Fix zstd 1.3.0 trouble: Be more respectful of its state machine

In zstd 1.3.0, once you have called ZSTD_endStream and been told
that your putput buffer is full, it really doesn't want you to call
ZSTD_compressStream again.  ZSTD 1.2.0 didn't seem to mind about
this.

This patch fixes the issue by making sure never to call
ZSTD_endStream if there's any more data on the input buffer to
process, by flushing even when we're about to call "endStream", and
by never calling "compress" or "flush" after "endStream".

changes/bug22927

@@ -0,0 +1,6 @@
+  o Minor bugfixes (compatibility, zstd):
+    - Write zstd epilogues correctly when the epilogue requires reallocation
+      of the output buffer, even with zstd 1.3.0. (Previously,
+      we worked on 1.2.0 and failed with 1.3.0).  Fixes bug 22927; bugfix on
+      0.3.1.1-alpha.
+

src/common/compress_zstd.c

@@ -98,6 +98,8 @@ struct tor_zstd_compress_state_t {
 #endif // HAVE_ZSTD.
 
   int compress; /**< True if we are compressing; false if we are inflating */
+  int have_called_end; /**< True if we are compressing and we've called
+                        * ZSTD_endStream */
 
   /** Number of bytes read so far.  Used to detect compression bombs. */
   size_t input_so_far;
@@ -270,9 +272,16 @@ tor_zstd_compress_process(tor_zstd_compress_state_t *state,
   ZSTD_inBuffer input = { *in, *in_len, 0 };
   ZSTD_outBuffer output = { *out, *out_len, 0 };
 
+  if (BUG(finish == 0 && state->have_called_end)) {
+    finish = 1;
+  }
+
   if (state->compress) {
-    retval = ZSTD_compressStream(state->u.compress_stream,
-                                 &output, &input);
+    if (! state->have_called_end)
+      retval = ZSTD_compressStream(state->u.compress_stream,
+                                   &output, &input);
+    else
+      retval = 0;
   } else {
     retval = ZSTD_decompressStream(state->u.decompress_stream,
                                    &output, &input);
@@ -300,7 +309,7 @@ tor_zstd_compress_process(tor_zstd_compress_state_t *state,
     return TOR_COMPRESS_ERROR;
   }
 
-  if (state->compress && !finish) {
+  if (state->compress && !state->have_called_end) {
     retval = ZSTD_flushStream(state->u.compress_stream, &output);
 
     *out = (char *)output.dst + output.pos;
@@ -314,16 +323,24 @@ tor_zstd_compress_process(tor_zstd_compress_state_t *state,
 
     // ZSTD_flushStream returns 0 if the frame is done, or >0 if it
     // is incomplete.
-    if (retval > 0)
+    if (retval > 0) {
       return TOR_COMPRESS_BUFFER_FULL;
+    }
   }
 
   if (!finish) {
-    // We're not done with the input, so no need to flush.
+    // The caller says we're not done with the input, so no need to write an
+    // epilogue.
     return TOR_COMPRESS_OK;
   } else if (state->compress) {
-    retval = ZSTD_endStream(state->u.compress_stream, &output);
+    if (*in_len) {
+      // We say that we're not done with the input, so we can't write an
+      // epilogue.
+      return TOR_COMPRESS_OK;
+    }
 
+    retval = ZSTD_endStream(state->u.compress_stream, &output);
+    state->have_called_end = 1;
     *out = (char *)output.dst + output.pos;
     *out_len = output.size - output.pos;