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package runners
import (
"fmt"
"sync"
)
// Processable defines a runnable process with error return
// that can be passed to a Processor instance for managed running.
type Processable func() error
// Processor acts similarly to a sync.Once object, except that it is reusable. After
// the first call to Process(), any further calls before this first has returned will
// block until the first call has returned, and return the same error. This ensures
// that only a single instance of it is ever running at any one time.
type Processor struct {
mutex sync.Mutex
state uint32
wait sync.WaitGroup
err *error
}
// Process will process the given function if first-call, else blocking until
// the first function has returned, returning the same error result.
func (p *Processor) Process(proc Processable) (err error) {
// Acquire state lock.
p.mutex.Lock()
if p.state != 0 {
// Already running.
//
// Get current err ptr.
errPtr := p.err
// Wait until finish.
p.mutex.Unlock()
p.wait.Wait()
return *errPtr
}
// Reset error ptr.
p.err = new(error)
// Set started.
p.wait.Add(1)
p.state = 1
p.mutex.Unlock()
defer func() {
if r := recover(); r != nil {
if err != nil {
rOld := r // wrap the panic so we don't lose existing returned error
r = fmt.Errorf("panic occured after error %q: %v", err.Error(), rOld)
}
// Catch any panics and wrap as error.
err = fmt.Errorf("caught panic: %v", r)
}
// Store error.
*p.err = err
// Mark done.
p.wait.Done()
// Set stopped.
p.mutex.Lock()
p.state = 0
p.mutex.Unlock()
}()
// Run process.
err = proc()
return
}
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