Make sure that if use_mmap_reads is on use_os_buffer is also on

Summary: The code assumes that if use_mmap_reads is on then use_os_buffer is also on.  This make sense as by using memory mapped files for reading you are expecting the OS to cache what it needs.  Add code to make sure the user does not turn off use_os_buffer when they turn on use_mmap_reads

Test Plan: New test: DBTest.MMapAndBufferOptions

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56397

db/db_impl.cc

@@ -201,6 +201,44 @@ Status SanitizeOptionsByTable(
   return Status::OK();
 }
 
+static Status ValidateOptions(
+    const DBOptions& db_options,
+    const std::vector<ColumnFamilyDescriptor>& column_families) {
+  Status s;
+
+  for (auto& cfd : column_families) {
+    s = CheckCompressionSupported(cfd.options);
+    if (s.ok() && db_options.allow_concurrent_memtable_write) {
+      s = CheckConcurrentWritesSupported(cfd.options);
+    }
+    if (!s.ok()) {
+      return s;
+    }
+    if (db_options.db_paths.size() > 1) {
+      if ((cfd.options.compaction_style != kCompactionStyleUniversal) &&
+          (cfd.options.compaction_style != kCompactionStyleLevel)) {
+        return Status::NotSupported(
+            "More than one DB paths are only supported in "
+            "universal and level compaction styles. ");
+      }
+    }
+  }
+
+  if (db_options.db_paths.size() > 4) {
+    return Status::NotSupported(
+        "More than four DB paths are not supported yet. ");
+  }
+
+  if (db_options.allow_mmap_reads && !db_options.allow_os_buffer) {
+    // Protect against assert in PosixMMapReadableFile constructor
+    return Status::NotSupported(
+        "If memory mapped reads (allow_mmap_reads) are enabled "
+        "then os caching (allow_os_buffer) must also be enabled. ");
+  }
+
+  return Status::OK();
+}
+
 CompressionType GetCompressionFlush(const ImmutableCFOptions& ioptions) {
   // Compressing memtable flushes might not help unless the sequential load
   // optimization is used for leveled compaction. Otherwise the CPU and
@@ -5388,28 +5426,10 @@ Status DB::Open(const DBOptions& db_options, const std::string& dbname,
     return s;
   }
 
-  for (auto& cfd : column_families) {
+  s = ValidateOptions(db_options, column_families);
-    s = CheckCompressionSupported(cfd.options);
-    if (s.ok() && db_options.allow_concurrent_memtable_write) {
-      s = CheckConcurrentWritesSupported(cfd.options);
-    }
   if (!s.ok()) {
     return s;
   }
-    if (db_options.db_paths.size() > 1) {
-      if ((cfd.options.compaction_style != kCompactionStyleUniversal) &&
-          (cfd.options.compaction_style != kCompactionStyleLevel)) {
-        return Status::NotSupported(
-            "More than one DB paths are only supported in "
-            "universal and level compaction styles. ");
-      }
-    }
-  }
-
-  if (db_options.db_paths.size() > 4) {
-    return Status::NotSupported(
-        "More than four DB paths are not supported yet. ");
-  }
 
   *dbptr = nullptr;
   handles->clear();

db/db_test.cc

@@ -5876,6 +5876,26 @@ TEST_F(DBTest, TableOptionsSanitizeTest) {
   ASSERT_OK(TryReopen(options));
 }
 
+TEST_F(DBTest, MmapAndBufferOptions) {
+  Options options = CurrentOptions();
+
+  // If allow_mmap_reads is on allow_os_buffer must also be on
+  options.allow_os_buffer = false;
+  options.allow_mmap_reads = true;
+  ASSERT_NOK(TryReopen(options));
+
+  // All other combinations are acceptable
+  options.allow_os_buffer = true;
+  ASSERT_OK(TryReopen(options));
+
+  options.allow_os_buffer = false;
+  options.allow_mmap_reads = false;
+  ASSERT_OK(TryReopen(options));
+
+  options.allow_os_buffer = true;
+  ASSERT_OK(TryReopen(options));
+}
+
 TEST_F(DBTest, ConcurrentMemtableNotSupported) {
   Options options = CurrentOptions();
   options.allow_concurrent_memtable_write = true;