GNATCOLL.OS.FS.Open: truncate files in write mode

Change-Id: If5c1cc1019a9844db211191bb0c1d5e23b983803
TN: V330-031

Author: pmderodat

src/os/gnatcoll-os-fs-open__unix.adb

@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
 --                              G N A T C O L L                             --
 --                                                                          --
---                     Copyright (C) 2020-2021, AdaCore                     --
+--                     Copyright (C) 2020-2022, AdaCore                     --
 --                                                                          --
 -- This library is free software;  you can redistribute it and/or modify it --
 -- under terms of the  GNU General Public License  as published by the Free --
@@ -48,7 +48,7 @@ begin
          O_Mode := O_RDONLY or O_CLOEXEC;
          Perm   := 0;
       when Write_Mode =>
-         O_Mode := O_WRONLY or O_CREAT or O_CLOEXEC;
+         O_Mode := O_WRONLY or O_CREAT or O_CLOEXEC or O_TRUNC;
          Perm   := S_IRUSR or S_IWUSR or S_IRGRP or
             S_IWGRP or S_IROTH or S_IWOTH;
       when Append_Mode =>

src/os/gnatcoll-os-fs-open__win32.adb

@@ -1,7 +1,7 @@
 ------------------------------------------------------------------------------
 --                              G N A T C O L L                             --
 --                                                                          --
---                     Copyright (C) 2020-2021, AdaCore                     --
+--                     Copyright (C) 2020-2022, AdaCore                     --
 --                                                                          --
 -- This library is free software;  you can redistribute it and/or modify it --
 -- under terms of the  GNU General Public License  as published by the Free --
@@ -54,7 +54,8 @@ begin
          O_Mode := Win32.Files.O_WRONLY
             or Win32.Files.O_CREAT
             or Win32.Files.O_NOINHERIT
-            or Win32.Files.O_BINARY;
+            or Win32.Files.O_BINARY
+            or Win32.Files.O_TRUNC;
          O_Perm := Win32.Files.S_IWRITE;
       when Append_Mode =>
          O_Mode := Win32.Files.O_WRONLY

testsuite/tests/os/fs/test.adb

@@ -57,6 +57,41 @@ begin
       end;
       Close (FD);
 
+      --  Check that opening a file in write mode truncates the file to size 0
+
+      declare
+         Name : constant String := "non_empty_file";
+      begin
+         --  First create a file that contains several KB of data (just in case
+         --  it makes a difference).
+
+         FD := Open (Name, Mode => Write_Mode);
+         for C in Character range 'A' .. 'Z' loop
+            for Dummy in 1 .. 10 loop
+               Write (FD, (1 .. 80 => C));
+               Write (FD, (1 => ASCII.LF));
+            end loop;
+         end loop;
+
+         Close (FD);
+
+         --  Then try to overwrite it
+
+         FD := Open (Name, Mode => Write_Mode);
+         Write (FD, "not much here");
+         Close (FD);
+
+         --  Now, check that it contains data from the last write session only
+
+         FD := Open (Name, Mode => Read_Mode);
+         declare
+            File_Content : constant String := Read (FD);
+         begin
+            A.Assert (File_Content, "not much here");
+         end;
+         Close (FD);
+      end;
+
    end;
    return A.Report;
 end Test;