Fix SFTP server hang on WS_WANT_WRITE with non-blocking sockets

.github/workflows/paramiko-sftp-test.yml

@@ -132,45 +132,99 @@ jobs:
           import os
           import time
           import sys
-
+          import hashlib
+          import signal
+
+          # Timeout handler for detecting hangs
+          class TimeoutError(Exception):
+              pass
+
+          def timeout_handler(signum, frame):
+              raise TimeoutError("SFTP operation timed out - possible hang detected!")
+
+          def get_file_hash(filepath):
+              """Calculate MD5 hash of a file."""
+              hash_md5 = hashlib.md5()
+              with open(filepath, "rb") as f:
+                  for chunk in iter(lambda: f.read(8192), b""):
+                      hash_md5.update(chunk)
+              return hash_md5.hexdigest()
+
           def run_sftp_test():
               # Create SSH client
               ssh = paramiko.SSHClient()
               ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
-              
+
               # Connect to server using password authentication with testuser
               print("Connecting to wolfSSHd server...")
               try:
                   ssh.connect('127.0.0.1', port=22222, username='testuser', password='testpassword')
               except Exception as e:
                   print(f"Connection error: {e}")
                   raise
-              
+
               # Open SFTP session
               print("Opening SFTP session...")
               sftp = ssh.open_sftp()
-              
+
               # Upload test
               print("Uploading 20MB test file...")
               start_time = time.time()
               sftp.put('/tmp/sftp_upload/test_upload.dat', '/tmp/test_upload.dat')
               upload_time = time.time() - start_time
               print(f"Upload completed in {upload_time:.2f} seconds")
-              
+
               # Download test
               print("Downloading 20MB test file...")
               start_time = time.time()
               sftp.get('/tmp/test_upload.dat', '/tmp/sftp_download/test_download.dat')
               download_time = time.time() - start_time
               print(f"Download completed in {download_time:.2f} seconds")
-
+
+              # Stress test: Repeated GET operations with prefetch
+              # This tests the WS_WANT_WRITE handling during repeated transfers
+              # which was the original bug trigger (SFTP hang on non-blocking sockets)
+              print("\n=== Starting repeated GET stress test (prefetch enabled) ===")
+              num_iterations = 10
+              timeout_seconds = 30  # Per-operation timeout to detect hangs
+
+              orig_hash = get_file_hash('/tmp/sftp_upload/test_upload.dat')
+
+              for i in range(num_iterations):
+                  signal.signal(signal.SIGALRM, timeout_handler)
+                  signal.alarm(timeout_seconds)
+                  try:
+                      download_path = f'/tmp/sftp_download/stress_test_{i}.dat'
+                      start_time = time.time()
+                      # Paramiko uses prefetch by default for get()
+                      sftp.get('/tmp/test_upload.dat', download_path)
+                      elapsed = time.time() - start_time
+
+                      # Verify integrity
+                      down_hash = get_file_hash(download_path)
+                      if orig_hash != down_hash:
+                          print(f"  Iteration {i+1}: FAILED - hash mismatch!")
+                          return False
+
+                      print(f"  Iteration {i+1}/{num_iterations}: OK ({elapsed:.2f}s)")
+                      os.remove(download_path)  # Cleanup
+
+                  except TimeoutError as e:
+                      print(f"  Iteration {i+1}: FAILED - {e}")
+                      print("  This may indicate the WS_WANT_WRITE hang bug!")
+                      return False
+                  finally:
+                      signal.alarm(0)  # Always cancel alarm
+
+              print(f"=== Stress test completed: {num_iterations} iterations OK ===\n")
+
               # Close connections
               sftp.close()
               ssh.close()
-              
+
               print("SFTP session closed")
               return True
-          
+
           if __name__ == "__main__":
               try:
                   success = run_sftp_test()

src/internal.c

@@ -3462,6 +3462,12 @@ int wolfSSH_SendPacket(WOLFSSH* ssh)
 }
 
 
+int wolfSSH_OutputPending(WOLFSSH* ssh)
+{
+    return (ssh != NULL && ssh->outputBuffer.length > ssh->outputBuffer.idx);
+}
+
+
 static int GetInputData(WOLFSSH* ssh, word32 size)
 {
     int in;

src/wolfsftp.c

@@ -145,7 +145,7 @@ enum WS_SFTP_LSTAT_STATE_ID {
     STATE_LSTAT_CLEANUP
 };
 
