Don't write ninja log header to log on every build on Windows.

src/build_log.cc

@@ -66,6 +66,10 @@ bool BuildLog::OpenForWrite(const string& path, string* err) {
   setvbuf(log_file_, NULL, _IOLBF, BUFSIZ);
   SetCloseOnExec(fileno(log_file_));
 
+  // Opening a file in append mode doesn't set the file pointer to the file's
+  // end on Windows. Do that explicitly.
+  fseek(log_file_, 0, SEEK_END);
+
   if (ftell(log_file_) == 0) {
     if (fprintf(log_file_, kFileSignature, kCurrentVersion) < 0) {
       *err = strerror(errno);

src/build_log_test.cc

@@ -14,6 +14,7 @@
 
 #include "build_log.h"
 
+#include "util.h"
 #include "test.h"
 
 #ifdef _WIN32
@@ -65,6 +66,36 @@ TEST_F(BuildLogTest, WriteRead) {
   ASSERT_EQ("out", e1->output);
 }
 
+TEST_F(BuildLogTest, FirstWriteAddsSignature) {
+  const char kExpectedVersion[] = "# ninja log vX\n";
+  const size_t kVersionPos = strlen(kExpectedVersion) - 2;  // Points at 'X'.
+
+  BuildLog log;
+  string contents, err;
+
+  EXPECT_TRUE(log.OpenForWrite(kTestFilename, &err));
+  ASSERT_EQ("", err);
+  log.Close();
+
+  ASSERT_EQ(0, ReadFile(kTestFilename, &contents, &err));
+  ASSERT_EQ("", err);
+  if (contents.size() >= kVersionPos)
+    contents[kVersionPos] = 'X';
+  EXPECT_EQ(kExpectedVersion, contents);
+
+  // Opening the file anew shouldn't add a second version string.
+  EXPECT_TRUE(log.OpenForWrite(kTestFilename, &err));
+  ASSERT_EQ("", err);
+  log.Close();
+
+  contents.clear();
+  ASSERT_EQ(0, ReadFile(kTestFilename, &contents, &err));
+  ASSERT_EQ("", err);
+  if (contents.size() >= kVersionPos)
+    contents[kVersionPos] = 'X';
+  EXPECT_EQ(kExpectedVersion, contents);
+}
+
 TEST_F(BuildLogTest, DoubleEntry) {
   FILE* f = fopen(kTestFilename, "wb");
   fprintf(f, "# ninja log v3\n");