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, 49 insertions, 6 deletions

diff --git a/lib/kernel/doc/src/file.xml b/lib/kernel/doc/src/file.xml
index 382262d1ee..a9ceac0bcf 100644
--- a/lib/kernel/doc/src/file.xml
+++ b/lib/kernel/doc/src/file.xml
@@ -603,7 +603,7 @@ f.txt:  {person, "kalle", 25}.
       <type>
         <v>Filename = name()</v>
         <v>Modes = [Mode]</v>
-        <v>&nbsp;Mode = read | write | append | raw | binary | {delayed_write, Size, Delay} | delayed_write | {read_ahead, Size} | read_ahead | compressed</v>
+        <v>&nbsp;Mode = read | write | append | exclusive | raw | binary | {delayed_write, Size, Delay} | delayed_write | {read_ahead, Size} | read_ahead | compressed</v>
         <v>&nbsp;&nbsp;Size = Delay = int()</v>
         <v>IoDevice = io_device()</v>
         <v>Reason = ext_posix() | system_limit</v>
@@ -630,6 +630,17 @@ f.txt:  {person, "kalle", 25}.
               file opened with <c>append</c> will take place at
               the end of the file.</p>
           </item>
+          <tag><c>exclusive</c></tag>
+          <item>
+            <p>The file, when opened for writing, is created if it
+              does not exist. If the file exists, open will return
+              <c>{error, eexist}</c>.</p>
+	    <warning><p>This option does not guarantee exclusiveness on
+	    file systems that do not support O_EXCL properly,
+	    such as NFS. Do not depend on this option unless you
+	    know that the file system supports it (in general, local
+	    file systems should be safe).</p></warning>
+          </item>
           <tag><c>raw</c></tag>
           <item>
             <p>The <c>raw</c> option allows faster access to a file,
diff --git a/lib/kernel/src/file.erl b/lib/kernel/src/file.erl
index a37614e424..1ba329a12b 100644
--- a/lib/kernel/src/file.erl
+++ b/lib/kernel/src/file.erl
@@ -81,7 +81,7 @@
 -type mode()      :: 'read' | 'write' | 'append' | 'raw' | 'binary' | 
 		     {'delayed_write', non_neg_integer(), non_neg_integer()} | 
 		     'delayed_write' | {'read_ahead', pos_integer()} | 
-		     'read_ahead' | 'compressed'.
+		     'read_ahead' | 'compressed' | 'exclusive'.
 -type name()      :: string() | atom() | [name()].
 -type posix()     :: atom().
 -type bindings()  :: any().
diff --git a/lib/kernel/test/file_SUITE.erl b/lib/kernel/test/file_SUITE.erl
index 15bab0dccd..17c47f871d 100644
--- a/lib/kernel/test/file_SUITE.erl
+++ b/lib/kernel/test/file_SUITE.erl
@@ -53,7 +53,7 @@
 -export([file_info/1, file_info_basic_file/1, file_info_basic_directory/1,
 	 file_info_bad/1, file_info_times/1, file_write_file_info/1]).
 -export([rename/1, access/1, truncate/1, datasync/1, sync/1,
-	 read_write/1, pread_write/1, append/1]).
+	 read_write/1, pread_write/1, append/1, exclusive/1]).
 -export([errors/1, e_delete/1, e_rename/1, e_make_dir/1, e_del_dir/1]).
 -export([otp_5814/1]).
 
@@ -466,7 +466,7 @@ files(suite) ->
      sync,datasync,advise].
 
 open(suite) -> [open1,old_modes,new_modes,path_open,close,access,read_write,
-	       pread_write,append,open_errors].
+	       pread_write,append,open_errors,exclusive].
 
 open1(suite) -> [];
 open1(doc) -> [];
@@ -840,6 +840,22 @@ open_errors(Config) when is_list(Config) ->
     ?line test_server:timetrap_cancel(Dog),
     ok.
 
+exclusive(suite) -> [];
+exclusive(doc) -> "Test exclusive access to a file.";
+exclusive(Config) when is_list(Config) ->
+    ?line Dog = test_server:timetrap(test_server:seconds(5)),
+    ?line RootDir = ?config(priv_dir,Config),
+    ?line NewDir = filename:join(RootDir,
+				 atom_to_list(?MODULE)
+				 ++"_exclusive"),
+    ?line ok = ?FILE_MODULE:make_dir(NewDir),
+    ?line Name = filename:join(NewDir, "ex_file.txt"),
+    ?line {ok, Fd} = ?FILE_MODULE:open(Name, [write, exclusive]),
+    ?line {error, eexist} = ?FILE_MODULE:open(Name, [write, exclusive]),
+    ?line ok = ?FILE_MODULE:close(Fd),
+    ?line test_server:timetrap_cancel(Dog),
+    ok.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 pos(suite) -> [pos1,pos2].
diff --git a/lib/kernel/test/prim_file_SUITE.erl b/lib/kernel/test/prim_file_SUITE.erl
index 21bdc06fdc..1688ec45ca 100644
--- a/lib/kernel/test/prim_file_SUITE.erl
+++ b/lib/kernel/test/prim_file_SUITE.erl
@@ -35,7 +35,7 @@
 	 file_write_file_info_a/1, file_write_file_info_b/1]).
 -export([rename_a/1, rename_b/1, 
 	 access/1, truncate/1, datasync/1, sync/1,
-	 read_write/1, pread_write/1, append/1]).
+	 read_write/1, pread_write/1, append/1, exclusive/1]).
 -export([errors/1, e_delete/1, e_rename/1, e_make_dir/1, e_del_dir/1]).
 
 -export([compression/1, read_not_really_compressed/1,
@@ -385,7 +385,7 @@ win_cur_dir_1(_Config, Handle) ->
 files(suite) -> [open,pos,file_info,truncate,sync,datasync,advise].
 
 open(suite) -> [open1,modes,close,access,read_write,
-	       pread_write,append].
+	       pread_write,append,exclusive].
 
 open1(suite) -> [];
 open1(doc) -> [];
@@ -610,6 +610,22 @@ append(Config) when is_list(Config) ->
     ?line test_server:timetrap_cancel(Dog),
     ok.
 
+exclusive(suite) -> [];
+exclusive(doc) -> "Test exclusive access to a file.";
+exclusive(Config) when is_list(Config) ->
+    ?line Dog = test_server:timetrap(test_server:seconds(5)),
+    ?line RootDir = ?config(priv_dir,Config),
+    ?line NewDir = filename:join(RootDir,
+				 atom_to_list(?MODULE)
+				 ++"_exclusive"),
+    ?line ok = ?PRIM_FILE:make_dir(NewDir),
+    ?line Name = filename:join(NewDir, "ex_file.txt"),
+    ?line {ok,Fd} = ?PRIM_FILE:open(Name, [write, exclusive]),
+    ?line {error, eexist} = ?PRIM_FILE:open(Name, [write, exclusive]),
+    ?line ok = ?PRIM_FILE:close(Fd),
+    ?line test_server:timetrap_cancel(Dog),
+    ok.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 pos(suite) -> [pos1,pos2].