github.com/opcr-io/oras-go/v2@v2.0.0-20231122155130-eb4260d8a0ae/internal/syncutil/once.go (about)

     1  /*
     2  Copyright The ORAS Authors.
     3  Licensed under the Apache License, Version 2.0 (the "License");
     4  you may not use this file except in compliance with the License.
     5  You may obtain a copy of the License at
     6  
     7  http://www.apache.org/licenses/LICENSE-2.0
     8  
     9  Unless required by applicable law or agreed to in writing, software
    10  distributed under the License is distributed on an "AS IS" BASIS,
    11  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  See the License for the specific language governing permissions and
    13  limitations under the License.
    14  */
    15  
    16  package syncutil
    17  
    18  import "context"
    19  
    20  // Once is an object that will perform exactly one action.
    21  // Unlike sync.Once, this Once allowes the action to have return values.
    22  type Once struct {
    23  	result interface{}
    24  	err    error
    25  	status chan bool
    26  }
    27  
    28  // NewOnce creates a new Once instance.
    29  func NewOnce() *Once {
    30  	status := make(chan bool, 1)
    31  	status <- true
    32  	return &Once{
    33  		status: status,
    34  	}
    35  }
    36  
    37  // Do calls the function f if and only if Do is being called first time or all
    38  // previous function calls are cancelled, deadline exceeded, or panicking.
    39  // When `once.Do(ctx, f)` is called multiple times, the return value of the
    40  // first call of the function f is stored, and is directly returned for other
    41  // calls.
    42  // Besides the return value of the function f, including the error, Do returns
    43  // true if the function f passed is called first and is not cancelled, deadline
    44  // exceeded, or panicking. Otherwise, returns false.
    45  func (o *Once) Do(ctx context.Context, f func() (interface{}, error)) (bool, interface{}, error) {
    46  	defer func() {
    47  		if r := recover(); r != nil {
    48  			o.status <- true
    49  			panic(r)
    50  		}
    51  	}()
    52  	for {
    53  		select {
    54  		case inProgress := <-o.status:
    55  			if !inProgress {
    56  				return false, o.result, o.err
    57  			}
    58  			result, err := f()
    59  			if err == context.Canceled || err == context.DeadlineExceeded {
    60  				o.status <- true
    61  				return false, nil, err
    62  			}
    63  			o.result, o.err = result, err
    64  			close(o.status)
    65  			return true, result, err
    66  		case <-ctx.Done():
    67  			return false, nil, ctx.Err()
    68  		}
    69  	}
    70  }