[trace] Keep track of a uint64_t tracing context

The most common information that you want to track between begin() and
end() is the timestamp of the begin function, so you can measure the
duration of the (de)compression call. Allow the tracing library to put
this information inside the `ZSTD_TraceCtx`, so it doesn't need to keep
a global map in this case. If a single uint64_t is not enough, the
tracing library can return a unique identifier (like the context
pointer) instead, and use it as a key in a map.

This keeps the simple case simple.

lib/common/zstd_trace.c

@@ -15,27 +15,27 @@
 
 #if ZSTD_TRACE && ZSTD_HAVE_WEAK_SYMBOLS
 
-ZSTD_WEAK_ATTR int ZSTD_trace_compress_begin(ZSTD_CCtx const* cctx)
+ZSTD_WEAK_ATTR ZSTD_TraceCtx ZSTD_trace_compress_begin(ZSTD_CCtx const* cctx)
 {
     (void)cctx;
     return 0;
 }
 
-ZSTD_WEAK_ATTR void ZSTD_trace_compress_end(ZSTD_CCtx const* cctx, ZSTD_trace const* trace)
+ZSTD_WEAK_ATTR void ZSTD_trace_compress_end(ZSTD_TraceCtx ctx, ZSTD_Trace const* trace)
 {
-    (void)cctx;
+    (void)ctx;
     (void)trace;
 }
 
-ZSTD_WEAK_ATTR int ZSTD_trace_decompress_begin(ZSTD_DCtx const* dctx)
+ZSTD_WEAK_ATTR ZSTD_TraceCtx ZSTD_trace_decompress_begin(ZSTD_DCtx const* dctx)
 {
     (void)dctx;
     return 0;
 }
 
-ZSTD_WEAK_ATTR void ZSTD_trace_decompress_end(ZSTD_DCtx const* dctx, ZSTD_trace const* trace)
+ZSTD_WEAK_ATTR void ZSTD_trace_decompress_end(ZSTD_TraceCtx ctx, ZSTD_Trace const* trace)
 {
-    (void)dctx;
+    (void)ctx;
     (void)trace;
 }
 

lib/common/zstd_trace.h

@@ -77,41 +77,67 @@ typedef struct {
      * The fully resolved CCtx parameters (NULL on decompression).
      */
     struct ZSTD_CCtx_params_s const* params;
-} ZSTD_trace;
+    /**
+     * The ZSTD_CCtx pointer (NULL on decompression).
+     */
+    struct ZSTD_CCtx_s const* cctx;
+    /**
+     * The ZSTD_DCtx pointer (NULL on compression).
+     */
+    struct ZSTD_DCtx_s const* dctx;
+} ZSTD_Trace;
+
+/**
+ * A tracing context. It must be 0 when tracing is disabled.
+ * Otherwise, any non-zero value returned by a tracing begin()
+ * function is presented to any subsequent calls to end().
+ *
+ * Any non-zero value is treated as tracing is enabled and not
+ * interpreted by the library.
+ *
+ * Two possible uses are:
+ * * A timestamp for when the begin() function was called.
+ * * A unique key identifying the (de)compression, like the
+ *   address of the [dc]ctx pointer if you need to track
+ *   more information than just a timestamp.
+ */
+typedef unsigned long long ZSTD_TraceCtx;
 
 /**
  * Trace the beginning of a compression call.
  * @param cctx The dctx pointer for the compression.
  *             It can be used as a key to map begin() to end().
- * @returns Non-zero if tracing is enabled.
+ * @returns Non-zero if tracing is enabled. The return value is
+ *          passed to ZSTD_trace_compress_end().
  */
-int ZSTD_trace_compress_begin(struct ZSTD_CCtx_s const* cctx);
+ZSTD_TraceCtx ZSTD_trace_compress_begin(struct ZSTD_CCtx_s const* cctx);
 
 /**
  * Trace the end of a compression call.
- * @param cctx The dctx pointer for the decompression.
+ * @param ctx The return value of ZSTD_trace_compress_begin().
  * @param trace The zstd tracing info.
  */
 void ZSTD_trace_compress_end(
-    struct ZSTD_CCtx_s const* cctx,
-    ZSTD_trace const* trace);
+    ZSTD_TraceCtx ctx,
+    ZSTD_Trace const* trace);
 
 /**
  * Trace the beginning of a decompression call.
  * @param dctx The dctx pointer for the decompression.
  *             It can be used as a key to map begin() to end().
- * @returns Non-zero if tracing is enabled.
+ * @returns Non-zero if tracing is enabled. The return value is
+ *          passed to ZSTD_trace_compress_end().
  */
-int ZSTD_trace_decompress_begin(struct ZSTD_DCtx_s const* dctx);
+ZSTD_TraceCtx ZSTD_trace_decompress_begin(struct ZSTD_DCtx_s const* dctx);
 
 /**
  * Trace the end of a decompression call.
- * @param dctx The dctx pointer for the decompression.
+ * @param ctx The return value of ZSTD_trace_decompress_begin().
  * @param trace The zstd tracing info.
  */
 void ZSTD_trace_decompress_end(
-    struct ZSTD_DCtx_s const* dctx,
-    ZSTD_trace const* trace);
+    ZSTD_TraceCtx ctx,
+    ZSTD_Trace const* trace);
 
 #endif /* ZSTD_TRACE */