Support for Conditionnal Write

This commit introduces support for conditional writes for
major cloud providers: aws, gcp, azure.

It does so my adding a new write options: IfNotExist. When
set to true, conditions are set for each cloud storage driver
to enable the desired behavior.

Author: ocampeau

blob/azureblob/azureblob.go

@@ -913,6 +913,14 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key, contentType string, op
 			BlobContentType:        &contentType,
 		},
 	}
+	if opts.IfNotExist {
+		etagAny := azcore.ETagAny
+		uploadOpts.AccessConditions = &azblob.AccessConditions{
+			ModifiedAccessConditions: &azblobblob.ModifiedAccessConditions{
+				IfNoneMatch: &etagAny,
+			},
+		}
+	}
 	if opts.BeforeWrite != nil {
 		asFunc := func(i any) bool {
 			p, ok := i.(**azblob.UploadStreamOptions)

blob/blob.go

@@ -1098,6 +1098,7 @@ func (b *Bucket) NewWriter(ctx context.Context, key string, opts *WriterOptions)
 		MaxConcurrency:              opts.MaxConcurrency,
 		BeforeWrite:                 opts.BeforeWrite,
 		DisableContentTypeDetection: opts.DisableContentTypeDetection,
+		IfNotExist:                  opts.IfNotExist,
 	}
 	if len(opts.Metadata) > 0 {
 		// Services are inconsistent, but at least some treat keys
@@ -1422,6 +1423,13 @@ type WriterOptions struct {
 	// asFunc converts its argument to driver-specific types.
 	// See https://gocloud.dev/concepts/as/ for background information.
 	BeforeWrite func(asFunc func(any) bool) error
+
+	// IfNotExist is used for conditional writes. When set to true, if
+	// a blob exists for the same key in the bucket, the write operation
+	// won't take place.
+	// It only works for AWS S3, Google Storage and Azure. Setting this to
+	// true for any other driver is a no-op.
+	IfNotExist bool
 }
 
 // CopyOptions sets options for Copy.

blob/driver/driver.go

@@ -110,6 +110,13 @@ type WriterOptions struct {
 	// asFunc allows drivers to expose driver-specific types;
 	// see Bucket.As for more details.
 	BeforeWrite func(asFunc func(any) bool) error
+
+	// IfNotExist is used for conditional writes. When set to true, if
+	// a blob exists for the same key in the bucket, the write operation
+	// won't take place.
+	// It only works for AWS S3, Google Storage and Azure. Setting this to
+	// true for any other driver is a no-op.
+	IfNotExist bool
 }
 
 // CopyOptions controls options for Copy.

blob/gcsblob/gcsblob.go

@@ -626,6 +626,9 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key, contentType string, op
 	bkt := b.client.Bucket(b.name)
 	obj := bkt.Object(key)
 
+	if opts.IfNotExist {
+		obj = obj.If(storage.Conditions{DoesNotExist: true})
+	}
 	// Add an extra level of indirection so that BeforeWrite can replace obj
 	// if needed. For example, ObjectHandle.If returns a new ObjectHandle.
 	// Also, make the Writer lazily in case this replacement happens.

blob/s3blob/s3blob.go

@@ -1062,6 +1062,10 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key, contentType string, op
 			Key:         aws.String(key),
 			Metadata:    md,
 		}
+		if opts.IfNotExist {
+			// See https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-writes.html
+			reqV2.IfNoneMatch = aws.String("*")
+		}
 		if opts.CacheControl != "" {
 			reqV2.CacheControl = aws.String(opts.CacheControl)
 		}