Exend XLOG_EVERY_N use

Summary:
the n argument in XLOG_EVERY_N is not captured by it's lambda, so
expressions using local variables can not be passed as n. EdenFS has
a use case where we would like to use local variables here. We want to use a
dynamic config value for n, so that we can easily adjust how often we log
see: D25505654.

It seems like it should be ok to capture the n here, but I am by no means a
macro expert, so please let me know if there is a tricky cpp thing here lurking
that makes capturing n bad.

Reviewed By: simpkins

Differential Revision: D25551033

fbshipit-source-id: e82fa465823d3d2ee2ac26924cc4a93b7b7242dd

folly/logging/test/XlogTest.cpp

@@ -274,6 +274,7 @@ TEST_F(XlogTest, perFileCategoryHandling) {
 }
 
 TEST_F(XlogTest, rateLimiting) {
+  auto SEVEN = 7;
   auto handler = make_shared<TestLogHandler>();
   LoggerDB::get().getCategory("xlog_test")->addHandler(handler);
   LoggerDB::get().setLevel("xlog_test", LogLevel::DBG1);
@@ -295,6 +296,15 @@ TEST_F(XlogTest, rateLimiting) {
           "msg 49"));
   handler->clearMessages();
 
+  for (size_t n = 0; n < 50; ++n) {
+    XLOG_EVERY_N(DBG1, SEVEN + 1, "msg ", n);
+  }
+  EXPECT_THAT(
+      handler->getMessageValues(),
+      ElementsAre(
+          "msg 0", "msg 8", "msg 16", "msg 24", "msg 32", "msg 40", "msg 48"));
+  handler->clearMessages();
+
   // Test XLOG_EVERY_N_EXACT
   for (size_t n = 0; n < 50; ++n) {
     XLOG_EVERY_N_EXACT(DBG1, 7, "msg ", n);
@@ -312,6 +322,15 @@ TEST_F(XlogTest, rateLimiting) {
           "msg 49"));
   handler->clearMessages();
 
+  for (size_t n = 0; n < 50; ++n) {
+    XLOG_EVERY_N_EXACT(DBG1, SEVEN + 1, "msg ", n);
+  }
+  EXPECT_THAT(
+      handler->getMessageValues(),
+      ElementsAre(
+          "msg 0", "msg 8", "msg 16", "msg 24", "msg 32", "msg 40", "msg 48"));
+  handler->clearMessages();
+
   // Test XLOG_EVERY_N_THREAD
   for (size_t n = 0; n < 50; ++n) {
     XLOG_EVERY_N_THREAD(DBG1, 7, "msg ", n);
@@ -329,6 +348,15 @@ TEST_F(XlogTest, rateLimiting) {
           "msg 49"));
   handler->clearMessages();
 
+  for (size_t n = 0; n < 50; ++n) {
+    XLOG_EVERY_N_THREAD(DBG1, SEVEN + 1, "msg ", n);
+  }
+  EXPECT_THAT(
+      handler->getMessageValues(),
+      ElementsAre(
+          "msg 0", "msg 8", "msg 16", "msg 24", "msg 32", "msg 40", "msg 48"));
+  handler->clearMessages();
+
   // Test XLOG_EVERY_MS and XLOG_N_PER_MS
   // We test these together to minimize the number of sleep operations.
   for (size_t n = 0; n < 10; ++n) {

folly/logging/xlog.h

@@ -141,7 +141,7 @@ FOLLY_EXPORT FOLLY_ALWAYS_INLINE bool xlogEveryNImpl(size_t n) {
 #define XLOG_EVERY_N(level, n, ...)                                       \
   XLOG_IF(                                                                \
       level,                                                              \
-      [] {                                                                \
+      [&] {                                                               \
         struct folly_detail_xlog_tag {};                                  \
         return ::folly::detail::xlogEveryNImpl<folly_detail_xlog_tag>(n); \
       }(),                                                                \
@@ -172,7 +172,7 @@ FOLLY_EXPORT FOLLY_ALWAYS_INLINE bool xlogEveryNExactImpl(size_t n) {
 #define XLOG_EVERY_N_EXACT(level, n, ...)                                      \
   XLOG_IF(                                                                     \
       level,                                                                   \
-      [] {                                                                     \
+      [&] {                                                                    \
         struct folly_detail_xlog_tag {};                                       \
         return ::folly::detail::xlogEveryNExactImpl<folly_detail_xlog_tag>(n); \
       }(),                                                                     \
@@ -212,7 +212,7 @@ FOLLY_EXPORT FOLLY_ALWAYS_INLINE bool xlogEveryNThreadImpl(size_t n) {
 #define XLOG_EVERY_N_THREAD(level, n, ...)                                   \
   XLOG_IF(                                                                   \
       level,                                                                 \
-      [] {                                                                   \
+      [&] {                                                                  \
         struct folly_detail_xlog_tag {};                                     \
         return ::folly::detail::xlogEveryNThreadImpl<folly_detail_xlog_tag>( \
             n);                                                              \