Delay actually opening log files until the first write, fix #1724

Calling DepsLog/BuildLog::OpenForWrite will now only save the file path.
The actual opening of the file (moved to OpenForWriteIfNeeded) happens
right before the first write attempt.

This is needed so that the files aren't held open when the generator
runs (i.e. RebuildManifest) as it may call tools like recompact which
won't be able to open the file on Windows.

The disadvantage is that now the error reporting happens at a later time
and will be reported as a failed write, not a failed open.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/build_log.h b/src/build_log.h
index ebe0530..6d060d1 100644
--- a/src/build_log.h
+++ b/src/build_log.h
@@ -45,7 +45,10 @@ struct BuildLog {
   BuildLog();
   ~BuildLog();
 
+  /// Prepares writing to the log file without actually opening it - that will
+  /// happen when/if it's needed
   bool OpenForWrite(const string& path, const BuildLogUser& user, string* err);
+
   bool RecordCommand(Edge* edge, int start_time, int end_time,
                      TimeStamp mtime = 0);
   void Close();
@@ -91,8 +94,13 @@ struct BuildLog {
   const Entries& entries() const { return entries_; }
 
  private:
+  /// Should be called before using log_file_. When false is returned, errno
+  /// will be set.
+  bool OpenForWriteIfNeeded();
+
   Entries entries_;
   FILE* log_file_;
+  std::string log_file_path_;
   bool needs_recompaction_;
 };