Bug 686691 - Plumbing to expose OS-level readhead. r=bsmedberg

xpcom/io/nsILocalFile.idl

@@ -95,6 +95,7 @@ interface nsILocalFile : nsIFile
      */
     attribute boolean followLinks;  
 
+    const unsigned long OS_READAHEAD = 0x40000000;
     const unsigned long DELETE_ON_CLOSE = 0x80000000;
 
     /**
@@ -102,9 +103,11 @@ interface nsILocalFile : nsIFile
      * responsible for calling PR_Close on the result.
      *
      * @param flags the PR_Open flags from prio.h, plus optionally
-     * DELETE_ON_CLOSE. DELETE_ON_CLOSE may be implemented by removing
-     * the file (by path name) immediately after opening it, so beware
-     * of possible races; the file should be exclusively owned by this
+     * OS_READAHEAD or DELETE_ON_CLOSE. OS_READAHEAD is a hint to the
+     * OS that the file will be read sequentially with agressive
+     * readahead. DELETE_ON_CLOSE may be implemented by removing the
+     * file (by path name) immediately after opening it, so beware of
+     * possible races; the file should be exclusively owned by this
      * process.
      */
     [noscript] PRFileDescStar openNSPRFileDesc(in long flags, in long mode);

xpcom/io/nsLocalFileUnix.cpp

@@ -88,6 +88,7 @@
 #include "prproces.h"
 #include "nsIDirectoryEnumerator.h"
 #include "nsISimpleEnumerator.h"
+#include "private/pprio.h"
 
 #ifdef MOZ_WIDGET_GTK2
 #include "nsIGIOService.h"
@@ -431,6 +432,11 @@ nsLocalFile::OpenNSPRFileDesc(PRInt32 flags, PRInt32 mode, PRFileDesc **_retval)
         PR_Delete(mPath.get());
     }
 
+#if defined(LINUX) && !defined(ANDROID)
+    if (flags & OS_READAHEAD) {
+        readahead(PR_FileDesc2NativeHandle(*_retval), 0, 0);
+    }
+#endif
     return NS_OK;
 }
 

xpcom/io/nsLocalFileWin.cpp

@@ -398,6 +398,10 @@ OpenFile(const nsAFlatString &name, PRIntn osflags, PRIntn mode,
       flag6 |= FILE_FLAG_DELETE_ON_CLOSE;
     }
 
+    if (osflags && nsILocalFile::OS_READAHEAD) {
+      flag6 |= FILE_FLAG_SEQUENTIAL_SCAN;
+    }
+
     HANDLE file = ::CreateFileW(name.get(), access,
                                 FILE_SHARE_READ|FILE_SHARE_WRITE,
                                 NULL, flags, flag6, NULL);