This tutorial talks about a powerful bug trap that was introduced in Ada 2012: the
Type invariant? What's that?
The idea of type invariant is pretty simple. Sometimes data structure are quite simple, just an aggregation of values, but often they are quite complex with an internal structure that needs to be preserved.
A type invariant associated to a given type is a Boolean expression that is true for every valid value of the type. Ada allows the association of a type invariant to a private type; the compiler — if instructed so — will insert code that checks that the invariant remains satisfied; if not, an exception will be raised.
An example: a table with two keys
Let's do a simple example. Suppose that
- We have a set of users
- Every user has a
Serial_ID(a 6 digit number) and a
SSN(Social Security Number). We will assume the Italian convention where the SSN is a 16 character string.
- Both the
SSNidentify uniquely the user, that is, there are no two users with the same
Serial_IDnor two users with the same
- We want to build a data structure
User_Directorythat will allow us to get (efficiently) an user both by
A possible solution is shown in the figure
The data structure is made of two
Ordered_Maps: one map is indexed by the
Serial_ID, the other one by the
SSN and both maps store pointers (access values in Ada jargon) to a
User_Record, a record that contains the user information. This structure clearly introduces some (quite obvious) redundancy
SSNstored in a
User_Recordmust be equal to the corresponding key in the
SSN_Map, the same for the
- The entries of the two maps corresponding to the same user must map to the same record
- The two maps have the same number of entries.
The spec file
This is an excerpt of the public part of the spec file (roughly the
*.h of C)
subtype ID_Char is Character range '0' .. '9'; type Serial_ID is array (1 .. 6) of ID_Char; type User_SSN is new String (1 .. 16); type User_Directory is private; procedure Add_User (Dir : in out User_Directory; ID : in Serial_ID; SSN : in User_SSN) with Pre => not Contains (Dir, Id) and not Contains (Dir, Ssn), Post => Contains (Dir, Id) and Contains (Dir, Ssn); procedure Delete (Dir : in out User_Directory; ID : in Serial_ID) with Pre => Contains (Dir, Id), Post => not Contains (Dir, Id);
Here we define
- The types
User_SSNas fixed lenght strings. Note that
Serial_IDcan contain only digits.
User_Directoryis the data structure we are defining.
Deleteshould be quite obvious. Note the use of preconditions (after
Pre =>) and postconditions (after
Post =>) that document the behavior of
The actual definition (in the
private part) of
User_Directory is basically the translation in Ada of the scheme shown above
type User_Record is record ID : Serial_ID; SSN : User_SSN; end record; type User_Record_Pt is access User_Record; package ID_To_User_Maps is new Ada.Containers.Ordered_Maps (Key_Type => Serial_ID, Element_Type => User_Record_Pt); package SSN_To_User_Maps is new Ada.Containers.Ordered_Maps (Key_Type => User_SSN, Element_Type => User_Record_Pt); type User_Directory is record ID_To_User : ID_To_User_Maps.Map; SSN_To_User : SSN_To_User_Maps.Map; end record;
- The record
User_Recordholding the user information and its access
- A package
ID_To_User_Maps(obtained by specializing the generic standard package
Ordered_Maps) that implements maps that associate
User_IDto access to
- A similar package
- Finally, we have the
User_Directorythat simply contains the two required maps.
Let's look at the implementation of the package. Let's start with
procedure Add_User (Dir : in out User_Directory; ID : in Serial_ID; SSN : in User_SSN) is New_User : User_Record_Pt; begin if Dir.ID_To_User.Contains (ID) then raise Constraint_Error; end if; New_User := new User_Record'(ID, SSN); Dir.ID_To_User.Include (Key => Id, New_Item => New_User); Dir.SSN_To_User.Include (Key => SSN, New_Item => New_User); end Add_User;
The implementation of
Add_User is quite straightforward: with
New_User := new User_Record'(ID => ID, SSN => SSN);
we allocate dynamically a record with the user information and with
Dir.ID_To_User.Include (Key => Id, New_Item => New_User); Dir.SSN_To_User.Include (Key => SSN, New_Item => New_User);
we update the two internal tables by associating the given
SSN with the address of the newly allocated record.
This is our first tentative to the implementation of
procedure Delete (Dir : in out User_Directory; ID : in Serial_ID) is To_Be_Removed : User_Record_Pt := Dir.ID_To_User (Id); begin Dir.ID_To_User.Delete (Id); Free (To_Be_Removed); end Delete;
This implementation has a bug 🐛, as it is clear from this figure that pictures the behavior of
The dashed line means that the memory area of the user record now has gone back to the heap. It is clear that we forgot to remove the entry from
Dir.SSN_To_User and now we have a dangling pointer referring to the old user record.
