_libssh2_error: Support allocating the error message

Before this patch "_libssh2_error" required the error message to be a
static string.

This patch adds a new function "_libssh2_error_flags" accepting an
additional "flags" argument and specifically the flag
"LIBSSH2_ERR_FLAG_DUP" indicating that the passed string must be
duplicated into the heap.

Then, the method "_libssh2_error" has been rewritten to use that new
function under the hood.

Signed-off-by: Salvador Fandino <sfandino@yahoo.com>
Signed-off-by: Salvador Fandiño <sfandino@yahoo.com>

src/libssh2_priv.h

@@ -631,6 +631,7 @@ struct _LIBSSH2_SESSION
     /* Error tracking */
     const char *err_msg;
     int err_code;
+    int err_flags;
 
     /* struct members for packet-level reading */
     struct transportpacket packet;
@@ -950,6 +951,10 @@ _libssh2_debug(LIBSSH2_SESSION * session, int context, const char *format, ...)
 /* Something very bad is going on */
 #define LIBSSH2_MAC_INVALID                     -1
 
+/* Flags for _libssh2_error_flags */
+/* Error message is allocated on the heap */
+#define LIBSSH2_ERR_FLAG_DUP                     1
+
 /* SSH Packet Types -- Defined by internet draft */
 /* Transport Layer */
 #define SSH_MSG_DISCONNECT                          1

src/misc.c

@@ -51,10 +51,29 @@
 #include <stdio.h>
 #include <errno.h>
 
-int _libssh2_error(LIBSSH2_SESSION* session, int errcode, const char* errmsg)
+int _libssh2_error_flags(LIBSSH2_SESSION* session, int errcode, const char* errmsg, int errflags)
 {
-    session->err_msg = errmsg;
+    if (session->err_flags & LIBSSH2_ERR_FLAG_DUP)
+        LIBSSH2_FREE(session, (char *)session->err_msg);
+
     session->err_code = errcode;
+    session->err_flags = 0;
+
+    if ((errmsg != NULL) && ((errflags & LIBSSH2_ERR_FLAG_DUP) != 0)) {
+        size_t len = strlen(errmsg);
+        char *copy = LIBSSH2_ALLOC(session, len + 1);
+        if (copy) {
+            memcpy(copy, errmsg, len + 1);
+            session->err_flags = LIBSSH2_ERR_FLAG_DUP;
+            session->err_msg = copy;
+        }
+        else
+            /* Out of memory: this code path is very unlikely */
+            session->err_msg = "former error forgotten (OOM)";
+    }
+    else
+        session->err_msg = errmsg;
+
 #ifdef LIBSSH2DEBUG
     if((errcode == LIBSSH2_ERROR_EAGAIN) && !session->api_block_mode)
         /* if this is EAGAIN and we're in non-blocking mode, don't generate
@@ -67,6 +86,11 @@ int _libssh2_error(LIBSSH2_SESSION* session, int errcode, const char* errmsg)
     return errcode;
 }
 
+int _libssh2_error(LIBSSH2_SESSION* session, int errcode, const char* errmsg)
+{
+    return _libssh2_error_flags(session, errcode, errmsg, 0);
+}
+
 #ifdef WIN32
 static int wsa2errno(void)
 {

src/misc.h

@@ -49,6 +49,7 @@ struct list_node {
     struct list_head *head;
 };
 
+int _libssh2_error_flags(LIBSSH2_SESSION* session, int errcode, const char* errmsg, int errflags);
 int _libssh2_error(LIBSSH2_SESSION* session, int errcode, const char* errmsg);
 
 void _libssh2_list_init(struct list_head *head);

src/session.c

@@ -1058,6 +1058,11 @@ session_free(LIBSSH2_SESSION *session)
         LIBSSH2_FREE(session, session->server_hostkey);
     }
 
+    /* error string */
+    if (session->err_msg && ((session->err_flags & LIBSSH2_ERR_FLAG_DUP) != 0)) {
+        LIBSSH2_FREE(session, (char *)session->err_msg);
+    }
+
     LIBSSH2_FREE(session, session);
 
     return 0;