != nullptr) {
+        // we need to free previously used buffer
+        free(buf);
+    }
+
+    buf = s;
+    buflen = total_len;
+    used = used_len;
+    allocation_failed = false;
+    external_buffer = true;
 }

libraries/AP_Common/ExpandingString.h

@@ -39,6 +39,9 @@ public:
     // append data to the string. s can be null for zero fill
     bool append(const char *s, uint32_t len);
 
+    // set address to custom external buffer
+    void set_buffer(char *s, uint32_t total_len, uint32_t used_len);
+
     // destructor
     ~ExpandingString();
 
@@ -51,6 +54,7 @@ private:
     uint32_t buflen;
     uint32_t used;
     bool allocation_failed;
+    bool external_buffer;
 
     // try to expand the buffer
     bool expand(uint32_t min_needed) WARN_IF_UNUSED;