This is a nasty bug.
Oh, yes, believe me. I have already seen this movie, actually I was the main character. What could go wrong? Well, for example, later you could want to update the entries of
Dir.SSN_To_User in some way. However, the memory that was assigned to the user record now belongs to some other structure that will be corrupted by your update.
Depending on your code, the bug can remain dormant for long time, then, suddenly, one day, if you are lucky, you get a SEGFAULT when you try to use the corrupted structure again. If you are unlucky you'll get funny random behaviors, possibly difficult to replicate.
Even if you are lucky (so to say...) you'll get the SEGFAULT in a place totally unrelated with the real bug in
Delete. Believe me, finding this bug can take days of stumbling around in the dark. Not even an army of rubber ducks can save you.
Although this is a specific example, this kind of time bomb 💣 (or Sword of Damocles) behavior is typical of bugs that cause a loss of internal coherence in some complex structure. The bug will not manifest itself when the structure is corrupted, but later as a delayed consequence of the loss of coherence, making bug hunting difficult.
Remember: there is one thing worse than a bug that causes a crash: a bug that causes the program to continue as nothing happened.
The solution: type invariant
Fear not, my friends and colleagues. Ada comes to the rescue with the
Type_Invariant. It suffices to add it to the type definition as in
type User_Directory is record ID_To_User : ID_To_User_Maps.Map; SSN_To_User : SSN_To_User_Maps.Map; end record with Type_Invariant => Is_Coherent (User_Directory);
Is_Coherent is a function that checks that the whole structure is coherent, that is, that the two maps have the same number of entries and that the data in the user record match the corresponding key in the tables. The source code of the (fairly long) function is included, for reference, at the end of this post.
Now if we run the following test program
with User_Directories; use User_Directories; procedure Main is Dir : User_Directory; begin Add_User (Dir => dir, ID => "123456", SSN => "ABCDEF64X12Q456R"); Add_User (Dir => dir, ID => "654321", SSN => "XBCDEF64X12Q456R"); Delete (Dir, Id => "123456"); end Main;
we get the following results
Execution of obj/main terminated by unhandled exception raised SYSTEM.ASSERTIONS.ASSERT_FAILURE : failed invariant from user_directories.ads:65 Call stack traceback locations: ⋯/s-assert.adb:46 ⋯/adainclude/a-coorma.adb:1561 ⋯/user_directories.adb:52 ⋯/user_directories.adb:59 ⋯/main.adb:18 ⋯
where, for the sake of readability I replaced part of the dump text with
⋯. The line
failed invariant from user_directories.ads:65
refers to the line where type
User_Directory is declared. In the stack dump the first two lines refer to some system files, we are interested in the third line
that refers to the first line of procedure
- Before calling
- After calling
Deleteit is not fine anymore, who are you gonna blame?
Ghostbusters! No, sorry, wrong citation... 😊😛 (this is also a hint to my age... 😊)
After a brief inspection of
Delete we discover the problem and we fix it quite easily
procedure Delete (Dir : in out User_Directory; ID : in Serial_ID) is To_Be_Removed : User_Record_Pt := Dir.ID_To_User (Id); begin Dir.SSN_To_User.Delete (To_Be_Removed.SSN); -- New line -- and here??? Dir.ID_To_User.Delete (Id); Free (To_Be_Removed); end Delete;
Wait a minute...
Someone maybe could observe that at the line
-- and here??? the variable
Dir is in a non coherent state since we updated one table, but not the other. The same happen in the
Add_User procedure. Wouldn't this raise an exception?
Well, no. The details are a bit complex, but the basic idea is that since
Delete are declared in the same package of
User_Directory, they are able to operate on the internal structure of the type and it is acceptable that during this manipulation the internal structure is not coherent for a moment. The type invariant will be checked only when the procedures end;
see the Reference manual if you want every single, gory, detail...
function Is_Coherent (Dir : User_Directory) return Boolean is use Ada.Containers; use ID_To_User_Maps; use SSN_To_User_Maps; begin if Dir.ID_To_User.Length /= Dir.SSN_To_User.Length then return False; end if; for Pos in Dir.ID_To_User.Iterate loop declare Usr_Entry : constant User_Record_Pt := Element (Pos); begin if Usr_Entry /= Dir.SSN_To_User (Usr_Entry.Ssn) then return False; end if; if Key (Pos) /= Usr_Entry.Id then return False; end if; end; end loop; return True; end Is_Coherent;