Coverage bytecode filter

[dmitrii-artuhov]

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Notice

This PR also changes the way classes bytecode is restore to the initially loaded into Lincheck agent. Instead of redefineClasses(...) now retransformClasses(...) is used in order to avoid double-instrumentation by other attached agents (e.g. intellij-coverage agent was throwing ClassFormatError because of this problem).

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Description

Relates to #254.

This PR adds a bytecode filter class, which "hides" bytecode generated by intellij-coverage agent (e.g. when using kover gradle plugin) from transformation that exist in Lincheck.

• [x] Hide bytecode generated by default kover settings (hits[index] = 1)
• [x] Hide bytecode generated by default "Run with coverage" settings in Intellij IDEA (hits[index] += 1)

The bytecode tracking is implemented as state machine with 5 states (+ symbol designates the direction of an arrow):

                           INITIAL  +----------------------------------------
                       /            +                                       |
                     /                \                                     |
                   +                    +                                   |
          HITS_INIT           HITS_INIT_FIELD                               |
                    \                    /                                  |
                     +                  +                                   |
                        HITS_IN_LOCAL   -------------------------------------  
                         |         +                
                         +         |                
                      HITS_BEFORE_ASSIGN 

Short overview:

• The state of automata is reset when starting to instrument a new method
• HITS_INIT and HITS_INIT_FIELD states occur, when the hits array (which stores coverage data) is loaded onto stack. The difference between state is the bytecode that they target: the 1st one occurs when hits was loaded via LDC instruction, the 2nd when GETSTATIC instruction was used.
• The way intellij coverage loads hits using LDC is as follows:

  LDC __$hits$__ : Ljava/lang/Object; ...
  CHECKCAST [Z
  ASTORE #n

  and using the GETSTATIC:

  GETSTATIC CLAZZ.__$hits$__ : [Z
  IFNONNULL L0
  ...
  INVOKESTATIC com/intellij/rt/coverage/instrumentation/CoverageRuntime.getHitsMask (Ljava/lang/String;)[Z
  PUTSTATIC CLAZZ.__$hits$__ : [Z
  L0
  GETSTATIC CLAZZ.__$hits$__ : [Z
  ASTORE #n

  Here GETSTATIC is called twice, that is why on the diagram above there is a double-sided arrow between INITIAL and LOADED_ON_STACK_STATIC states (first time hits is loaded on stack to check if it is null, then it is discarded, and lastly it is put on stack once again).

• HITS_IN_LOCAL occurs when the hits array is already saved to local variables of the method and the automata knows its index.
• When the ALOAD #n instruction occurs automata goes into HITS_BEFORE_ASSIGN state, then it searches for bytecode of hits[index] = 1 or/and hits[index] += 1. When the assignment completed the state goes back to HITS_IN_LOCAL.
• When the method is fully transformed, the automata is put to INITIAL state again.