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How would the multiple copies find the global memory segment? In theory each copy could create such a segment and check that all handles received by APIs live i

How would the multiple copies find the global memory segment? In theory each copy could create such a segment and check that all handles received by APIs live in it, but I can’t see how you could convince multiple versions to get a handle to the same shared segment (well, without a file handle).

zvrba 5y ago
> How would the multiple copies find the global memory segment? They would all use the same reserved "name". It's possible to atomically create or open a SHM segment with a given "name". Then have a header at the start of the segment with critical metadata like signature, version, etc. Barf unconditionally if the metadata is found to be invalid (i.e., some other process deliberately used the name to sabotage sqlite [1]), or, in addition, have an option to not use SHM. [1] The situation is not different from some process sabotaging another by deleting or corrupting well-known files.
- CGamesPlay 5y ago
  I might be misunderstanding something, but doesn’t that mean that only one SQLite library could be loaded per system? That hardly seems like an acceptable compromise for an embedded software.
  - SQLite 5y ago
    You misunderstand. It means that a single application should not use two or more copies of the SQLite library to open the same database file. As a practical matter, you really have to work hard to get an application to use two different copies of SQLite at the same time - all the while avoiding symbol collisions on link. You can do it, but it takes some work. And then on top of that your application has to decide to open two or more connections to the same database file, using different copies of SQLite in each case.
    - mst 5y ago
      Yeah, managing to do that is ... almost impressive. The more common failure mode I've seen is a library bundling and inlining an old version of a database API in a way that means it overrides the shared object I was expecting my code to link to, and alarums and excursions resulting thereby. (I don't imagine SQLite would -break- in that situation, but I doubt my application code would be any less unhappy than in the cases of that problem I've encountered in the wild)
    - zvrba 5y ago
      > You can do it, but it takes some work. It's easy: create DLL/.so A with statically linked sqlite. create DLL/.so B with statically linked sqlite. Make a program using both DLLs.
      - CGamesPlay 5y ago
        I bet this situation is even relatively common amongst users of SQLite from Node, which is notorious for having multiple versions of the same dependency.
  - zvrba 5y ago
    No. That's why it's called _shared_ memory segment. Between processes. It's like an in-memory file.

zvrba 5y ago

> How would the multiple copies find the global memory segment? They would all use the same reserved "name". It's possible to atomically create or open a SHM segment with a given "name". Then have a header at the start of the segment with critical metadata like signature, version, etc. Barf unconditionally if the metadata is found to be invalid (i.e., some other process deliberately used the name to sabotage sqlite [1]), or, in addition, have an option to not use SHM. [1] The situation is not different from some process sabotaging another by deleting or corrupting well-known files.

CGamesPlay 5y ago
I might be misunderstanding something, but doesn’t that mean that only one SQLite library could be loaded per system? That hardly seems like an acceptable compromise for an embedded software.
- SQLite 5y ago
  You misunderstand. It means that a single application should not use two or more copies of the SQLite library to open the same database file. As a practical matter, you really have to work hard to get an application to use two different copies of SQLite at the same time - all the while avoiding symbol collisions on link. You can do it, but it takes some work. And then on top of that your application has to decide to open two or more connections to the same database file, using different copies of SQLite in each case.
  - mst 5y ago
    Yeah, managing to do that is ... almost impressive. The more common failure mode I've seen is a library bundling and inlining an old version of a database API in a way that means it overrides the shared object I was expecting my code to link to, and alarums and excursions resulting thereby. (I don't imagine SQLite would -break- in that situation, but I doubt my application code would be any less unhappy than in the cases of that problem I've encountered in the wild)
  - zvrba 5y ago
    > You can do it, but it takes some work. It's easy: create DLL/.so A with statically linked sqlite. create DLL/.so B with statically linked sqlite. Make a program using both DLLs.
    - CGamesPlay 5y ago
      I bet this situation is even relatively common amongst users of SQLite from Node, which is notorious for having multiple versions of the same dependency.
- zvrba 5y ago
  No. That's why it's called _shared_ memory segment. Between processes. It's like an in-memory file.