-/* This structure is to help with nonblocking and keeping track of state.
+/* SFTP buffer for nonblocking state tracking.
  * If adding any read/writes use the wolfSSH_SFTP_buffer_read/send functions */
 typedef struct WS_SFTP_BUFFER {
     byte*  data;
@@ -526,6 +526,19 @@ static int wolfSSH_SFTP_buffer_send(WOLFSSH* ssh, WS_SFTP_BUFFER* buffer)
         return WS_BUFFER_E;
     }
 
+    /* Flush any pending data in SSH output buffer first.
+     * Handles case where previous send returned WS_WANT_WRITE
+     * and data is still buffered at the SSH layer. */
+    if (wolfSSH_OutputPending(ssh)) {
+        ret = wolfSSH_SendPacket(ssh);
+        if (ret == WS_WANT_WRITE) {
+            return ret;
+        }
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
     /* Call wolfSSH worker if rekeying or adjusting window size */
     err = wolfSSH_get_error(ssh);
     if (err == WS_WINDOW_FULL || err == WS_REKEYING) {
@@ -546,6 +559,20 @@ static int wolfSSH_SFTP_buffer_send(WOLFSSH* ssh, WS_SFTP_BUFFER* buffer)
 }
 
 
+#ifdef WOLFSSH_TEST_INTERNAL
+int wolfSSH_TestSftpBufferSend(WOLFSSH* ssh,
+        byte* data, word32 sz, word32 idx)
+{
+    WS_SFTP_BUFFER buffer;
+
+    buffer.data = data;
+    buffer.sz = sz;
+    buffer.idx = idx;
+    return wolfSSH_SFTP_buffer_send(ssh, &buffer);
+}
+#endif
+
+
 /* returns the amount read on success */
 static int wolfSSH_SFTP_buffer_read(WOLFSSH* ssh, WS_SFTP_BUFFER* buffer,
         int readSz)
@@ -1603,6 +1630,13 @@ int wolfSSH_SFTP_read(WOLFSSH* ssh)
                     ssh->error = WS_WANT_WRITE;
                     return WS_FATAL_ERROR;
                 }
+                /* Check if SSH layer still has pending data from WS_WANT_WRITE.
+                 * Even if SFTP buffer is fully consumed, the data may still be
+                 * sitting in the SSH output buffer waiting to be sent. */
+                if (wolfSSH_OutputPending(ssh)) {
+                    ssh->error = WS_WANT_WRITE;
+                    return WS_FATAL_ERROR;
+                }
                 ret = WS_SUCCESS;
                 state->toSend = 0;
             }

tests/regress.c

@@ -40,6 +40,9 @@
 #include <wolfssh/port.h>
 #include <wolfssh/ssh.h>
 #include <wolfssh/internal.h>
+#ifdef WOLFSSH_SFTP
+    #include <wolfssh/wolfsftp.h>
+#endif
 #include "apps/wolfssh/common.h"
 
 #ifndef WOLFSSH_NO_ABORT
@@ -482,6 +485,78 @@ static void TestWorkerReadsWhenSendWouldBlock(void)
 #endif
 
