SERVER-10172 Refactor RamLog to make it thread-safe.

Also prevent accidental creation of duplicate ramlogs of the same name.

1 files changed, 93 insertions, 12 deletions

diff --git a/src/mongo/logger/ramlog.h b/src/mongo/logger/ramlog.h
index 2b78d91daaa..16263055c38 100644
--- a/src/mongo/logger/ramlog.h
+++ b/src/mongo/logger/ramlog.h
@@ -17,7 +17,6 @@
 
 #pragma once
 
-#include <map>
 #include <sstream>
 #include <string>
 #include <vector>
@@ -32,23 +31,61 @@
 
 namespace mongo {
 
+    /**
+     * Fixed-capacity log of line-oriented messages.
+     *
+     * Holds up to RamLog::N lines of up to RamLog::C bytes, each.
+     *
+     * RamLogs are stored in a global registry, accessed via RamLog::get() and
+     * RamLog::getIfExists().
+     *
+     * RamLogs and their registry are self-synchronizing.  See documentary comments.
+     * To read a RamLog, instantiate a RamLog::LineIterator, documented below.
+     */
     class RamLog : public logger::Tee {
         MONGO_DISALLOW_COPYING(RamLog);
     public:
-        RamLog( const std::string& name );
+        class LineIterator;
+        friend class RamLog::LineIterator;
+
+        /**
+         * Returns a pointer to the ramlog named "name", creating one if it did not already exist.
+         *
+         * Synchronizes on the RamLog catalog lock, _namedLock.
+         */
+        static RamLog* get(const std::string& name);
+
+        /**
+         * Returns a pointer to the ramlog named "name", or NULL if no such ramlog exists.
+         *
+         * Synchronizes on the RamLog catalog lock, _namedLock.
+         */
+        static RamLog* getIfExists(const std::string& name);
+
+        /**
+         * Writes the names of all existing ramlogs into "names".
+         *
+         * Synchronizes on the RamLog catalog lock, _namedLock.
+         */
+        static void getNames( std::vector<std::string>& names );
 
+        /**
+         * Writes "str" as a line into the RamLog.  If "str" is longer than the maximum
+         * line size, RamLog::C, truncates the line to the first C bytes.  If "str"
+         * is shorter than RamLog::C and has a terminal '\n', it omits that character.
+         *
+         * Synchronized on the instance's own mutex, _mutex.
+         */
         void write(const std::string& str);
 
-        void get( std::vector<const char*>& v);
 
+        /**
+         * Writes an HTML representation of the log to "s".
+         *
+         * Synchronized on the instance's own mutex, _mutex.
+         */
         void toHTML(std::stringstream& s);
 
-        static RamLog* get( const std::string& name );
-        static void getNames( std::vector<std::string>& names );
-
-        time_t lastWrite();
-        long long getTotalLinesWritten();
-
     private:
         static int repeats(const std::vector<const char *>& v, int i);
         static string clean(const std::vector<const char *>& v, int i, string line="");
@@ -57,6 +94,7 @@ namespace mongo {
         /* turn http:... into an anchor */
         static string linkify(const char *s);
 
+        explicit RamLog( const std::string& name );
         ~RamLog(); // want this private as we want to leak so we can use them till the very end
 
         enum {
@@ -64,6 +102,8 @@ namespace mongo {
             C = 512 // max size of line
         };
 
+        const char* getLine_inlock(unsigned lineNumber) const;
+
         boost::mutex _mutex;  // Guards all non-static data.
         char lines[N][C];
         unsigned h; // current position
@@ -71,12 +111,53 @@ namespace mongo {
         string _name;
         long long _totalLinesWritten;
 
-        typedef std::map<string,RamLog*> RM;
-        static mongo::mutex* _namedLock;
-        static RM*  _named;
         time_t _lastWrite;
     };
 
+    /**
+     * Iterator over the lines of a RamLog.
+     *
+     * Also acts as a means of inspecting other properites of a ramlog consistently.
+     *
+     * Instances of LineIterator hold the lock for the underlying RamLog for their whole lifetime,
+     * and so should not be kept around.
+     */
+    class RamLog::LineIterator {
+        MONGO_DISALLOW_COPYING(LineIterator);
+    public:
+        explicit LineIterator(RamLog* ramlog);
+
+        /**
+         * Returns true if there are more lines available to return by calls to next().
+         */
+        bool more() const { return _nextLineIndex < _ramlog->n; }
+
+        /**
+         * Returns the next line and advances the iterator.
+         */
+        const char* next() {
+            return _ramlog->getLine_inlock(_nextLineIndex++);  // Postfix increment.
+        }
+
+        /**
+         * Returns the time of the last write to the ramlog.
+         */
+        time_t lastWrite();
+
+        /**
+         * Returns the total number of lines ever written to the ramlog.
+         */
+        long long getTotalLinesWritten();
+
+    private:
+        const RamLog* _ramlog;
+        boost::lock_guard<boost::mutex> _lock;
+        unsigned _nextLineIndex;
+    };
+
+    /**
+     * Appender for appending MessageEvents to a RamLog.
+     */
     class RamLogAppender : public logger::Appender<logger::MessageEventEphemeral> {
     public:
         explicit RamLogAppender(RamLog* ramlog);