Disable stdout buffering in real_main

Previously stdout buffering was disabled in the LinePrinter constructor.
This worked for a long time but ultimately this side-effect caused a
performance bug (issue #2018) in tools such as -t deps. Moving the
disabling of buffering into real_main and only disabling buffering
when a tool is not used makes the desired semantics clearer and restores
the lost performance.

This fixes issue #2018. It has been tested and a 10x speedup was seen
relative to the tip-of-tree version.

2 files changed, 11 insertions, 5 deletions

diff --git a/src/line_printer.cc b/src/line_printer.cc
index 3138960..a3d0528 100644
--- a/src/line_printer.cc
+++ b/src/line_printer.cc
@@ -37,14 +37,9 @@ LinePrinter::LinePrinter() : have_blank_line_(true), console_locked_(false) {
 #ifndef _WIN32
   smart_terminal_ = isatty(1) && term && string(term) != "dumb";
 #else
-  // Disable output buffer.  It'd be nice to use line buffering but
-  // MSDN says: "For some systems, [_IOLBF] provides line
-  // buffering. However, for Win32, the behavior is the same as _IOFBF
-  // - Full Buffering."
   if (term && string(term) == "dumb") {
     smart_terminal_ = false;
   } else {
-    setvbuf(stdout, NULL, _IONBF, 0);
     console_ = GetStdHandle(STD_OUTPUT_HANDLE);
     CONSOLE_SCREEN_BUFFER_INFO csbi;
     smart_terminal_ = GetConsoleScreenBufferInfo(console_, &csbi);
diff --git a/src/ninja.cc b/src/ninja.cc
index 07b1f1e..3e5c971 100644
--- a/src/ninja.cc
+++ b/src/ninja.cc
@@ -1436,6 +1436,17 @@ NORETURN void real_main(int argc, char** argv) {
     exit((ninja.*options.tool->func)(&options, argc, argv));
   }
 
+#ifdef WIN32
+  // It'd be nice to use line buffering but MSDN says: "For some systems,
+  // [_IOLBF] provides line buffering. However, for Win32, the behavior is the
+  //  same as _IOFBF - Full Buffering."
+  // Buffering used to be disabled in the LinePrinter constructor but that
+  // now disables it too early and breaks -t deps performance (see issue #2018)
+  // so we disable it here instead, but only when not running a tool.
+  if (!options.tool)
+    setvbuf(stdout, NULL, _IONBF, 0);
+#endif
+
   // Limit number of rebuilds, to prevent infinite loops.
   const int kCycleLimit = 100;
   for (int cycle = 1; cycle <= kCycleLimit; ++cycle) {