From fa9c56e913259101dfd054c56f3af708b002294f Mon Sep 17 00:00:00 2001
From: Patrick Ohly <patrick.ohly@intel.com>
Date: Wed, 3 Sep 2014 14:39:36 +0200
Subject: binfile: avoid unnecessary write and truncate

The goal is to leave binfile completely untouched, including its "last
modified" time stamp, if nothing changes.

There are cases where the upper layers will call
TBinFileBase::updateRecord() and write the same data that already
is in the file. Instead of catching those case-by-case, let's check
at the lowest level.

On btrfs (and possibly other file systems), ftruncate() bumps the
"last modified" time stamp of a file even if the requested size
matches the existing one. Avoid that by skipping the ftruncate() when
the file already has the desired size.
---
 src/platform_adapters/binfile.cpp | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/platform_adapters/binfile.cpp b/src/platform_adapters/binfile.cpp
index e1da717..02d6c52 100755
--- a/src/platform_adapters/binfile.cpp
+++ b/src/platform_adapters/binfile.cpp
@@ -21,6 +21,7 @@
 #endif
 
 #include <string.h>
+#include <vector>
 
 namespace sysync {
 
@@ -141,7 +142,20 @@ bool TBinFile::platformWriteFile(const void *aBuffer, uInt32 aBytes)
     return true;
   }
 
-  return fwrite(aBuffer,aBytes,1,fCBinFile) == 1;
+  // Don't overwrite content which is already exactly what we want it
+  // to be.
+  std::vector<char> buffer(aBytes);
+  char *addr = &buffer.front();
+  long offset = ftell(fCBinFile);
+  if (offset == -1)
+    return false;
+  if (fread(addr,aBytes,1,fCBinFile) != 1 ||
+      memcmp(addr,aBuffer,aBytes)) {
+    return fseek(fCBinFile,offset,SEEK_SET) == 0 &&
+      fwrite(aBuffer,aBytes,1,fCBinFile) == 1;
+  } else {
+    return true;
+  }
 } // TBinFile::platformWriteFile
 
 
@@ -167,7 +181,10 @@ bool TBinFile::platformTruncateFile(uInt32 aNewSize)
   #if defined(LINUX) || defined(MACOSX)
     fflush(fCBinFile); // unbuffer everything
     int fd = fileno(fCBinFile); // get file descriptor
-    if (ftruncate(fd,aNewSize)) {
+    struct stat buffer;
+    if (fstat(fd, &buffer) ||
+        buffer.st_size == (off_t)aNewSize || // avoid modifying file if it already has the right size
+        ftruncate(fd,aNewSize)) {
       ; // error ignored
     }
   #elif defined(WIN32)
-- 
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