Fix Zstd-less Build with Macro Guards

Summary:
D18890515 broke folly builds when zstd isn't present. (My bad!!)

This should fix it.

Reviewed By: lukaspiatkowski

Differential Revision: D19029293

fbshipit-source-id: 2ede5a8402e60af61a294c9db35ca15dffdf4c49

folly/compression/Compression.h

@@ -548,11 +548,10 @@ bool hasCodec(CodecType type);
 bool hasStreamCodec(CodecType type);
 
 /**
- * Added here so users of folly can figure out whether the singletons are
- * usable, and therefore whether to even try to include the header.
- *
- * 1 means that the header exists and ZSTD_CCtx and ZSTD_DCtx pools are defined.
+ * Added here so users of folly can figure out whether the header
+ * folly/compression/CompressionContextPoolSingletons.h is present, and
+ * therefore whether it can be included.
  */
-#define FOLLY_COMPRESSION_HAS_CONTEXT_POOL_SINGLETONS 1
+#define FOLLY_COMPRESSION_HAS_CONTEXT_POOL_SINGLETONS
 } // namespace io
 } // namespace folly

folly/compression/CompressionContextPoolSingletons.cpp

@@ -20,6 +20,7 @@ namespace folly {
 namespace compression {
 namespace contexts {
 
+#if FOLLY_HAVE_LIBZSTD
 namespace {
 // These objects have no static dependencies and therefore no SIOF issues.
 ZSTD_CCtx_Pool zstd_cctx_pool_singleton;
@@ -57,6 +58,8 @@ ZSTD_CCtx_Pool::Ref getNULL_ZSTD_CCtx() {
 ZSTD_DCtx_Pool::Ref getNULL_ZSTD_DCtx() {
   return zstd_dctx_pool_singleton.getNull();
 }
+#endif // FOLLY_HAVE_LIBZSTD
+
 } // namespace contexts
 } // namespace compression
 } // namespace folly

folly/compression/CompressionContextPoolSingletons.h

@@ -16,14 +16,26 @@
 
 #pragma once
 
+#include <folly/folly-config.h>
+
+#if FOLLY_HAVE_LIBZSTD
 #include <zstd.h>
+#endif
 
 #include <folly/compression/CompressionCoreLocalContextPool.h>
 
+// When this header is present, folly/compression/Compression.h defines
+// FOLLY_COMPRESSION_HAS_CONTEXT_POOL_SINGLETONS.
+
 namespace folly {
 namespace compression {
 namespace contexts {
 
+#if FOLLY_HAVE_LIBZSTD
+
+// Additional feature test macro for zstd singletons.
+#define FOLLY_COMPRESSION_HAS_ZSTD_CONTEXT_POOL_SINGLETONS
+
 struct ZSTD_CCtx_Creator {
   ZSTD_CCtx* operator()() const noexcept;
 };
@@ -59,6 +71,8 @@ ZSTD_CCtx_Pool::Ref getNULL_ZSTD_CCtx();
 
 ZSTD_DCtx_Pool::Ref getNULL_ZSTD_DCtx();
 
+#endif // FOLLY_HAVE_LIBZSTD
+
 } // namespace contexts
 } // namespace compression
 } // namespace folly

folly/compression/test/CompressionContextPoolTest.cpp

@@ -260,6 +260,8 @@ TEST_F(CompressionCoreLocalContextPoolTest, testMultithread) {
   EXPECT_LE(numFoos.load(), numThreads);
 }
 
+#ifdef FOLLY_COMPRESSION_HAS_ZSTD_CONTEXT_POOL_SINGLETONS
+
 TEST(CompressionContextPoolSingletonsTest, testSingletons) {
   EXPECT_NE(contexts::getZSTD_CCtx(), nullptr);
   EXPECT_NE(contexts::getZSTD_DCtx(), nullptr);
@@ -269,5 +271,8 @@ TEST(CompressionContextPoolSingletonsTest, testSingletonsNull) {
   EXPECT_EQ(contexts::getNULL_ZSTD_CCtx(), nullptr);
   EXPECT_EQ(contexts::getNULL_ZSTD_DCtx(), nullptr);
 }
+
+#endif // FOLLY_COMPRESSION_HAS_ZSTD_CONTEXT_POOL_SINGLETONS
+
 } // namespace compression
 } // namespace folly