 
+#ifdef WOLFSSH_SFTP
+/* Test that wolfSSH_SFTP_buffer_send() properly handles WS_WANT_WRITE when
+ * SSH output buffer has pending data. This is a regression test for
+ * the SFTP hang issue with non-blocking sockets.
+ *
+ * The fix checks for pending data in ssh->outputBuffer at the start of
+ * wolfSSH_SFTP_buffer_send() and returns WS_WANT_WRITE if the flush fails. */
+static int sftpWantWriteCallCount = 0;
+
+static int SftpWantWriteSendCb(WOLFSSH* ssh, void* buf, word32 sz, void* ctx)
+{
+    (void)ssh; (void)buf; (void)ctx;
+    sftpWantWriteCallCount++;
+    /* First call returns WANT_WRITE, subsequent calls succeed */
+    if (sftpWantWriteCallCount == 1) {
+        return WS_CBIO_ERR_WANT_WRITE;
+    }
+    return (int)sz;
+}
+
+static int SftpDummyRecv(WOLFSSH* ssh, void* buf, word32 sz, void* ctx)
+{
+    (void)ssh; (void)buf; (void)sz; (void)ctx;
+    return WS_CBIO_ERR_WANT_READ;
+}
+
+static void TestSftpBufferSendPendingOutput(void)
+{
+    WOLFSSH_CTX* ctx;
+    WOLFSSH* ssh;
+    byte testData[16];
+    int ret;
+
+    ctx = wolfSSH_CTX_new(WOLFSSH_ENDPOINT_SERVER, NULL);
+    AssertNotNull(ctx);
+
+    wolfSSH_SetIOSend(ctx, SftpWantWriteSendCb);
+    wolfSSH_SetIORecv(ctx, SftpDummyRecv);
+
+    ssh = wolfSSH_new(ctx);
+    AssertNotNull(ssh);
+
+    WMEMSET(testData, 0x42, sizeof(testData));
+
+    /* Simulate pending data in SSH output buffer (as if previous send
+     * returned WS_WANT_WRITE and data was buffered).
+     * Note: outputBuffer is initialized by BufferInit() with bufferSz set
+     * to at least STATIC_BUFFER_LEN (16 bytes), so we use a smaller value. */
+    ssh->outputBuffer.length = 8;   /* 8 bytes pending */
+    ssh->outputBuffer.idx = 0;      /* none sent yet */
+
+    sftpWantWriteCallCount = 0;
+
+    /* Call wolfSSH_TestSftpBufferSend - should return WS_WANT_WRITE because
+     * the fix detects pending data in outputBuffer and tries to flush it,
+     * which fails with WS_WANT_WRITE from our callback.
+     *
+     * Before the fix, the function would ignore the pending SSH output buffer
+     * data and proceed to send new SFTP data, leading to a hang because the
+     * pending data was never flushed. */
+    ret = wolfSSH_TestSftpBufferSend(ssh, testData, sizeof(testData), 0);
+    AssertIntEQ(ret, WS_WANT_WRITE);
+
+    /* Verify the SSH output buffer still has pending data */
+    AssertTrue(ssh->outputBuffer.length > ssh->outputBuffer.idx);
+
+    wolfSSH_free(ssh);
+    wolfSSH_CTX_free(ctx);
+}
+#endif /* WOLFSSH_SFTP */
+
+
 int main(int argc, char** argv)
 {
     WOLFSSH_CTX* ctx;
@@ -515,6 +590,10 @@ int main(int argc, char** argv)
     TestWorkerReadsWhenSendWouldBlock();
 #endif
 
+#ifdef WOLFSSH_SFTP
+    TestSftpBufferSendPendingOutput();
+#endif
+
     /* TODO: add app-level regressions that simulate stdin EOF/password
      * prompts and mid-session socket closes once the test harness can
      * drive the wolfssh client without real sockets/tty. */

wolfssh/internal.h

@@ -1072,6 +1072,7 @@ enum ChannelOpenFailReasons {
 WOLFSSH_LOCAL int DoReceive(WOLFSSH*);
 WOLFSSH_LOCAL int DoProtoId(WOLFSSH*);
 WOLFSSH_LOCAL int wolfSSH_SendPacket(WOLFSSH*);
+WOLFSSH_LOCAL int wolfSSH_OutputPending(WOLFSSH*);
 WOLFSSH_LOCAL int SendProtoId(WOLFSSH*);
 WOLFSSH_LOCAL int SendKexInit(WOLFSSH*);
 WOLFSSH_LOCAL int SendKexDhInit(WOLFSSH*);

wolfssh/wolfsftp.h

@@ -282,6 +282,11 @@ WOLFSSL_LOCAL int SFTP_RemoveHandleNode(WOLFSSH* ssh, byte* handle,
 
 WOLFSSH_LOCAL void wolfSSH_SFTP_ShowSizes(void);
 
+#ifdef WOLFSSH_TEST_INTERNAL
+    WOLFSSH_API int wolfSSH_TestSftpBufferSend(WOLFSSH* ssh,
+            byte* data, word32 sz, word32 idx);
+#endif
+
 #ifdef __cplusplus
 }
 #endif