Enable (safe) use of zstd static-only APIs

We'll only use these when the compile-time version and the run-time
version of the zstd library match.  Part of ticket 25162.

changes/ticket25162

@@ -0,0 +1,6 @@
+  o Minor features (compression, zstd):
+    - When running with zstd, Tor now considers using advanced functions that
+      the zstd maintainers have labeled as potentially unstable. To
+      prevent breakage, Tor will only use this functionality when
+      the runtime version of the zstd library matches the version
+      with which it were compiled.  Closes ticket 25162.

src/common/compress_zstd.c

@@ -18,6 +18,13 @@
 #include "compress.h"
 #include "compress_zstd.h"
 
+/* This is a lie, but we make sure it doesn't get us in trouble by wrapping
+ * all invocations of zstd's static-only functions in a check to make sure
+ * that the compile-time version matches the run-time version.
+ *
+ * Note: Make sure that this file still builds with this macro disabled. */
+#define ZSTD_STATIC_LINKING_ONLY
+
 #ifdef HAVE_ZSTD
 #include <zstd.h>
 #endif
@@ -85,6 +92,26 @@ tor_zstd_get_header_version_str(void)
 #endif
 }
 
+#ifdef TOR_UNIT_TESTS
+static int static_apis_disable_for_testing = 0;
+#endif
+
+/** Return true iff we can use the "static-only" APIs. */
+int
+tor_zstd_can_use_static_apis(void)
+{
+#if defined(ZSTD_STATIC_LINKING_ONLY) && defined(HAVE_ZSTD)
+#ifdef TOR_UNIT_TESTS
+  if (static_apis_disable_for_testing) {
+    return 0;
+  }
+#endif
+  return (ZSTD_VERSION_NUMBER == ZSTD_versionNumber());
+#else
+  return 0;
+#endif
+}
+
 /** Internal Zstandard state for incremental compression/decompression.
  * The body of this struct is not exposed. */
 struct tor_zstd_compress_state_t {
@@ -440,3 +467,12 @@ tor_zstd_init(void)
   atomic_counter_init(&total_zstd_allocation);
 }
 
+#ifdef TOR_UNIT_TESTS
+/** Testing only: disable usage of static-only APIs, so we can make sure that
+ * we still work without them. */
+void
+tor_zstd_set_static_apis_disabled_for_testing(int disabled)
+{
+  static_apis_disable_for_testing = disabled;
+}
+#endif

src/common/compress_zstd.h

@@ -17,6 +17,8 @@ const char *tor_zstd_get_version_str(void);
 
 const char *tor_zstd_get_header_version_str(void);
 
+int tor_zstd_can_use_static_apis(void);
+
 /** Internal state for an incremental Zstandard compression/decompression. */
 typedef struct tor_zstd_compress_state_t tor_zstd_compress_state_t;
 
@@ -42,5 +44,9 @@ size_t tor_zstd_get_total_allocation(void);
 
 void tor_zstd_init(void);
 
+#ifdef TOR_UNIT_TESTS
+void tor_zstd_set_static_apis_disabled_for_testing(int disabled);
+#endif
+
 #endif /* !defined(TOR_COMPRESS_ZSTD_H) */