lib: add overload Mmsg() for std::vector<char>

Previously Mmsg() would only write into a POOLMEM or PoolMem. This patch
adds another overload that allows Mmsg() to write to a std::vector<char>
and to automatically resize that vector as needed.

core/src/include/baconfig.h

@@ -448,6 +448,7 @@ class PoolMem;
 int Mmsg(POOLMEM*& msgbuf, const char* fmt, ...);
 int Mmsg(PoolMem& msgbuf, const char* fmt, ...);
 int Mmsg(PoolMem*& msgbuf, const char* fmt, ...);
+int Mmsg(std::vector<char>& msgbuf, const char* fmt, ...);
 
 class JobControlRecord;
 void d_msg(const char* file, int line, int level, const char* fmt, ...);

core/src/lib/message.cc

@@ -31,6 +31,8 @@
  * Kern Sibbald, April 2000
  */
 
+#include <vector>
+
 #include "include/bareos.h"
 #include "include/jcr.h"
 #include "lib/berrno.h"
@@ -1564,6 +1566,35 @@ int Mmsg(PoolMem*& pool_buf, const char* fmt, ...)
   return len;
 }
 
+int Mmsg(std::vector<char>& msgbuf, const char* fmt, ...)
+{
+  va_list ap;
+
+  size_t maxlen = msgbuf.size();
+  size_t len = strlen(fmt);
+
+  /* resize msgbuf so at least fmt fits in there.
+   * this makes sure the rest of the code works with a zero-sized vector
+   */
+  if (maxlen < len) {
+    msgbuf.resize(len);
+    maxlen = len;
+  }
+
+  while (1) {
+    va_start(ap, fmt);
+    len = Bvsnprintf(msgbuf.data(), maxlen, fmt, ap);
+    va_end(ap);
+
+    if (len < 0 || len >= (maxlen - 5)) {
+      maxlen += maxlen / 2;
+      msgbuf.resize(maxlen);
+      continue;
+    }
+    return len;
+  }
+}
+
 /*
  * We queue messages rather than print them directly. This
  * is generally used in low level routines (msg handler, bnet)

core/src/lib/message.h

@@ -36,8 +36,9 @@
 #include "lib/rwlock.h"
 #include "lib/message_destination_info.h"
 
-#include <string>
 #include <functional>
+#include <string>
+#include <vector>
 
 class JobControlRecord;