minor behavior refinements

specifically, there is no obligation to start streaming compression with pos=0.
stableSrc mode is now compatible with this setup.

lib/compress/zstd_compress.c

@@ -5630,6 +5630,8 @@ static size_t ZSTD_CCtx_init_compressStream2(ZSTD_CCtx* cctx,
     return 0;
 }
 
+/* @return provides a minimum amount of data remaining to be flushed from internal buffers
+ */
 size_t ZSTD_compressStream2( ZSTD_CCtx* cctx,
                              ZSTD_outBuffer* output,
                              ZSTD_inBuffer* input,
@@ -5644,13 +5646,15 @@ size_t ZSTD_compressStream2( ZSTD_CCtx* cctx,
 
     /* transparent initialization stage */
     if (cctx->streamStage == zcss_init) {
+        size_t const inputSize = input->size - input->pos;  /* no obligation to start from pos==0 */
         if ( (cctx->requestedParams.inBufferMode == ZSTD_bm_stable) /* input is presumed stable, across invocations */
-          && (endOp == ZSTD_e_continue)                             /* more to come */
+          && (endOp == ZSTD_e_continue)                             /* no flush requested, more input to come */
-          && (input->pos < ZSTD_BLOCKSIZE_MAX) ) {                  /* not even reached one block yet */
+          && (inputSize < ZSTD_BLOCKSIZE_MAX) ) {                   /* not even reached one block yet */
+            /* just wait, allows lazy adaptation of compression parameters */
             cctx->expectedInBuffer = *input;
-            return (ZSTD_BLOCKSIZE_MAX - input->pos);               /* don't do anything : allows lazy adaptation of compression parameters */
+            return ZSTD_FRAMEHEADERSIZE_MIN(cctx->requestedParams.format);  /* at least some header to produce */
         }
-        FORWARD_IF_ERROR(ZSTD_CCtx_init_compressStream2(cctx, endOp, input->size), "CompressStream2 initialization failed");
+        FORWARD_IF_ERROR(ZSTD_CCtx_init_compressStream2(cctx, endOp, inputSize), "CompressStream2 initialization failed");
         ZSTD_setBufferExpectations(cctx, output, input);    /* Set initial buffer expectations now that we've initialized */
     }
     /* end of transparent initialization stage */

lib/zstd.h

@@ -1851,7 +1851,7 @@ ZSTDLIB_STATIC_API size_t ZSTD_CCtx_refPrefix_advanced(ZSTD_CCtx* cctx, const vo
  * memory. However, compression WILL fail if conditions are not respected.
  *
  * WARNING: The data in the ZSTD_inBuffer in the range [src, src + pos) MUST
- * not be modified during compression or it sill result in data corruption.
+ * not be modified during compression or it will result in data corruption.
  * This is because zstd needs to reference data in the ZSTD_inBuffer to find
  * matches. Normally zstd maintains its own window buffer for this purpose,
  * but passing this flag tells zstd to rely on user provided buffer instead.

tests/zstreamtest.c

@@ -873,20 +873,21 @@ static int basicUnitTests(U32 seed, double compressibility)
         DISPLAYLEVEL(3, "test%3i : ZSTD_c_stableInBuffer compatibility with compressStream, flushStream and endStream : ", testNb++);
         CHECK_Z( ZSTD_initCStream(cctx, 1) );
         CHECK_Z( ZSTD_CCtx_setParameter(cctx, ZSTD_c_stableInBuffer, 1) );
-        in.src = CNBuffer;
+        {   ZSTD_inBuffer inBuf;
-        in.size = 100;
+            ZSTD_outBuffer outBuf;
-        in.pos = 0;
+            inBuf.src = CNBuffer;
-        {   ZSTD_outBuffer outBuf;
+            inBuf.size = 100;
+            inBuf.pos = 0;
             outBuf.dst = (char*)(compressedBuffer)+cSize;
             outBuf.size = ZSTD_compressBound(500);
             outBuf.pos = 0;
-            CHECK_Z( ZSTD_compressStream(cctx, &outBuf, &in) );
+            CHECK_Z( ZSTD_compressStream(cctx, &outBuf, &inBuf) );
-            in.size = 200;
+            inBuf.size = 200;
-            CHECK_Z( ZSTD_compressStream(cctx, &outBuf, &in) );
+            CHECK_Z( ZSTD_compressStream(cctx, &outBuf, &inBuf) );
             CHECK_Z( ZSTD_flushStream(cctx, &outBuf) );
-            in.size = 300;
+            inBuf.size = 300;
-            CHECK_Z( ZSTD_compressStream(cctx, &outBuf, &in) );
+            CHECK_Z( ZSTD_compressStream(cctx, &outBuf, &inBuf) );
-            if (ZSTD_endStream(cctx, &outBuf) != 0) goto _output_error;  /* error, or some data not flushed */
+            CHECK(ZSTD_endStream(cctx, &outBuf) != 0, "compression should be successful and fully flushed");
         }
         DISPLAYLEVEL(3, "OK \n");
 
@@ -902,6 +903,7 @@ static int basicUnitTests(U32 seed, double compressibility)
         CHECK_Z(ZSTD_CCtx_reset(cctx, ZSTD_reset_session_and_parameters));
         CHECK_Z(ZSTD_CCtx_setParameter(cctx, ZSTD_c_checksumFlag, 1));
         CHECK_Z(ZSTD_CCtx_setParameter(cctx, ZSTD_c_stableOutBuffer, 1));
+        in.src = CNBuffer;
         in.pos = out.pos = 0;
         in.size = MIN(CNBufferSize, 10);
         out.size = compressedBufferSize;