add static context object which has no capabilities #553

1. 
   apoelstra commented at 9:27 pm on August 15, 2018: contributor
2. apoelstra force-pushed on Aug 15, 2018
3. apoelstra cross-referenced this on Aug 20, 2018 from issue remove `Secp256k1::without_caps` from public API and all no-capability context args by apoelstra
4. 
   gmaxwell commented at 3:59 pm on September 4, 2018: contributor

   Should we document that you can’t call set_illegal_callback on this and perhaps add some checks on some of the disallowed calls to fail if you try?
5. 
   sipa commented at 4:00 pm on September 4, 2018: contributor

   @gmaxwell I believe the set of permitted functions on this object is exactly the functions which take a const context pointer.
6. 
   apoelstra commented at 4:02 pm on September 4, 2018: contributor

   Sure, we can do a VERIFY_CHECK on those functions that the ctx is not the static one, much like how we compare to NULL now.
7. 
   apoelstra commented at 5:53 pm on September 20, 2018: contributor

   Added a commit which VERIFY_CHECKs on all functions that modify the context object.

   I considered using ARG_CHECK but I didn’t see the point, since you can’t change the callback function from its default CHECK-like behaviour. Also, since it is actually UB to call these functions on static data (without synchronization primitives anyway), I wanted to prevent it more emphatically.

   Edit Changed from VERIFY_CHECK to CHECK because Greg pointed out that VERIFY is only defined during unit testing.

8. apoelstra force-pushed on Sep 20, 2018
9. apoelstra cross-referenced this on Sep 22, 2018 from issue Eliminate scratch memory used when generating contexts by apoelstra
10. in include/secp256k1.h:187 in 9461ae6f02 outdated

    178@@ -179,6 +179,13 @@ typedef int (*secp256k1_nonce_function)(
    179 #define SECP256K1_TAG_PUBKEY_HYBRID_EVEN 0x06
    180 #define SECP256K1_TAG_PUBKEY_HYBRID_ODD 0x07
    181 
    182+/** A simple secp256k1 context object with no precomputed tables. These are useful for
    183+ *  type serialization/parsing functions which require a context object to maintain
    184+ *  API consistency, but currently do not require expensive precomputations or dynamic
    185+ *  allocations.
    186+ */
    187+SECP256K1_API extern secp256k1_context *secp256k1_context_no_precomp;
    

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    sipa commented at 5:01 am on October 4, 2018:

    Can you mark this object const? If that works you don’t need any of the CHECK invocations I think as they all require a non-const argument. It would also alleviate any concerns about synchronization etc.

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    apoelstra commented at 3:17 pm on October 4, 2018:

    Done
11. add static context object which has no capabilities ed7c08417a
12. prevent attempts to modify `secp256k1_context_no_precomp` 40fde611bd
13. apoelstra force-pushed on Oct 4, 2018
14. in src/secp256k1.c:102 in 40fde611bd

     98@@ -91,6 +99,7 @@ secp256k1_context* secp256k1_context_clone(const secp256k1_context* ctx) {
     99 }
    100 
    101 void secp256k1_context_destroy(secp256k1_context* ctx) {
    102+    CHECK(ctx != secp256k1_context_no_precomp);
    

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    sipa commented at 7:56 pm on October 16, 2018:

    Are these necessary? A non-const ctx pointer is used, which can’t be secp256k1_context_no_precomp (as that one is const).

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    apoelstra commented at 8:23 pm on October 16, 2018:

    Oh, good point. I guess they’re not necessary. I can remove them if you want, though I don’t mind having them there.
15. 
    sipa commented at 7:17 pm on October 17, 2018: contributor

    utACK 40fde611bd145bdafb78a52af6d197489101bafb
16. 
    real-or-random commented at 10:33 am on October 18, 2018: contributor

    ACK 40fde611bd145bdafb78a52af6d197489101bafb
17. apoelstra cross-referenced this on Oct 27, 2018 from issue 0.13 tracking issue by apoelstra
18. sipa merged this on Nov 6, 2018
19. sipa closed this on Nov 6, 2018

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20. sipa referenced this in commit 314a61d724 on Nov 6, 2018
21. dongcarl cross-referenced this on Nov 6, 2018 from issue PublicKey de/serialization should support io::{Read,Write} traits. by dongcarl
22. sipa referenced this in commit e34ceb333b on Nov 26, 2018
23. etscrivner cross-referenced this on Jan 5, 2019 from issue Investigate using static context objects by etscrivner
24. gmaxwell cross-referenced this on Jan 23, 2021 from issue UB in tests: Invalid pointer comparison in secp256k1_fe_inv_all_var by practicalswift
25. sipa cross-referenced this on Jan 24, 2021 from issue Remove unused secp256k1_fe_inv_all_var by sipa

Contributors
apoelstra gmaxwell sipa real-or-random