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Source file src/oras.land/oras-go/pkg/registry/remote/internal/syncutil/once.go

Documentation: oras.land/oras-go/pkg/registry/remote/internal/syncutil

     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  package syncutil
    16  
    17  import "context"
    18  
    19  // Once is an object that will perform exactly one action.
    20  // Unlike sync.Once, this Once allowes the action to have return values.
    21  type Once struct {
    22  	result interface{}
    23  	err    error
    24  	status chan bool
    25  }
    26  
    27  // NewOnce creates a new Once instance.
    28  func NewOnce() *Once {
    29  	status := make(chan bool, 1)
    30  	status <- true
    31  	return &Once{
    32  		status: status,
    33  	}
    34  }
    35  
    36  // Do calls the function f if and only if Do is being called first time or all
    37  // previous function calls are cancelled, deadline exceeded, or panicking.
    38  // When `once.Do(ctx, f)` is called multiple times, the return value of the
    39  // first call of the function f is stored, and is directly returned for other
    40  // calls.
    41  // Besides the return value of the function f, including the error, Do returns
    42  // true if the function f passed is called first and is not cancelled, deadline
    43  // exceeded, or panicking. Otherwise, returns false.
    44  func (o *Once) Do(ctx context.Context, f func() (interface{}, error)) (bool, interface{}, error) {
    45  	defer func() {
    46  		if r := recover(); r != nil {
    47  			o.status <- true
    48  			panic(r)
    49  		}
    50  	}()
    51  	for {
    52  		select {
    53  		case inProgress := <-o.status:
    54  			if !inProgress {
    55  				return false, o.result, o.err
    56  			}
    57  			result, err := f()
    58  			if err == context.Canceled || err == context.DeadlineExceeded {
    59  				o.status <- true
    60  				return false, nil, err
    61  			}
    62  			o.result, o.err = result, err
    63  			close(o.status)
    64  			return true, result, err
    65  		case <-ctx.Done():
    66  			return false, nil, ctx.Err()
    67  		}
    68  	}
    69  }
    70  

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