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Merge 67dfd18f44 into merged_master (Bitcoin PR #16902)

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Andrew Poelstra 2020-11-26 01:07:55 +00:00
commit ed5795f10a
2 changed files with 90 additions and 3 deletions
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70
src/script/interpreter.cpp
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@ -286,6 +286,70 @@ int FindAndDelete(CScript& script, const CScript& b)
return nFound;
}
namespace {
/** A data type to abstract out the condition stack during script execution.
*
* Conceptually it acts like a vector of booleans, one for each level of nested
* IF/THEN/ELSE, indicating whether we're in the active or inactive branch of
* each.
*
* The elements on the stack cannot be observed individually; we only need to
* expose whether the stack is empty and whether or not any false values are
* present at all. To implement OP_ELSE, a toggle_top modifier is added, which
* flips the last value without returning it.
*
* This uses an optimized implementation that does not materialize the
* actual stack. Instead, it just stores the size of the would-be stack,
* and the position of the first false value in it.
*/
class ConditionStack {
private:
//! A constant for m_first_false_pos to indicate there are no falses.
static constexpr uint32_t NO_FALSE = std::numeric_limits<uint32_t>::max();
//! The size of the implied stack.
uint32_t m_stack_size = 0;
//! The position of the first false value on the implied stack, or NO_FALSE if all true.
uint32_t m_first_false_pos = NO_FALSE;
public:
bool empty() { return m_stack_size == 0; }
bool all_true() { return m_first_false_pos == NO_FALSE; }
void push_back(bool f)
{
if (m_first_false_pos == NO_FALSE && !f) {
// The stack consists of all true values, and a false is added.
// The first false value will appear at the current size.
m_first_false_pos = m_stack_size;
}
++m_stack_size;
}
void pop_back()
{
assert(m_stack_size > 0);
--m_stack_size;
if (m_first_false_pos == m_stack_size) {
// When popping off the first false value, everything becomes true.
m_first_false_pos = NO_FALSE;
}
}
void toggle_top()
{
assert(m_stack_size > 0);
if (m_first_false_pos == NO_FALSE) {
// The current stack is all true values; the first false will be the top.
m_first_false_pos = m_stack_size - 1;
} else if (m_first_false_pos == m_stack_size - 1) {
// The top is the first false value; toggling it will make everything true.
m_first_false_pos = NO_FALSE;
} else {
// There is a false value, but not on top. No action is needed as toggling
// anything but the first false value is unobservable.
}
}
};
}
bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror)
{
static const CScriptNum bnZero(0);
@ -301,7 +365,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
CScript::const_iterator pbegincodehash = script.begin();
opcodetype opcode;
valtype vchPushValue;
std::vector<bool> vfExec;
ConditionStack vfExec;
std::vector<valtype> altstack;
set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
if (script.size() > MAX_SCRIPT_SIZE)
@ -313,7 +377,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
{
while (pc < pend)
{
bool fExec = !count(vfExec.begin(), vfExec.end(), false);
bool fExec = vfExec.all_true();
//
// Read instruction
@ -506,7 +570,7 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
{
if (vfExec.empty())
return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
vfExec.back() = !vfExec.back();
vfExec.toggle_top();
}
break;