Support opening files in exclusive mode

Add an option that atomically tests for the existence of a file and
creates it if the file does not exist, by passing the O_EXCL flag
to open() on Unix and CREATE_NEW flag on Windows. Support for O_EXCL
varies across platforms and filesystems.

{ok, Fd} = file:open("/tmp/foo", [write,exclusive]),
{error, eexist} = file:open("/tmp/foo", [write,exclusive]).

4 files changed, 12 insertions, 2 deletions

diff --git a/erts/emulator/drivers/common/erl_efile.h b/erts/emulator/drivers/common/erl_efile.h
index bbc973d58b..ac95c1f949 100644
--- a/erts/emulator/drivers/common/erl_efile.h
+++ b/erts/emulator/drivers/common/erl_efile.h
@@ -32,7 +32,8 @@
 #define EFILE_MODE_READ_WRITE 	3
 #define EFILE_MODE_APPEND	4
 #define EFILE_COMPRESSED 	8
-#define EFILE_NO_TRUNCATE      16 /* Special for reopening on VxWorks */
+#define EFILE_MODE_EXCL        16
+#define EFILE_NO_TRUNCATE      32 /* Special for reopening on VxWorks */
 
 /*
  * Seek modes for efile_seek().
diff --git a/erts/emulator/drivers/unix/unix_efile.c b/erts/emulator/drivers/unix/unix_efile.c
index ea016526ef..0052ac0739 100644
--- a/erts/emulator/drivers/unix/unix_efile.c
+++ b/erts/emulator/drivers/unix/unix_efile.c
@@ -706,6 +706,9 @@ efile_openfile(Efile_error* errInfo,	/* Where to return error codes. */
 #endif
     }
 
+    if (flags & EFILE_MODE_EXCL) {
+	mode |= O_EXCL;
+    }
 
 #ifdef VXWORKS
     if (*name != '/') {
diff --git a/erts/emulator/drivers/win32/win_efile.c b/erts/emulator/drivers/win32/win_efile.c
index 24b6fb30dc..04bd1139f5 100644
--- a/erts/emulator/drivers/win32/win_efile.c
+++ b/erts/emulator/drivers/win32/win_efile.c
@@ -689,6 +689,9 @@ Sint64* pSize;			/* Where to store the size of the file. */
     if (flags & EFILE_MODE_APPEND) {
 	crFlags = OPEN_ALWAYS;
     }
+    if (flags & EFILE_MODE_EXCL) {
+	crFlags = CREATE_NEW;
+    }
     fd = CreateFile(name, access,
 		    FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
 		    NULL, crFlags, FILE_ATTRIBUTE_NORMAL, NULL);
diff --git a/erts/preloaded/src/prim_file.erl b/erts/preloaded/src/prim_file.erl
index 2d177bf80e..7f24889bb2 100644
--- a/erts/preloaded/src/prim_file.erl
+++ b/erts/preloaded/src/prim_file.erl
@@ -116,9 +116,10 @@
 -define(EFILE_MODE_READ_WRITE, 3).  
 -define(EFILE_MODE_APPEND,     4).
 -define(EFILE_COMPRESSED,      8).
+-define(EFILE_MODE_EXCL,       16).
 
 %% Use this mask to get just the mode bits to be passed to the driver.
--define(EFILE_MODE_MASK, 15).
+-define(EFILE_MODE_MASK, 31).
 
 %% Seek modes for the driver's seek function.
 -define(EFILE_SEEK_SET, 0).
@@ -958,6 +959,8 @@ open_mode([compressed|Rest], Mode, Portopts, Setopts) ->
 open_mode([append|Rest], Mode, Portopts, Setopts) ->
     open_mode(Rest, Mode bor ?EFILE_MODE_APPEND bor ?EFILE_MODE_WRITE, 
 	      Portopts, Setopts);
+open_mode([exclusive|Rest], Mode, Portopts, Setopts) ->
+    open_mode(Rest, Mode bor ?EFILE_MODE_EXCL, Portopts, Setopts);
 open_mode([delayed_write|Rest], Mode, Portopts, Setopts) ->
     open_mode([{delayed_write, 64*1024, 2000}|Rest], Mode,
 	      Portopts, Setopts);