WiFiClientSecure: don't trash unread decrypted data when writing

When application requests to write data, check if there is any unread
decrypted data left. If there is, don't write immediately, but save the
data to be written. When all decrypted data has been consumed by the
application, send out the saved outgoing data.

Fixes #2256.

libraries/ESP8266WiFi/src/WiFiClientSecure.cpp

@@ -26,6 +26,7 @@ extern "C"
 {
 #include "osapi.h"
 #include "ets_sys.h"
+#include "queue.h"
 }
 #include <errno.h>
 #include "debug.h"
@@ -50,6 +51,28 @@ extern "C"
 #define SSL_DEBUG_OPTS 0
 #endif
 
+
+typedef struct BufferItem
+{
+    size_t size;
+    STAILQ_ENTRY(BufferItem) next;
+    uint8_t data[0];
+
+    static BufferItem* Create(const uint8_t* data, size_t size)
+    {
+        BufferItem* newItem = (BufferItem*) calloc(1, sizeof(BufferItem) + size);
+        if (newItem == nullptr) {
+            return nullptr;
+        }
+        newItem->size = size;
+        memcpy(newItem->data, data, size);
+        return newItem;
+    }
+
+} BufferItem;
+
+typedef STAILQ_HEAD(BufferList, BufferItem) BufferList;
+
 class SSLContext
 {
 public:
@@ -59,6 +82,7 @@ class SSLContext
             _ssl_ctx = ssl_ctx_new(SSL_SERVER_VERIFY_LATER | SSL_DEBUG_OPTS | SSL_CONNECT_IN_PARTS | SSL_READ_BLOCKING | SSL_NO_DEFAULT_KEY, 0);
         }
         ++_ssl_ctx_refcnt;
+        STAILQ_INIT(&_writeBuffers);
     }
 
     ~SSLContext()
@@ -139,6 +163,10 @@ class SSLContext
         _available -= will_copy;
         if (_available == 0) {
             _read_ptr = nullptr;
+            /* Send pending outgoing data, if any */
+            if (_hasWriteBuffers()) {
+                _writeBuffersSend();
+            }
         }
         return will_copy;
     }
@@ -155,10 +183,34 @@ class SSLContext
         --_available;
         if (_available == 0) {
             _read_ptr = nullptr;
+            /* Send pending outgoing data, if any */
+            if (_hasWriteBuffers()) {
+                _writeBuffersSend();
+            }
         }
         return result;
     }
 
+    int write(const uint8_t* src, size_t size)
+    {
+        if (!_available) {
+            if (_hasWriteBuffers()) {
+                int rc = _writeBuffersSend();
+                if (rc < 0) {
+                    return rc;
+                }
+            }
+            return _write(src, size);
+        }
+        /* Some received data is still present in the axtls fragment buffer.
+           We can't call ssl_write now, as that will overwrite the contents of
+           the fragment buffer, corrupting the received data.
+           Save a copy of the outgoing data, and call ssl_write when all
+           recevied data has been consumed by the application.
+        */
+        return _writeBufferAdd(src, size);
+    }
+
     int peek()
     {
         if (!_available) {
@@ -282,12 +334,71 @@ class SSLContext
         return _available;
     }
 
+    int _write(const uint8_t* src, size_t size)
+    {
+        if (!_ssl) {
+            return 0;
+        }
+
+        int rc = ssl_write(_ssl, src, size);
+        if (rc >= 0) {
+            return rc;
+        }
+        return rc;
+    }
+
+    int _writeBufferAdd(const uint8_t* data, size_t size)
+    {
+        if (!_ssl) {
+            return 0;
+        }
+
+        BufferItem* newItem = BufferItem::Create(data, size);
+        if (newItem == nullptr) {
+            return SSL_NOT_OK;
+        }
+        STAILQ_INSERT_TAIL(&_writeBuffers, newItem, next);
+        return size;
+    }
+
+    int _writeBuffersSend()
+    {
+        BufferItem* it;
+        BufferItem* tmp;
+        STAILQ_FOREACH_SAFE(it, &_writeBuffers, next, tmp) {
+            STAILQ_REMOVE_HEAD(&_writeBuffers, next);
+            int rc = _write(it->data, it->size);
+            free(it);
+            if (rc < 0) {
+                _writeBuffersDiscard();
+                return rc;
+            }
+        }
+        return 0;
+    }
+
+    void _writeBuffersDiscard()
+    {
+        BufferItem* it;
+        BufferItem* tmp;
+        STAILQ_FOREACH_SAFE(it, &_writeBuffers, next, tmp) {
+            STAILQ_REMOVE_HEAD(&_writeBuffers, next);
+            free(it);
+        }
+    }
+
+    bool _hasWriteBuffers()
+    {
+        return !STAILQ_EMPTY(&_writeBuffers);
+    }
+
     static SSL_CTX* _ssl_ctx;
     static int _ssl_ctx_refcnt;
     SSL* _ssl = nullptr;
     int _refcnt = 0;
     const uint8_t* _read_ptr = nullptr;
     size_t _available = 0;
+    BufferList _writeBuffers;
     bool _allowSelfSignedCerts = false;
     static ClientContext* s_io_ctx;
 };
@@ -371,7 +482,7 @@ size_t WiFiClientSecure::write(const uint8_t *buf, size_t size)
         return 0;
     }
 
-    int rc = ssl_write(*_ssl, buf, size);
+    int rc = _ssl->write(buf, size);
     if (rc >= 0) {
         return rc;
     }