Bug 1564900 - [OS.File] Fix size of statvfs::f_frsize;r=Yoric

Make Firefox not assume type of member statvfs::f_frsize from sys head, which may vary

Differential Revision: https://phabricator.services.mozilla.com/D38306

--HG--
extra : moz-landing-system : lando

dom/system/OSFileConstants.cpp

@@ -11,6 +11,14 @@
 
 #include "prsystem.h"
 
+// Short macro to get the size of a member of a
+// given struct at compile time.
+// t is the type of struct, m the name of the
+// member:
+// DOM_SIZEOF_MEMBER(struct mystruct, myint)
+// will give you the size of the type of myint.
+#define DOM_SIZEOF_MEMBER(t, m) sizeof(((t*)0)->m)
+
 #if defined(XP_UNIX)
 #  include "unistd.h"
 #  include "dirent.h"
@@ -602,6 +610,9 @@ static const dom::ConstantSpec gLibcProperties[] = {
 
     {"OSFILE_SIZEOF_STATVFS", JS::Int32Value(sizeof(struct statvfs))},
 
+    // We have no guarantee how big "f_frsize" is, so we have to calculate that.
+    {"OSFILE_SIZEOF_STATVFS_F_FRSIZE",
+     JS::Int32Value(DOM_SIZEOF_MEMBER(struct statvfs, f_frsize))},
     {"OSFILE_OFFSETOF_STATVFS_F_FRSIZE",
      JS::Int32Value(offsetof(struct statvfs, f_frsize))},
     {"OSFILE_OFFSETOF_STATVFS_F_BAVAIL",

toolkit/components/osfile/modules/osfile_unix_back.jsm

@@ -257,6 +257,16 @@
       Type.fsblkcnt_t = Type.uintn_t(Const.OSFILE_SIZEOF_FSBLKCNT_T).withName(
         "fsblkcnt_t"
       );
+      // There is no guarantee of the size or order of members in sys-header structs
+      // It mostly is "unsigned long", but can be "unsigned int" as well.
+      // So it has its own "type".
+      // NOTE: This is still only partially correct, as signedness is also not guaranteed,
+      //       so assuming an unsigned int might still be wrong here.
+      //       But unsigned seems to have worked all those years, even though its signed
+      //       on various platforms.
+      Type.statvfs_f_frsize = Type.uintn_t(
+        Const.OSFILE_SIZEOF_STATVFS_F_FRSIZE
+      ).withName("statvfs_f_rsize");
 
       // Structure |statvfs|
       // Use an hollow structure
@@ -269,7 +279,7 @@
         statvfs.add_field_at(
           Const.OSFILE_OFFSETOF_STATVFS_F_FRSIZE,
           "f_frsize",
-          Type.unsigned_long.implementation
+          Type.statvfs_f_frsize.implementation
         );
         statvfs.add_field_at(
           Const.OSFILE_OFFSETOF_STATVFS_F_BAVAIL,