Add file:allocate/3 operation

This operation allows pre-allocation of space for files.
It succeeds only on systems that support such operation.

The POSIX standard defines the optional system call
posix_fallocate() to implement this feature. However,
some systems implement more specific functions to
accomplish the same operation.

On Linux, if the more specific function fallocate() is
implemented, it is used instead of posix_fallocate(),
falling back to posix_fallocate() if the fallocate()
call failed (it's only supported for the ext4, ocfs2,
xfs and btrfs file systems at the moment).

On Mac OS X it uses the specific fcntl() operation
F_PREALLOCATE, falling back to posix_fallocate() if
it's available (at the moment Mac OS X doesn't provide
posix_fallocate()).

On any other UNIX system, it uses posix_fallocate() if it's
available. Any other system not providing this system call
or any function to pre-allocate space for files, this operation
always fails with the ENOTSUP POSIX error.

1 files changed, 71 insertions, 1 deletions

diff --git a/lib/kernel/test/file_SUITE.erl b/lib/kernel/test/file_SUITE.erl
index 9c507fd437..914f0d6127 100644
--- a/lib/kernel/test/file_SUITE.erl
+++ b/lib/kernel/test/file_SUITE.erl
@@ -84,6 +84,8 @@
 
 -export([advise/1]).
 
+-export([allocate/1]).
+
 -export([standard_io/1,mini_server/1]).
 
 %% Debug exports
@@ -116,7 +118,7 @@ groups() ->
      {files, [],
       [{group, open}, {group, pos}, {group, file_info},
        {group, consult}, {group, eval}, {group, script},
-       truncate, sync, datasync, advise]},
+       truncate, sync, datasync, advise, allocate]},
      {open, [],
       [open1, old_modes, new_modes, path_open, close, access,
        read_write, pread_write, append, open_errors,
@@ -1617,6 +1619,74 @@ advise(Config) when is_list(Config) ->
     ?line test_server:timetrap_cancel(Dog),
     ok.
 
+allocate(suite) -> [];
+allocate(doc) -> "Tests that ?FILE_MODULE:allocate/3 at least doesn't crash.";
+allocate(Config) when is_list(Config) ->
+    ?line Dog = test_server:timetrap(test_server:seconds(5)),
+    ?line PrivDir = ?config(priv_dir, Config),
+    ?line Allocate = filename:join(PrivDir,
+			       atom_to_list(?MODULE)
+			       ++"_allocate.fil"),
+
+    Line1 = "Hello\n",
+    Line2 = "World!\n",
+
+    ?line {ok, Fd} = ?FILE_MODULE:open(Allocate, [write, binary]),
+    allocate_and_assert(Fd, 1, iolist_size([Line1, Line2])),
+    ?line ok = io:format(Fd, "~s", [Line1]),
+    ?line ok = io:format(Fd, "~s", [Line2]),
+    ?line ok = ?FILE_MODULE:close(Fd),
+
+    ?line {ok, Fd2} = ?FILE_MODULE:open(Allocate, [write, binary]),
+    allocate_and_assert(Fd2, 1, iolist_size(Line1)),
+    ?line ok = io:format(Fd2, "~s", [Line1]),
+    ?line ok = io:format(Fd2, "~s", [Line2]),
+    ?line ok = ?FILE_MODULE:close(Fd2),
+
+    ?line {ok, Fd3} = ?FILE_MODULE:open(Allocate, [write, binary]),
+    allocate_and_assert(Fd3, 1, iolist_size(Line1) + 1),
+    ?line ok = io:format(Fd3, "~s", [Line1]),
+    ?line ok = io:format(Fd3, "~s", [Line2]),
+    ?line ok = ?FILE_MODULE:close(Fd3),
+
+    ?line {ok, Fd4} = ?FILE_MODULE:open(Allocate, [write, binary]),
+    allocate_and_assert(Fd4, 1, 4 * iolist_size([Line1, Line2])),
+    ?line ok = io:format(Fd4, "~s", [Line1]),
+    ?line ok = io:format(Fd4, "~s", [Line2]),
+    ?line ok = ?FILE_MODULE:close(Fd4),
+
+    ?line [] = flush(),
+    ?line test_server:timetrap_cancel(Dog),
+    ok.
+
+allocate_and_assert(Fd, Offset, Length) ->
+    % Just verify that calls to ?PRIM_FILE:allocate/3 don't crash or have
+    % any other negative side effect. We can't really asssert against a
+    % specific return value, because support for file space pre-allocation
+    % depends on the OS, OS version and underlying filesystem.
+    %
+    % The Linux kernel added support for fallocate() in version 2.6.23,
+    % which currently works only for the ext4, ocfs2, xfs and btrfs file
+    % systems. posix_fallocate() is available in glibc as of version
+    % 2.1.94, but it was buggy until glibc version 2.7.
+    %
+    % Mac OS X, as of version 10.3, supports the fcntl operation F_PREALLOCATE.
+    %
+    % Solaris supports posix_fallocate() but only for the UFS file system
+    % apparently (not supported for ZFS).
+    %
+    % FreeBSD 9.0 is the first FreeBSD release supporting posix_fallocate().
+    %
+    % For Windows there's apparently no way to pre-allocate file space, at
+    % least with same semantics as posix_fallocate(), fallocate() and
+    % fcntl F_PREALLOCATE.
+    Result = ?FILE_MODULE:allocate(Fd, Offset, Length),
+    case os:type() of
+        {win32, _} ->
+            ?line {error, enotsup} = Result;
+        _ ->
+            ?line _ = Result
+    end.
 
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